The Algae - Phaepophycae - Laminariales - Laminaria

Phylum Thallophyta - The Algae - Phaepophycae - Laminariales - Laminaria 
The Laminariales are Phaeophyceae in ,yhich there is a marked alternation of dissimilar generations in which the sporophyte is large and conspicuous while the gametophyte is small and filamentous.
Although species of the Laminariales are widely distributed in temperate seas, the bulk of them occur in the colder waters of the Korth Pacific and North Atlantic oceans. Several species are common just below low-tide mark around the British coast.
The thallus is large, sometimes exceptionally so, for the order includes the largest of all the sea,yeeds, such as Lessonia and Macrocystis; the latter reaches zoo ft. in length and grows at a depth of 60 to 100 ft. In other species, however, the thallus may be much smaller and more comparable with other brown seaweeds. The largest British species is Sacco1'hiza bulbosa, the « Sea Furbelovvs," which may gro,y up to I5 ft. in length.
The gametophyte is small and filamentous, and has only been studied critically in a few species. The sex organs are borne on separate plants and consist of antheridia and oogonia. The former liberate antherozoids, but the oosphere after fertilization remains within or attached to the oogonium. The sporophytic plant may be either annual or perennial.
Laminaria
There are a number of common British species, all of which agree in general structure and life history, but- as they differ from one another very markedly in external form it ,yill be desirable to outline briefly these differ­ences at the outset so far as the common species are concerned.
Laminaria saccharina (Tangles). In this species the thallus consists of an undivided frond with a wavy margin, arising from a rather short, thin, round stalk, which is attached by a rhizoidal holdfast. It is perennial, grows up to 6 ft. long and occurs from low-water mark to a depth of 10 fathoms.
Laminaria digitata (Kelp) (Fig. IIs).-In this species there is a smooth, thick stalk which widens out gradually into a broad frond which is divided palmately into a number of separate fingers It is a considerably larger species than the last and the degree of dissection of the lamina varies considerably.
It may be up to 12 ft. long and is attached by an elaborate rhizoidal system. It occurs between low water and 15 fathoms.
Laminaria hyperborea (Tangles ).- This species resembles L. digitata in the shape of the thallus, but differs from it in that the stipe is not smooth and it expands abruptly into the palmate frond. It is a large plant, and though not as large as the last, grows up to 10 ft. long. It occurs bet\yeen low-tide mark and 12 fathoms.
In all species the expanded lamina has no midrib and is borne on a basal stipe, which is attached to the rock surface by a hold fast of very variable form. This holdfast is made up of a number of separate branches of decreas­ing thickness which adhere very tightly, forming flattened discoid masses immediately in contact with the rock and attached by densely packed rhizoids. The lamina grmys from a meristematic zone at its base, which annually forms a new frond, displacing that of the previous season,which then dies off.
Many of the species are used as food in Asia, especially by the Chinese, Japanese and Russians. In Japan about ten species are eaten, and the gathering between July and October is an important industry. These kelps contain considerable quantities of Iodine, which is important for the functioning of the thyroid gland, and it is a noticeable fact that goitre is almost unknown among the Japanese. Apart from their use as food the kelps are perhaps the most important commercial source of Iodine, which is extracted from the ash after the weeds have been burned.
STRUCTURE OF THE THALLUS 
The thallus of Larninaria shows an exceptionally complex structure, the most elaborate in any group of the Algae. Anatomically stipe and lamina are alike and both show a separation into three distinct zones (Fig. 117) although these are more clearly marked in the stipe owing to its greater thickness. The stipe may also show annual zones of growth. Near the apex the blade is only one cell thick, but it soon becomes two-layered, after which the primary tubes are formed, which constitute the medulla and separate the two external layers. By division of the cells of the outer layers parallel to the surface a zone of cortex is cut off. These cortical cells then elongate longitudinally and the common walls between them swell and so separate the cells from one another, except at certain points of union where they become drawn out into short secondary tubes. In this way three separate zones of tissue become differentiated. On the outside lies the external layer, the cells of which are primarily concerned with assimilation. These divide only perpendicular to the surface. Inside this comes the cortex, composed of elongated cells separated from one another by mucilage, and finally in the middle is the central medulla composed of larger, longitudinally running filaments whose function appears to be primarily that of conducting materials in solution. In the stipe these zones can be clearly seen.
The cells of the medulla may become greatly modified. This applies particularly to certain cells which cease to divide at an early stage.
They are drawn out by the growth of the tissues, into long straight filaments, considerably attenuated, except at their ends, which retain the original width, thus producing the appearance which has given them the name trumpet hyphae. The end walls are penetrated by numerous protoplasmic con­nections, sometimes sheathed in callus, which later extends to cover both sides of the perforated plate. The structures have been compared to the sieve tubes of higher plants.
Apart from this the trumpet hyphae possess spiral bands of cellulose thickening on their walls, a feature which again recalls the thickening of a wood vessel. The function of these trumpet hyphae is still uncertain. Many suggest that they serve for conduction of fluids, while others think of them as storage organs, and others again prefer to regard them mainly as organs of support. It may be pointed out that pitting is not restricted to the trumpet hyphae, as in some species other cells in the inner cortex occur with pitted walls, similar to those in the medulla. These may also facilitate the diffusion of food material.
A system of anastomosing, intercellular mucilage ducts occurs in the stipe and frond of several species, but only in the fronds of L. saccharina and L.digitata. There are periodic openings from these ducts to the exterior and they are lined with isolated groups of secretor; cells. They arise schizogenously between cells of the surface layer and deepen and extend with growth, becoming connected with each other into a continuous network.
The structure of the attaching organ or holdfast is markedly different from the rest 0 the thallus. Growth is localized in the apices of the branches which spread out in contac with the rock. It differs anatomically by the absence of a medulla and of trumpet hyphae.
ASEXUAL REPRODUCTION 
The asexual reproductive organs are developed in widely extended sori, which may cover the greater part of the surface on both sides of the lamina. The zoosporangia arise from the superficial cells in the following way. Each cell divides into two, forming a basal cell and a terminal cell. The terminal cell is at first assimi­latory, but later enlarges considerably, becomes club shaped and is invested at the top by a  mucilaginous cap. This body becomes a para­physis, and the caps of all the paraphyses adhere ct and serve to keep them together. Meanwhile the 19 basal cell enlarges laterally and from its outer ends cuts off two cells, one on each side of the
terminal cell. Each enlarges considerabely, becomes oval and forms a sporangium. These sporangia thus come to lie between the paraphyses, and the whole is covered by the mucilage derived from the latter. In­side the zoosporangium thirty-two zoospores are differentiated, and are finally liberated through the apex of the sporangium. According to most workers these zoospores are all of the same size, though in one or two cases zoospores of different sizes have been described.
The zoospores are almost pear­shaped bodies with two long, laterally placed flagella, and each may possess a tiny eye spot. They are actively motile but soon settle down and germinate.
THE GAlVIETOPHYTES 
The result of germination is the formation of very small male and female gametophytic plants, both of which are filamentous and differ completely from the sporophytic thallus. They also differ from each other in shape and size.
On germination the zoospore first forms a tube which terminates in an enlargement into which the contents of the zoospore migrate. The nuclelli in the zoospore divides, and one daughter nucleus passes into the enlargeme-­while the other degenerates. The cell formed by the enlargement divide several times, and the mature gametophyte consists of a short filamen: of cells with shorter branches. It may even be reduced to two or three cells. The male gametophyte is composed of smaller cells than the female gametophyte.
The sex organs arise from the lateral branches. The antheridium is 2 small, more or less spherical, cell which gives rise to one antherozoid. This antherozoid is a small, oval cell provided with a pair of fairly long, unequal. laterally placed flagella. The oogonium is also formed from a cell of a lateral branch, inside which is a single oosphere. The oogonium is surrounded by a thick mucilaginous investment which is prolonged at the apex into a cup comparable with an egg cup. The oosphere emerges from the oogonium and lies in the cup with the greater part of the oosphere exposed.
Fertilization is effected in the usual way by the migration of the anthero­zoid to the oosphere and the union of the male and female nuclei, after which a wall is formed around the oosphere, resulting in the formation of an oospore. Male and female gametophytes are found in equal numbers, and it has been shown experimentally that from any zoosporangium an average of sixteen male and sixteen female gametophytes will be produced. It follows that normally each oosphere should become fertilized, and this takes place quite rapidly. Should the oosphere fail to receive a male gamete it may be capable of independent development producing a parthenogenetic sporophyte.
The development of the oospore begins with its division into two cells from which is produced a filament \vith an apical cell. At first the young plant remains attached to the top of the oogonium, but it later become detached and one or more of the basal cells of the filament elongate into unicellular rhizoids which form the primary attachment to the substratum. Next the upper part widens into a mono stromatic blade on a filamentous stipe. The blade then becomes distromatic and the stipe becomes polysiphonous. A new meristematic region appears between stipe and blade; apical growth gradually ceases and the apex of the frond is eroded. The first appearance of the cortex is as a single layer of cells between the two layers of the blade. The cells are large and parenchymatous, and are increased by the addition of new cells in the meristematic zone. The medullary hyphae appear as the cortex becomes double-layered, and the expansion of the trumpet hyphae takes place in the intercellular spaces.
ALTERNATION OF GENERATIONS 
Laminaria thus exhibits an alternation of generations comparable with that in Dictyota, but whereas in Dictyota the two generations are morpho­logically alike, in Laminaria the gametophyte is reduced to a minimum, producing little more than the essential sex organs, while, on the other hand, the sporophytic generation shows the greatest tissue elaboration found in the Phaeophyceae. It must not be thought, however, that there is a definite progression within the Phaeophyceae tovvards a reduction of the gametophyte for in certain other orders the gametophyte is large and fairly elaborately developed, while the sporophyte is reduced to a filamentous structure bearin the zoosporangia. It might appear therefore that in the Phaeophyceae there have been two contrasting tendencies operating, the one to reduce the sporophyte and the other to reduce the gametophyte.

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The Algae - Phaepophycae - Dictyotales - Dictyota dichotoma

Phylum Thallophyta - The Algae - Phaepophycae - Dictyotales  - Dictyota dichotoma
These Algae are Phaeophyceae in which a definite alternation of genera­tions occurs and in \yhich the sporophytic and gametophytic plants are morphologically indistinguishable. Sexual reproduction is oogamous, while the organ of asexual reproduction is a non-motile spore termed a tetraspore, so called because only four spores are produced in each sporangium.
This is a small order with only a few genera, and \ye shall consider Dictyota dichotoma as our example.
Dictyota dichotoma
This Alga is common around the coast of Britain, growing in pools between tide-marks. The strap-like thallus is 10 to 20 em. long and consists of rectangular cells arranged in a single layer, with a superficial layer of smaller cells on each side of the thallus. Small tufts of hairs develop from scattered groups of these surface cells. The thallus is attached to the rock by a basal holdfast.
The thallus branches repeatedly, each division giving rise to two equal branches. Such a type of branching is termed dichotomous. Growth is by means of one large apical cell on each branch, which divides vertically into two equal halves when branching is about to take place. The male and female organs are borne on separate plants, and the asexual reproductive organs are produced on different plants from the sexual ones; but, as far as their general appearance is concerned, all three kinds of plants are exactly the same.
ASEXUAL REPRODUCTION 
The sporangium arises from a superficial cell of the thallus, which grows into a small spherical projection from the surface. Its nucleus enlarges and then divides meiotically, so that four monoploid nuclei are produced and the cytoplasm divides into four portions. No separating walls are formed, but the four monoploid spores are eventually liberated by a breakdown of the wall of the sporangium. These spores have no flagella.
The term tetraspore, which is applied to them, is used also for the asexual spores in the Rhodophyceae which are also formed in fours, but the number is not constant in the Dictyotaceae, some genera producing eight in each sporangium. The tetrasporangia are, of course, equivalent to the sporangia of Ectocarpus.
On germination a tetraspore produces a fresh Dictyota plant, resembling in appearance the parent from which it was developed, but bearing gametes.
SEXUAL REPRODUCTION 
The sex organs are antheridia and oogonia. They are arranged in groups or sori on the surface of the thallus on separate plants. Each antheridium arises from a superficial cell of the thallus. This cuts off a lower stalk cell and an upper or antheridial cell which enlarges and divides, like a gametangium of Ectocmpus, into a large number of parts, so that, at maturity, some 1,500 antherozoids are liberated. As there are between 100 and 300 antheridia in each sorus, and as the number of sari on a full-grown plant may be well over 3,000, there is a fortnightly production of something like 500 million antherozoids from each mature plant, which means that there are on the average about 6,000 male gametes to each female. These antherozoids are pear-shaped and very small, and each is provided with two lateral flagella, the one which is directed backwards being much reduced in length. Each antheridial sorus is surrounded by two or three rings of sterile cells, called the paraphyses.
The oogonium also develops from a superficial cell of the female thallus. This cell divides into two, the lower forming a stalk cell, and the upper the oogonium. Inside the oogonium a single oosphere is formed, which is liberated as a naked mass of protoplasm by the breakdown of the oogonial wall. Each female sorus contains twenty-five to fifty oogonia which are not surrounded by paraphyses.
Both kinds of gametes are set free in large quantities at intervals of fourteen days, shortly after the highest spring tides. This periodicity is hereditary and is maintained even under culture in aquaria. Fertilization occurs in the sea water and is effected by the movement of the antherozoids in large numbers to the oosphere, followed by the penetration of the oosphere by one of the antherozoids. Even if an oosphere fails to be fertilized it is capable of development to a limited extent.
Under normal circumstances the zygote germinates at once to produce a fresh diploid plant, like the parents in appearance, but on ,vhich eventually tetrasporangia will be produced.
It will be seen, then, that Dictyota exhibits a typical alternation of genera­tions between a diploid tetrasporic plant on the one hand, which is regarded as the sporophyte, and two haploid sexual plants, the male and the female, which constitute the gametophytes.

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The Algae - Phaepophycae - Cutleriales - Cutleria multifida

Phylum Thallophyta - The Algae - Phaepophycae - Cutleriales - Cutleria multifida 
The Cutleriales are Phae­ophyceae in which there is an alternation of generations be­tween a large upright gamet­ophyte and a small prostrate sporophyte. The gametangia are borne on short filaments arising from the surface layer I of separate gametophyte plants, and the number of gamete cells produced in the male gametangium is considerably greater than those in the female. It is only on the structure of the sex organs that the sexes can be distinguished. The asexual plants give rise to zoosporangia, from which a number of zoospores are liberated. Growth of the thallus is peculiar. The apices of the branches are composed of clusters of uniseriate filaments. These filaments coalesce below to form a solid tissue. At or near the base of each filament is a zone of actively dividing cells, which cut off above more cells to the filaments, while at the same time contributing below to the solid tissue of the thallus. Such a type of intercalary growth is sometimes referred to as trichothallic.
Only one family, the Cutleriaceae, is included in this order, and it contains only two well-known genera of which we shall consider one, Cutleria.
Cutleria multifida 
This Brown Alga occurs somewhat rarely around the British coasts below low-tide mark, and, though apparently widely distributed, the plants are usually found isolated. The male and female plants vary with the locality, so that it is difficult to find any feature whereby the sexes can be distinguished clearly from one another.
These plants are attached to the rock on which they grow by a small' holdfast. The thallus is thin and irregularly branched, the branches varying much in width and frequency. Each branch terminates in a collection' of filaments of cells of varying length, as described above. In section the thallus is seen to be composed of large central parenchymatous cells surrounded by several layers of much smaller cells, which contain the pigments and form the assimilatory tissue of the plant. In addition to the terminal filaments or hairs, tufts of similar hairs may be developed upon the whole surface of the thallus.
THE MALE GAMETANGIUM
The male gametangia occur over the whole smface of the plant either in small or large clusters, usually in association with the hairs. Both the hairs and the gametangia arise from superficial cells of the thallus. The cell grows out and divides to give a stalk-cell below and the gametangium above. Further divisions of the stalk-cell may occur so that a filament is produced, In this case not only does the terminal cell become a gametangium but one or more of the cells of the filament may produce lateral branches terminating in gametangia.
Inside the gametangium the nucleus divides and two cells are formed separated by a wall; more divisions follow, until the greatly enlarged gametangium consists of a number of parallel rows, each containing twenty­four or more cells. Each of these cells finally becomes a male gamete mother cell. Each cell has a single nucleus and associated with it a red pigmented eye spot. When mature a portion of the wall of the mother cell dissolves, leaving a small pore through which the gamete is discharged. In this process the flagella emerge first and show active movement, while the contents of the cell are still emerging.
THE FEMALE GAMETANGIUM 
Like the male, the female gametangia are scattered over the whole surface of the female plant and may be developed alone or associated with hairs. These hairs may develop long before any evidence of the gametangia is seen. Both the hairs and the gametangia arise from superficial cells of the thallus, but usually the filaments are shorter and not so much branched as in the male. The development of the gametangium is similar to that of the male, but the number of mother cells produced is considerably smaller. A mature female gametangium consists of from four to seven tiers, each being composed of from four to eight cells. Thus from sixteen to about fifty-six female gametes are ultimately liberated from each gametangium.
The mature female gamete is an oval structure with two long flagella, the longer one pointing forward and the other backwards, attached to its side in the region of a red pigment eye spot. The gamete also contains about thirty oval plastids. When first liberated it is actively motile but it soon becomes sluggish, the shape becomes spherical and the flagella are withdrawn.
FERTILIZATION
Both kinds of gametes are discharged at any time of the day or night, though it has been found in the Mediterranean that they are most abundant between 5 and 7 A.M.
The male gamete is considerably smaller than the female; it possesses two unequal flagella like those of the female, and a red pigment.spot, but few or no plastids. It may remain active for as long as twenty hours. The active male gamete usually be­comes attached to a female gamete only when the latter has become sluggish, but before it has produced any en­veloping wall. The flagella of the male gamete are then with­drawn, and the cytoplasms of the two cells coalesce and fusion of the two nuclei follows.
Should a female gamete fail to be fertilized it is still capable of growth, though only after becoming enclosed in a cell wall. A period of rest ustfally follovvs before any division takes place.
Compared with the rate of cell division in the fertilized zygote the rate of eelopment of these apogamous sporelings is very slow.
DEVELOPMENT OF THE ZYGOTE 
The development of the zygote and of the apogamous female gamete is similar. The cell first elongates, forming a spherical apex attached to a filamentous lower end. Division follows cutting off the spherical apex; repeated transverse divisions of the apical region follow, so that by the time the sporeling is about ten days old it consists of a columnar structure standing upright on the substratum with a stalk tomposed of one or two cells and a head made up of three or four cells. The direction of growth now changes and there is formed a flat plate of tissue derived from the basal end of the talk. The basal expansion grows by division of the marginal cells, which causes zonation of growth to appear similar to that in Coleochaete. As grow1:h continues the outline of this flat plate of cells becomes more and more irregular, and by the time it is about a month old it is large enough to be seen with the naked eye. Meanwhile the columnar tissue has ceased to grow, though the prostrate thallus continues to increase by marginal growth.
THE MATURE SPOROPHYTE (Aglaozonia reptans) 
For a long time the sporophyte of Cutleria was thought to be an independent seaweed and was known as Aglaozonia reptans. It possesses a creeping habit, and gro\ys on rocks and sea urchins' shells to which it becomes attached by means of rhizoids, developed from the superficial layer of its lower surface. It may reach a diameter of two or three inches. I t was not until 1898 that it was· shown that Aglaozonia reptans was really the sporophyte of Cutleria multifida.
A section through the thallus reveals one or two layers of small cells on the upper and lower surfaces, and in the middle two or three layers of very large parenchyma cells. From any superficial cell of the lower surface a rhizoidal holdfast may arise.
THE ZOOSPORANGIUM 
The zoosporangia are produced on the upper surface of the thallus and are formed in patches, each composed of hundreds or thousands of zoo­sporangia, which are packed close together in a palisade arrangement. Each zoosporangium arises from a superficial cell of the thallus, which elongates slightly and divides to give a small basal stalk cell and an upper cell, which becomes the zoosporangium. This latter cell enlarges and elongates until finally it is about three times as long as it is wide, with a somewhat swollen top. The nucleus of the zoo sporangium now divides by reduction division, which is followed by further mitotic divisions, until from eight to thirty-two nuclei are formed. Cleavage furrows now appear in the cytoplasm, which divide the contents into zoospores. A pore develops at the top of the zoosporangium and through this the zoospores escape.
THE GERMINATION OF THE ZOOSPORES 
When liberated the zoospore is an oval body contammg about twenty plastids, associated with which is a single red pigment spot. It possesses two lateral flagella of unequal length, one being about twice the length of the other. The longer one is directed forwards, while the shorter trails behind. The zoospore may remain active for up to an hour and a half, after which it becomes sluggish, the flagella are withdrawn, the cell becomes spherical and a wall is laid down.
Division soon follows and a filament is formed. As growth continues the cells at the posterior end begin to divide longitudinally so that a widening of the filament occurs and it begins to assume a thalloid form, while rhizoids develop from the lower end. Gradually this lower portion, developing by lateral growth, forms a funnel-shaped structure. From the margin of this structure delicate multicellular filaments grow out, which become the apical tufts of branches, and gradually the typical Cutleria plant becomes differentiated.
ALTERNATION OF GENERATIONS 
Cutleria shows an alternation of generations between a large gameto­phyte plant which is unisexual and a small sporophytic plant. In the development of its reproductive organs it is essentially comparable with Ectocarpus, though there is more differentiation both in the number and size of the two kinds of gametes. It may be therefore regarded as a specialized type derived from a form like Ectocarpus, in which the asexual plants have been reduced in size, while the gametophyte plants have become more complex, especially in their vegetative structure. In certain genera of the Ectocarpales we have examples in which longitudinal division of some of the cells of the filaments occur, and thus it is possible to see the way in which the thallus of Cutleria may have originated. The zoospores produced as a result of meiosis give rise to male and female plants, and it is reasonable to assume . that as a result of genetic segregation these male and female plants should be produced in equal numbers. The Aglaozonia stage is comparable with the asexual stage of an Ectocarpus, and the early development of the zygote produces a filament  compareable with Ectocarpus plant. The flat thallus is  a secondary development facilitating a greater production  of zoosporangia.

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The Algae - Phaepophycae - Ectocarpales - Ectocarpus

Phylum Thallophyta - The Algae - Phaepophycae - Ectocarpales - Ectocarpus
The Ectocarpales are a small group of relatively simple Phaeophyceae in which the thallus is filamentous in its fundamental structure, though very varied in form. In the simplest cases the filaments are entirely free, but in some other forms the lower parts of the filaments are variously aggregated together to form more or less solid bodies, the ends alone remaining free.
The vegetative cells in Ectocarpales are shortly cylindrical, containing irregularly shaped chromoplasts with pyrenoids. There is no special apical cell. Growth takes place by the division of an intermediate group of cells in the filament, which form a meristematic zone. Such a method of growth, which is neither apical nor basal, is called intercalary.
There are two characteristic reproductive structures. Firstly, unilocular sporangia, which consist of single enlarged cells, the contents of which divide up, without the formation of any cell walls, to liberate a large number of motile cells which are pear-shaped and have two flagella, one directed forwards and the other backwards. Secondly, gametangia (plurilocular sporangia), derived from the cells of a short side branch. These divide up by transverse and longitudinal walls into a large number of small cubical cells, each of which produces a biflagellated motile gamete, similar in structure to the products of the unilocular sporangia.
Meiosis takes place at the first nuclear division in the unilocular sporangium, so that the spores from this type are definitely monoploid. In general, they function as asexual zoospores, giving rise to haploid plants. The plurilocular sporangia, on the other hand, are generally regarded as gametangia, the cells from which fuse in pairs, and from the zygote thus formed diploid plants arise. Variations of this behaviour are, however, known, as explained below.
One example of this order is Ectocarpus, but as there is considerable variation in details between species, the description must necessarily be a general one.
Ectocarpus
Species of the genus Ectocarpus are very common in sea water around the British coast. Some are attached to rocks and stones, but many grow attached to other Algae, and are therefore said to live as epiphytes. '''Te must dis­tinguish between an epiphyte, in which the plant obtains no nourishment from the organism on which it lives, and a parasite in which the reverse is the case; for the parasites live at the expense of the second plant, which is then termed the host plant.
The thallus of Ectocarpus is generally filamentous and repeatedly branched. It is made up of two parts, a creeping portion which covers the substratum and serves as the holdfast, and a number of upright branches which arise from it. These upright branches  generally consist of a single row of cells, though in some species longitudinal cell divisions occur. The cells are small and rectangular, each con­taining a single nucleus and a number of small spherical bodies containing the brown pigment, which are called chromatophores or chromoplasts. Pyrenoids are present in the chromo­plasts. This is interesting because these bodies, which are characteristic of the Chlorophyceae, are only found in the most primitive members of the Red and Brown Algae.
Growth takes place by the division of a series of small cells situated near the base of the vertical branches. These cells are generally small and continue throughout the life of the branch to cut off additional cells from their upper sur­faces. The cells of the more apical part of the branch do not divide and are more elongated than those of the actively divid­ing or meristematic region. This is therefore an example of intercalary growth .
Asexual Reproduction in Ectocarpus
The asexual reproductive organs consist of unilocular zoosporangia which are developed on the side branches (Fig. 94). They originate as simple globular or pear-shaped cells which become densely filled with protoplasm. The original nucleus divides first meiotically and then repeatedly by mitosis. Around each daughter nucleus a small quantity of protoplasm collects to form a zoospore. It consists of a small, rather elongated cell, and contains besides the nucleus a single brown plastid. Two flagella are developed at one side of the zoospore, one pointing forwards and the other backwards. On liberation the zoospore swims away, eventually settles down and grows into a monoploid plant.
Sexual Reproduction in Ectocarpus
The sex organs are borne on monoploid plants and consist of gametangia , which develop laterally, either sessile on a branch or at the end of a short row of sterile cells.* Each gametangium is composed of a number of chambers, separated from one another by transverse and longitudinal walls. In each of these com­partments one, or sometimes two, gametes are formed.
They do not differ mor­phologically from the asexual zoospores, but on liberation they fuse in pairs. In some species, e.g., E. secundus, there is a difference in the size of the gametes, and in such cases it is seen that the smaller ones cluster around the larger ones. It is con­cluded that the larger repre­sent the female gametes, while the smaller are regarded as the males. After conjugation the zygote develops into a diploid plant, which eventu­ally produces sporangia.
Sexual plants are as a rule dioecious, gametes from one plant behaving for the most part consistently either as males or as females. There are, however, exceptions, cases of irregular behaviour, which led Hartmann to the doctrine Fr of relative sexuality, which regards sex as a quantitative character, which may be either <:trnno- nr weak. so that a  given cell may behave as either male or female according to the strength of sex in the cell with which it pairs.
As mentioned above under the heading of Ectocarpales, certain variations in the life history of Ectocarpus exist, which have given rise to considerable controversy in regard to its methods of reproduction. The principal point to be noticed is that both monoploid and diploid plants occur, which are morphologically indistinguishable. There is evidence that in some species they may be geographically separated. Monoploid plants appear to form only gametangia, which produce gametes that fuse in pairs. They may therefore be considered as true gametophytes. Diploid plants, on the other hand, may produce both gametangia and zoosporangia. In this case the products of the gametangia are diploid and germinate without conjugation like zoospores. The products of the unilocular sporangia are, however, monoploid.  Most observers agree that they too germinate without fusion and regard them as true zoospores. It has been claimed, however, that they may fuse in pairs, producing diploid plants, but the actual develop­ment of such apparent conjugations has never been observed. If it occurs we have here a complete inversion of the normal reproductive order
The occurrence of con­jugation between the motile 2 cells produced in sporangia has been interpreted as im­plying, in these cases, the complete suppression of the gametophyte stage, the pro­ducts of the sporangia be­having directly as gametes instead of germinating to produce another vegetative generation. This special case in Ectocarpus would therefore be comparable to that in the normal life-history of Fucus.

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The Algae - Phaeophycae

Phylum Thallophyta - The Algae - Phaeophycae
The Phaeophyceae are Algae in which an aditional pigment, fucoxanthin, is developed, more or less completely masking the chlorophyll which is also present in the cells. Unicellular types occur, and there are a number of genera in which the majority of the species are filamentous, but the bulk of the species included in this group have a more complex thallus built up of either cellular tissue or of a number of central filaments bound together by mucilage, each of which gives off lateral branches in such numbers that they form a tissue surrounding the central filaments. In such types the central filaments are termed the medulla, while the surrounding tissue is spoken of as the cortex. In the most advanced types the cells of the medulla become modified into specialized conducting elements, which assist in the transference of the food material from one part of the plant to another. We find, then, in the higher Phaeophyceae much greater morphological differentiation than is met with in the Chlorophyceae.
Sexual reproduction by means of isogametes is the method most common among the lower orders, while in the higher orders reproduction may be either isogamous or oogamous. Asexual reproduction is by means of zoo­spores, which differ from those of the Chlorophyceae in being pear-shaped with two laterally placed flagella.
Many of the Phaeophyceae exhibit alternation of generations, that is to say, there is a definite and immutable alternation of sexual and asexual individuals. The zygote develops into a plant which mayor may not resemble the parent and on which only asexual reproductive bodies are borne. These on germination produce a plant, not necessarily like its parent, but like the " grandparent," on which sexual reproductive bodies are developed. The plant which produces the gametes is termed the gametophyte, and that which develops the asexual spores is called the sporophyte. In alternation of generations therefore there is an alternation of gametophyte and sporo­phyte generations. This obligatory alternation of generations is extremely important, and although the Phaeophyceae are the first group in which we meet it as a general characteristic, we shall find that in all higher plants it invariably occurs, though it may not be easy to recognize it at first sight, owing to secondary modifications which have taken place in the course of evolution.
The Phaeophyceae are divided into a number of orders of which we shall
consider examples from five :-

1. Ectocarpales (e.g., Ectocarpus). 
2. Cutleriales (e.g., Cutleria). 
3. Dictyotales (e.g., Dictyota). 
4. Laminariales (e.g., Laminaria). 
5. Fucales (e.g., Fucus). 

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Phylum Thallophyta - The Algae - CYANOPHYCEAE (Myxophyceae)

Phylum Thallophyta - The Algae - CYANOPHYCEAE (Myxophyceae) 
The Cyanophyceae are Algae in which the thallus may be either unicellular or filamentous. It is always enclosed in a gelatinous sheath. The cells possess phycocyanin (blue) and carotin (orange) in addition to chloro­phyll, and the varying proportions of these pigments not only mask the green colour but cause a wide variation of tint from almost pure blue to brown. They occur chiefly in fresh water and on damp surfaces; some are found in the soil, whilst a minority are marine. They constitute the principal vegetation of hot springs, some being able to live in water as high as 85° C.
Apart from their interest as the simplest of the Algae they are remarkable for their cytological structure. The cell wall is usually very thin and is composed either of pure cellulose or a hemi-cellulose. External to the wall is the gelatinous sheath, composed of pectic compounds.* It is sometimes thin and tough, sometimes massive and diffuse. It may also consist of multiple lamellae, and not infrequently forms a common investment round a group of filaments, or trichomes as they are called in this family. These sheaths may become very tough, and it is undoubtedly due to them that the plants can withstand high temperatures and considerable desiccation.
The protoplast is remarkably simple. It is more or less uniform, without any vacuole or plastids. The central region is almost colourless, and scattered in it are chromatin granules which represent all there is of a nucleus. These granules can divide, and they separate into two groups prior to cell division, but in some instances there appears to be little, if any, co-ordination between the grouping of these granules and the division of the cells. There is certainly no regular mitosis. In the external zone of the protoplasm the pigments are diffused throughout its substance and there are also present numerous minute granules, including oil drops, glycogen and others which are probably volutin and are called metachromatic granules.
Many of the filamentous species develop either terminal or intercalary heterocysts, which are easily recognized by their larger size, thick wall and homogeneous contents. The function of these bodies is not known.
Reproduction may take place by the breaking of the trichome, within the sheath, into short filaments of cells which are termed hormogones. The points of disjunction are marked by collapsed biconcave cells. The hormo­gones are set free by a breakdown of the sheath at the end of the parent filament. They are capable of slow movement though they have no flagella, and finally settle down and form fresh filaments. In one or two species reproduction may be eft'ected by gonidia, which are tiny non-motile cells derived by the division of one or more cells of the parent plant. These are liberated and finally grow into fresh filaments. Resting spores may also be produced. These bodies arise from vegetative cells which increase in size, become gorged with reserve material, and are often yellow or brown in colour. They are frequently developed in association with heterocysts. The unicellular species generally multiply only by simple cell division.
Many species of the Cyanophyceae are world-wide in distribution. They are among the first organisms to colonize new ground and they were the first organisms found on the island of Krakatau after the eruption of 1883. They are found in the Arctic, even though they may be covered for months on end with snow and ice. On the other hand, as we have seen, they occur very commonly in hot springs. Some species live within the tissues or even within the cells of higher plants. Species of Anabaena are found in cavities in the fronds of the water-fern Azolla, and also in the roots of the gymno­sperm Cycas. Species of Nostoc form colonies in the underground stem of the giant angiospermic marsh plant Gunnera manicata. Many as we shall see later, are concerned in the formation of Lichens. Their wide distribution, coupled with their peculiar structure, seems to indicate that they are probably the most ancient and primitive of the Algae.
The Cyanophyceae are divided into several orders of which we shall consider only the Hormogonales, which includes most of the filamentous types.
Hormogonales 
The Hormogonales are Cyanopqyceae in which the filaments have no individual membrane on the vegetative cells. The trichomes are simple, free and sometimes coiled. They are septate, but the septa are frequently obscure. The cells are very uniform, short and discoid or rectangular in shape, with a uniform granular cytoplasm, divisible into two regions, a peripheral one containing the pigment together with glycogen and oil­drops, and a central region which is colour less and contains metachromatic granules. The trichomes may be free-floating, or may form a more or less mucilaginous mass on solid surfaces. Heterocysts occur only in certain families.
Reproduction is by means of hormogones, short lengths cut off from the parent filament which are capable of growing into mature trichomes. Thick­walled resting spores or akinetes are often developed, usually next to the heterocysts. No method of sexual reproduction occurs.
We shall consider one common type-Nostoc.·
Nostoc 
There are a number of common species which occur either terrestrially or as aquatics in mucilaginous masses          (" moon-spit "), living either attached to a substratum or free floating. Each mass is composed of a more or less firm jelly or mucilage, with a denser surface layer, inside which are numerous twisted trichomes, which are yellowish-green in colour. These gelatinous masses may be either solid or hollow and as much as several centimetres in diameter; they frequently break open to form flattened expanses with lacerated margins.
The trichomes are much contorted and form an intricately woven mass which is more crowded towards the exterior. Each is composed of nearly spherical cells which are only loosely joined together. The heterocysts are intercalary and solitary, and are oval in shape and slightly larger than the vegetative cells.
Reproduction in CYANOPHYCEAE (Myxophyceae) 
Hormogone formation is very common and results from the rupture of the trichome at points where a vegetative cell and a heterocyst join. The large numbers of trichomes in a colony is probably due to the development of the hormogones into fresh trichomes without being liberated from the colony.
When the colony is mature akinetes may be formed from any or all of the vegetative cells, and it is not unusual to find all the cells between two heterocysts converted into akinetes. Each germinates to form a new trichome.
While many species grow on bare soil among mosses, others are sub­terranean, and have been found as much as three feet deep in the soil. Many are aquatic and live free-floating, attached to vegetation or on the bottom of the pond. A few species favour running water, particularly fast-flowing mountain streams.
Nostoc frequently forms the algal component of Lichens, while one species is found in symbiotic association with Anthoceros.

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The Algae - Siphonocladiales - Cladophora

Phylum Thallophyta - The Algae - Siphonocladiales - Cladophora
The Siphonocladiales are Chlorophyceae in which each branch of the ' thallus is composed of one or more coenocytes. Though in the simpler members the thallus consists of a single coenocyte and therefore may be comparable with that of the Siphonales, the group as a whole shows a pro­gressive complexity, brought about partly by the development of septa which cut the large coenocytes into smaller ones and partly by the develop­ment of the branching system.
Asexual reproduction is by means of zoosporangia in 'which are developed zoospores, or by aplanospores. Sexual reproduction is by the fusion of isogametes.
Most of the species are marine and are more characteristic of the warmer temperate and tropical seas than of this country.Many are found in the Mediterranean while others occur particularly in the West Indies. A few genera are found in fresh water. A number of species are calcified, fossil types of which have been described.
We shall consider only one type, Cladophora, which is among the more advanced members of the group. In fact recent classifications have tended to favour separating it as an independent order, the Cladophorales, while relegating all the other families to the Siphonales, from which they have most probably arisen.
Cladophora 
Species of this genus occur characteristically both in fresh water and also in the sea, and a few species are epiphytic or even epizoic. The species are very numerous and difficult to separate. The thallus is composed of branched,septate filaments formed of cylindrical coenocytes joined end to end. It is attached to the substratum by means of branched, septate rhizoids. Each branch grows by means of an apical segment. The separate coeno­cytes contain numerous nuclei which are either embedded in the meshes of the chloroplast or lie internal to it. The structure of the chloroplast is complex, and divergent views exist regarding its nature. It appears to consist of a parietal reticulum which lies in the cytoplasm, though additional chloroplast segments may develop inwards into the central vacuole. The meshes of this reticulum cover the longitudinal and transverse walls of the coenocyte and vary much in width. There are numerous pyre­noids scattered in it which can FI multiply by division. In unfavourable conditions the chlorophyll may contract around the pyrenoids in more or less spherical masses, thus giving the impression of separate discoid chloroplasts, which may explain the view expressed by some workers that the chloroplast is not a single reticulate structure but a number of small connected, discoid bodies. Fragmentation of the chloroplast has been observed not infrequently.
The wall of the coenocyte is composed of a series of thick, stratified membranes, which consist of an inner and an outer lamella, with a superficial pellicle which can be separated by treatment with acetic acid. Both lamellae show stratification which usually runs obliquely to the long axis of the coenocyte. There is little or no mucilage, which probably explains the great abundance of epiphytes which normally occur on the plants.
Branches usually arise at the upper end of a coenocyte and at first form a wide angle with the parent branch, for the septum cutting off the branch is developed nearly perpendicular to it. Sometimes two branches arise together, one on either side of the main axis.
Growth is by means of the apical segment, which, after elongating sufficiently, forms a transverse septum cutting off the greater part of the segment. At the beginning of such a division the protoplast is withdrawn from the region where the new septum will develop, the space so left being apparently filled with mucilaginous material. An annular bar of thickening is laid down here, extending transversely all round the cell. This forms the beginning of the cross wall which then gradually extends inwards towards the centre. At the same time it extends into and fuses with the inner lamella of . the longitudinal wall. The formation of septa is in no way related to nuclear division.
The attaching organs consist of a number of septate, rhizoidal branches. These may sometimes spread out as stolons which either give rise to new upright filaments or their tips may grow out into a number of short cells, which later fall apart and serve as a method of propagation. The short cells formed on the rhizoids have thick walls and are abundantly supplied with food reserves. After separation they may undergo further wall thickening and function as over-wintering organs. Many species are, however, free-floating, as can be seen in the floating tangles (" flannel weed") which are so characteristic of ponds. In the free-floating species smaller or larger parts of the vegetative branches may separate, develop thick walls, and being well supplied with food reserves may function, in some species, as the only means of reproduction.
Asexual Reproduction in Cladophora
The zoospores are small pyriform bodies with a small anterior beak and two apical flagella, or in some species four. These flagella are differentiated before the emergence of the spores. There are two granules at the point where the flagella are inserted and the chloroplast appears as a ring in the posterior part of the cell and shows a slow streaming movement. Those zoospores nearest the orifice escape first, squeezing their way through the opening with their flagella behind them, and they are followed by a steady stream of others. After about twenty minutes they settle down by their anterior ends and elongate. A septum appears and gradually the coenocyte structure of the thallus is re-established.
Sexual Reproduction in Cladophora
The formation of gametes occurs in most species and probably in all. In some species it has been definitely established that the thallii are dioecious. Any segment of the thallus is potentially capable of functioning as a gamet­angium, and its development is similar to that of the zoosporangium. The gametes are iso­gamous and differ in no marked way from the zoospores, except that they are invariably biflagellate. They are liberated in a similar manner to the zoospores, and fuse externally to give rise to a zygote which germinates immediately to form a fresh Cladophora filament.
Alteration of Generation
It has now been definitely established that in almost all species there is a strict alternation of sexual and asexual generations. These generations are morphologically alike. Meiosis occurs prior to the formation of the zoospores which therefore give rise to male and female monoploid plants, the diploid phase being established after gametic fusion, resulting in the formation of a sporophytic plant, on which the zoospores are produced. A similar type of alternation has been found in a small number of related genera.
The life-cycle of Cladophora may therefore be represented by the following diagram :-
In Cladophoraglomerata it has been shown recently that zoospore formation occurs at intervals all through the year, while gamete formation happens only in the spring and at the end of a long series of zoospore discharges. Gametes and zoospores are developed in distinct plants, but there is no reduction division in the formation of the zoospores. It appears that this species can produce a succession of diploid zoospore generations, after which meiosis occurs during the formation of the gametes. Conjugation restores the diploid condition. This condition resembles that \\'hich has been described in some members of the Siphonales.

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The Algae - Chlorophycae - Siphonales - Vaucheria

Phylum Thallophyta - The Algae - Chlorophycae - Siphonales - Vaucheria 
This Alga is largely terrestrial, occurring frequently on damp soils, as, for example, on the soil in flower pots, where it forms a green felt-like covering. It may also be found in fresh water, and it is under these conditions that asexual reproduction most frequently occurs.
Vaucheria consists of a non-septate, tubular coenocyte with lateral branches. It is dark green in colour and is very sensitive to changes of environment. The coenocyte is frequently attached to the substratum by
means of a branched holdfast or hapteron, which may be brown or white in colour and is devoid of chloroplasts. The remainder of the coenocyte contains a lining layer of protoplasm closely applied to the cell wall, and in it are embedded numerous nuclei and chloroplasts. The central part of the coenocyte consists of a vacuole containing cell sap. The chloroplasts are small and discoid, but in some species they become spherical when exposed to intense light.
In Vaucheria the reserve material is oil, which is stored in countless tiny droplets in the cytoplasm. Under normal circumstances no starch is formed, but under constant intense illumination it can be produced, which seems to indicate that the appropriate enzymes are present.
Injury to the thallus results in the formation of a septum sufficing to isolate the injured part, but otherwise, with the exception of those formed in relation to the sex organs, no septa occur.
Asexual Reproduction in Vaucheria
Asexual reproduction takes place by the formation of zoospores (Fig. 77), which are produced singly in club-shaped zoosporangia. The sporangia develop singly from the swollen ends of the branches by the accumulation of a large amount of cytoplasm with many nuclei and chloroplasts in the swollen tip, which, as a result, diminishes the size of the central vacuole, and thus these tips appear dark green in colour. Inside the zoosporangium a single large mass is formed which is termed the zoospore. It contains many nuclei which are arranged in a single layer near the surface and opposite each nucleus a pair of flagella is developed. The zoospore is liberated by the breakdown of the apical part of the wall of the sporangium to form a small pore through Which the large zoospore forces Its way. This zoospore IS generally mter­preted as a compound structure, resulting from the failure of the protoplast within the sporangium to divide into a number of uninucleate biflagellate zoospores. Zoospores are formed only by aquatic species or by terrestrial ones which have become flooded. They are usually developed during the night, or they may be induced by a sudden transference from light to darkness, or from running to quiet water. On liberation the zoospores are sluggish and only swim for a short time. On coming to rest the flagella are immedi­ately withdrawn and a thin membrane is secreted around the zoospore. Germination begins with the protrusion of two or more tubular outgrowths, one of which attaches itself to the substratum and forms the holdfast, whilst the other produces the filament.
Not all species of Vaucheria produce zoospores. Some develop what are termed aplanospores. These are simple non-flagellate structures which are produced in special sporangia called aplanosporangia. These aplano­sporangia develop as swellings either on the main or lateral branches of the thallus. When mature the aplanospores simply drop out of the aplano­sporangium through a perforation in the wall. This second method of asexual reproduction occurs most commonly among terrestrial species.
Sexual Reproduction in Vaucheria
In Vaucheria the sexual reproduction is oogamous and occurs most frequently in plants growing on damp soil or in quiet water, but is rarely found in plants growing in streams. The antheridia and oogonia are borne close to one another, either together on the same filament or on a common lateral branch or on adjoining lateral ones. Development of the oogonium begins (Fig. 78) with the formation of a swelling, which becomes filled with nuclei, oil and chloroplasts, though ultimately only a single central nucleus remains, which reaches a considerable size. The rest migrate back into the filament. The whole oogonium then becomes cut off from the rest of the coenocyte by a transverse wall or septum. Later a beak, or lateral swelling, is developed at one side of the oogonium, and in this region the wall becomes gelatinous and dissolves, leaving a pore. It is at this point that, subsequently, the antherozoids enter the oogonium. When mature the chloroplasts and oil take up a central position, leaving either a clear peripheral area of protoplasm or at least a clear area under the beak, the receptive spot, and the contents as a whole contract and become the oosphere.
The antheridia are formed at the ends of short lateral branches and develop simultaneously with the oogonia. Each antheridium is an elongated, strongly curved structure and is cut off from the thallus by a septum. The nuclei within the antheridium divide up and ultimately each becomes surrounded by a small mass of cytoplasm and constitutes an antherozoid. These antherozoids form a mass in the centre of the antheridium, whilst the periphery is filled with chloroplasts and residual cytoplasm. The antherozoids are liberated apically shortly before daybreak, as very minute, colourless, oval bodies with two laterally inserted flagella.
At the time of fertilization the oosphere exudes a droplet of gelatinous cytoplasm through the pore in the oogonium wall, and the antherozoids accumulate in numbers around the droplet. Several antherozoids may enter the oogonium, but only one enters the oosphere. The small male nucleus migrates to that of the oosphere, which is considerably larger, but does not immediately fuse \yith it. Whilst lying side by side the male nucleus increases in size, and when its volume equals that of the female nucleus the two fuse together.
After fertilization a membrane develops across the oogonial aperture, and subsequently a thick, several-layered envelope is formed around the zygote, thus producing an oospore. At the same time the oil globules unite to form one or more large central globules, and the zygote enters a resting period of several months before germination. It germinates directly to produce a new filament without the intervention of zoospores.

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The Algae - Chlorophycae - Siphonales - Bryopsis plumosa

Phylum Thallophyta - The Algae - Chlorophycae - Siphonales - Bryopsis plumosa
The Siphonales are Chlorophyceae in which the thallus consists of a single multinucleate cell or coenocyte, which often grows to form a structure of macroscopic size. Asexual reproduction occurs in a few genera by the formation of zoospores. The sexual reproduction in many of the marine genera is by the fusion of unlike motile gametes; that in the fresh-water genera IS oogamous.
Some of these coenocytic Green Algae show great complexity. For example, in the genus Caulerpa, there is a complex development of coenocyte branches to form a thallus, which may be as much as 10 em. in height, with a definite external form. The thallus in Caulerpa is especially remarkable, since the branches of the single coenocyte build up thalli that simulate in appearance the differentiation into roots, stems and leaves found in the higher land plants.
Bryopsis plumosa
The genus Bryopsis is distributed mainly in tropical seas, but one species, B.plumosa, is commonly found in spring and early summer on the British coasts. The thallus consists of a single coenocyte, but this enormous ce shows differentiation into a main axis, from which arise, towards the up end, two rows of lateral branches or pinnae. From the lower end of the axis there is formed a little-branched horizontal rhizome anchored by rhizoids. This rhizome may produce numerous upright axes, so that each plant actually consists of a little tuft of vertically growing filaments. The pinnae vary in length, those nearest the base being the longest and decreasin regularly towards the apex. Each pinna is an elongated sac, and there is a constriction at the base where it joins the main axis. No true septa are formed in the coenocyte prior to the reproductive phase and the entire plant has one continuous vacuole lined by cytoplasm and containing numerous minute, round chloroplasts and nuclei.
Vegetative propagation may be effected by the detachment of pinnae which become plugged at the point of abstriction. These are able to develop new rhizoids and grow into fresh plants. It is interesting to note that in conditions of dull light or when plants are placed upside down the apices of the pinnae develop rhizoids.
The only known method of reproduction is by means of gametes. There is no asexual method of reproduction.
SEXUAL REPRODUCTION
The gametes are not alike and are usually produced on separate plants.
The first stage consists of the cutting off of a pinna from the main axis by means of a septum which arises as a ring-like thickening. The whole of the pinna thus forms a 'gametangium whose protoplasmic contents increase and the chloroplasts multiply by division. In the male gametangium the pyrenoids disappear from the chloroplasts, but they remain in those of the female gametangium. Later the contents divide up by simultaneous cleavages to form gametes, which are liberated by the gelatinization of the apex of the pinna. The gametes are pyriform with two equal, apical flagella. The female gamete is about three times as large as the male and is provided with a deep-green chloroplast. In the male the cWoroplast is yellowish in colour and probably not functional. Gametes fuse in pairs and produce a zygote which may, for a time, retain all four flagella. Later it rounds off and germinates directly to produce a new plant. The plants are diploid and reduction division occurs in the formation of the gametes

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The Algae - Chlorophycae - Conjugales - Spirogyra

Phylum Thallophyta - The Algae - Chlorophycae - Conjugales - Spirogyra
Conjugales are Chlorophyceae devoid of flagellated reproductive boies. Sexual reproduction is by means of amoeboid gametes. Asexual reproduction is unknown. The vegetative cells are generally large, and possess large, elaborately shaped chloroplasts. The plants may be unicellular or the cells may be united together into filaments, which may fragment into separate unicellular parts.
Included in this group are the Desmids, which are among the most beautiful of the Green Algae. Most of them are unicellular, but the wall of the cell is divided into two halves, or semi-cells, each of which is often extremely complex in outline and is connected to the other by a narrow region called the isthmus.
Spirogyra 
Spirogyra is a very common bright-green Alga which may be found in large masses growing in ponds and slow-flowing rivers. The filaments are usually free, but are sometimes attached at one end to a substratum by a branched and colourless rhizoidal cell or holdfast. These masses are very slimy to the touch, owing to a mucous sheath surrounding the cells, and consist of enormous numbers of long, unbranched, green filaments.
There is no' morphological differentiation between the cells of the filament. A delicate sheath of cuticle surrounds the entire filament inside the mucous sheath, while each cell has its own polysaccharide wall, inside which is a peripheral layer of cytoplasm. The central part of the cell is occupied by a large vacuole containing cell sap. The nucleus is suspended in the centre of the vacuole by cytoplasmic strands. The most conspicuous structure in the cell is the chloroplast, which consists of one or more spiral bands lying in the peripheral layer of cytoplasm. It is the appearance of these bands that has given the plant its name. Embedded along the chloroplast are numerous pyrenoids around which is stored up the starch acquired during photosynthesis. In section the chloroplast is usually T -shaped, the inner flange being connected by fine filaments of cytoplasm to the nucleus.
The filaments grow by simple cell division, each cell dividing by a wall transverse to its length, and any cell of the filament being capable of division. Such a type of growth, where any cell of the filament can divide, is spoken of as intercalary growth, and may be contrasted with the apical growth of higher plants, where division is restricted chiefly to the cells of the growing point.
Cell division in Spirogyra begins before nuclear division, not after it as in higher plants, and the two processes are completed simultaneously. The new transverse wall starts as a ring on the inner surface of the longitudinal walls and grows inwards towards the centre. The cell does not necessarily divide into two equal parts, for the new wall lies exactly half-way between the nuclei, wherever that may happen to be in the cell. The position of the new wall is therefore fixed by the position of the nuclei and this position is determined while the mother nucleus is still in the metabolic condition. A similar condition exists in Cladophora.
Nuclear division in Spimgyra is also exceptional in that the nuclear membrane persists and divides \yith the nucleus, the spindle being formed inside the nucleus instead of in the cytoplasm.
Sexual Reproduction in Spyrogyra
Spirogyra grows in dense masses, and in this condition the filaments at the time of sexual reproduction come to lie side by side, probably as a result of surface tension changes. In this position sexual reproduction is possible by a process known as conjugation. Most frequently this process takes place in the spring, but it can be induced by the presence of a liberal supply of sugar, or by a deficiency of nitrogen. The filaments adhere closely to one another in pairs surrounded by a common investment of mucilage, and then, preliminary to conjugation, opposite cells in adjacent filaments develop short protuberances from their side walls, which are in contact with each other from the start, and as they grovv push the filaments apart. Each pair of protuberances when united forms a conjugation canal. The wall between them partially breaks down and a small conjugation pore is formed, through which the protoplasts of the two opposite cells come into contact. Conjugation by this method is called the ladder type from the appearance of the two parallel filaments linked by the conjugation tubes. At this stage the mucous sheath of the filaments disappears.
Meanwhile the contents of the conjugating cell which is about to function the male gamete contract away from the outer cell wall but without losing contact with the female. In this process many contractile vacuoles are formed, which pump water from the central cell vacuole into the space between the cell wall and the contracting cytoplasm. Subsequently the female gamete also contracts from the conjugation tube, leaving a space into which the male gamete passes, enters the other cell and fuses with the female gamete. It appears that the migration of the male gamete along the conjugation tube and the contraction of the female
are brought about by the pumping action of contractile vacuoles. In Mougeotia and certain callied genera the process is lightly different, both gametes Zygospore passing into the conjugation tube, where the zygote is formed. Such types are therefore isogamous.
The active gamete is looked upon as the male and the passive one as the female, although they do not show any morpho­logical differences. We have therefore here a case of physi­ological heterogamy. In some species of Spirogyra all the male gametes are formed in the cells
of one filament, and all the females in the other, with the result that after conjugation we find one filament entirely devoid of cell contents, while the other contains all the zygotes. In other species some cells function as males and others as females, with the result that the zygotes are distributed between both filaments. The former species are dioecious and the latter are monoecious.
There is also another type of conjugation called the chain type, in which adjacent cells of the same filament unite through tubes which are formed like side-links between cell and cell. In this case alternate cells behave as males and females respectively, and there is no sexual differentiation between the filaments as a whole. In some species this method is the rule.
Whatever the position of the formation of the zygote its subsequent development is the same. Immediately after the fusion of the gamete nuclei a zygospore is formed. A zygospore is a zygote enclosed in a thick wall. It is specially adapted for withstanding dry conditions, and is easily recognized by the thick yellow-brown wall which is usually produced. In Spirogyra the wall consists of three distinct layers, the characteristic chloroplast dis­appears and the cell becomes filled with brownish oil-drops.
The further development of the zygospore depends upon the decay of the parent filament. As this consists simply of the dead cell wall, its decom­position in water is fairly rapid, and in time the zygospores are set free and fall to the bottom of the pond. Activity only begins when the zygospore is lying in water: it will not germinate in a dried-up pond. Under suitable conditions the thick wall is burst open and the contents emerge as a green cell which divides transversly, the lower half forming a rhizoidal cell, while the upper half divides repeatedly to form a new filament. At the beginning of germination the nucleus of the zygospore divides into four by meiosis, but only one of the four monoploid nuclei survives, to become the nucleus of the first cell of the new plant.
There is no asexual method of reproduction in Spirogyra, and this and the entire absence of any truly motile gametes constitute the fundamental characteristics which distinguish this genus, as well as the whole of the Conjugales, from the rest of the Green Algae. It seems probable that the absence of motile gametes is a feature associated with the habit of the plant of living in dense masses where little or no difficulty is experienced in the association of cells potentially able to conjugate, as a result of which the gametes have, in the course of evolution, lost the power of motility.
Spirogyra has, however, a method of multiplying vegetatively by the fragmentation of the filaments into single cells, each of which becomes the starting point of new growth. In many species this process, which occurs mostly in early spring, is assisted by the action of a ring-like fold on the end walls of the cells, somewhat like that in Oedogonium, which expands laterally and thus ruptures the cuticular sheath of the filament.
Although the filaments of Spirogyra are multicellular they are not, biologically speaking, individuals, but rather colonies of comparatively independent cells. At sexual reproduction they behave like unicellular types in that the conjugating units are used up and disappear as such. This limits the reproductive capacity of the plant in comparison with higher forms in which a vegetative body persists and may reproduce many times.

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The Algae - Chlorophycae - Oedogoniales - Oedogonium

Phylum Thallophyta - The Algae - Chlorophycae - Oedogoniales - Oedogonium 
The Oedogoniales are Chlorophyceae in which both the zoospores and the antherozoids are characterized by having a whorl of many flagella at their anterior end. Sexual reproduction is always of an advanced type; the male organs, or antheridia, giving rise to antherozoids, and the female organs, or oogonia, each producing a single oosphere.
The vegetative cells are elongated and form a filament; growth is by division of the cells, and in this process there is a characteristic annular splitting of the cell wall prior to the formation of the new cell. The filaments may be simple or branched.
Oedogonium 
Included in this genus are a large number of fresh-water species which frequently occur in ponds and tanks but are uncommon in running water. The Alga is composed of a filament of rather elongated cells, and is usually attached at one end by a rhizoid cell which is often expanded as a terminal disc or holdfast. This cell contains less chlorophyll than the other cells of the filament and is probably unable to assimilate food material.
The structure of the vegetative cell is simple; it consists of a cellulose wall, bounding the inner surface of which is a layer of protoplasm. The centre of the cell is occupied by a large vacuole with cell sap. The chloroplast is a large cylindrical network, with several included pyrenoids formed from protein granules, around which starch grains are deposited. Embedded in the peripheral protoplasm of each cell is a single large nucleus.
Growth consists in the division of the cells of the filament. The apical cell does not divide, and in some species is elongated as a fine slender terminal hair. Only certain cells of the filament divide, and these are distinguished by the formation of an internal ring on the wall, near the upper end.
The cell divides transversely and then the cell wall splits at the level of the ring, which stretches out longitudinally to form a new portion of cell wall. Another ring then forms at the upper edge of this new length of wall. Only the cell which contains the ring divides again, arid it possesses overlapping caps at its upper end, each marking the position of a ring and equal in number to the times the cell has divided. The occurrence of these cap cells in the filaments makes Oedogonium easily recognizable.
Reproduction is both sexual and asexual, the latter occurring during the summer months under conditions favourable for growth, while the former, since it results in the production of resting spores, is suitable for tiding the Alga over periods unfavourable for growth, such as, for example, drought.
Asexual Reproduction in Oedoogonium
In asexual reproduction any cell of the filament may function. The protoplast is withdrawn from the cell wall and forms a mass in the centre of the cell. One area at the side of th.is mass becomes colourless
and round this numerous fine flagella are developed. The cell wall splits across and the protoplasmic mass passes out through the opening. On being liberated the zoospore slips out of the gelatinous membrane which originally surrounded it and becomes spherical. There is no true wall around the zoospore, for the whole mass is purely protoplasmic in structure. It swims by means of the flagella, possesses an eye spot and is sensitive to light in just the same way as we have described in the case of Ulothrix. After swimming for an hour or so it settles down on its colourless end, withdraws its flagella and secretes a cell wall. The free end grows out and is divided by a transverse wall, thus beginning a new filament. The colourless portion usually develops into the branched holdfast cells. 
Sexual Reproduction in Oedogonium
The sexual reproduction of Oedogonium is of an advanced and com­plicated type. The distribution of the sexes varies in the diHerent species. In some both sexes occur in the same filament, when it is said to be monoecious, while in others the male and female organs are borne on different filaments, and the plants are spoken of as dioecious. The male cells are formed by repeated transverse divisions of a vegetative cell, which in monoecious species is close to the female organ. These divisions continue until a row of about a dozen small cells are produced, each of which is termed an antheridium. The contents of each antheridium divide into two, and each portion becomes an antherozoid. These antherozoids resemble the zoospores, but are much smaller.
The female organ, or oogonium, is formed from a single vegetative cell which divides into two, the upper cell being much richer in protoplasm than the lower one. It has a large nucleus and forms the oogonium, while the lower cell is termed the supporting cell. The oogonium swells out to about twice its original diameter and becomes rounded or oval in outline. The cell contents are withdrawn from the wall and from a free, rounded, proto­plasmic body or oosphere. A small clear patch appears on one side, which is called the receptive spot. The wall of the oogonium now opens either by a split or by a thinning of the wall above the receptive spot, and is thereupon ready for the entry of the antherozoid.
In addition to the normal monoecious and dioecious types, certain dioecious forms of Oedogonium are known in which Cell caps the male filaments are very much smaller than the female. Such plants are spoken
of as dwarf male or nannandrous forms. Cells of the parent filament divide by repeated transverse division, and from each of the cells formed a ingle zoospore escapes. I t is intermediate in size between the asexual zoospores and the antherozoids and is termed an androspore. The androspores swim till they reach the female filaments, attaching themselves either to the oogonium itself or to a cell close to it. Here they germinate and produce a filament of two or three cells. With the exception of the lowest, which forms a holdfast, each becomes an antheridium liberating two antherozoids.
Fertilization is effected by the entrance of the antherozoid through the opening in the wall of the oogonium, after which it unites with the oosphere. After fertilization the oosphere secretes a thick wall and is now known as an oospore. The contents of the oospore undergo considerable change; the green colour disappears and is replaced by a brown or red pigment, while large quantities of oil appear in the protoplasm. Such a structure is regarded as a resting spore, and it may remain for a considerable time in this con­dition before germinating. It is thus an extremely suitable structure for tiding the Alga over a period of unfavourable conditions.
When conditions are favourable for its germination the cell wall bursts and the contents slip out surrounded by a thin membrane. It now normally divides by meiosis into four monoploid cells, and from each of these a flagellated zoospore is produced exactly resembling the asexual zoospores. These zoospores free themselves from the membrane and swim away, giving rise eventually to separate plants.
In Oedogonium, then, we have an example of a green Alga, with an advanced type of sexual reproduction as compared with Ulothrix. Its method of sexual reproduction is in some respects comparable with that of the higher plants, but it is peculiar in the production of nannandrous male filaments.
Despite the fact that it is itself highly evolved, it is doubtful if Oedogonium is related to any higher type of plant. It would appear rather that it is an end line, specialized along certain directions, but not along those which produced the higher forms of plant life.

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The Algae - Chlorophycae - Ulotrichales - Ulothrix zonata

Phylum Thallophyta - The Algae - Chlorophycae - Ulotrichales - Ulothrix zonata 

The Ulotrichales are Chlorophyceae in which the cells are usually uni­nucleate and form simple or branched filaments. Asexual reproduction takes place by zoospores, or more rarely by non-motile spores. In the simpler members sexual reproduction is by isogametes. There is generally a single chloroplast in each cell which lies in close proximity to the wall and is therefore said to be parietal. The members of the group occur chiefly in fresh water. 

Ulothrix zonata 

This Alga is extremely common in running water and also in rain puddles. It is bright green in colour and consists of extremely fine filaments rarely exceeding one thousandth of an inch (0.025 mm.) in diameter. These filaments consist of a single row of small cells, often as many as a thousand in a single filament. The threads are generally attached to stones, but they can live equally well when unattached. 

The cell has a cellulose wall, inside which is a lining layer of protoplasm in which the nucleus is embedded. The single chloroplast is in the form of a broad transverse band lying close against the wall and curved round the cell in the form of a ring. Situated in the chloroplast are several pyrenoids. The centre of the cell consists of a large vacuole containing cell sap. 

In cases where the filaments are attached at the end, the anchoring cells are devoid of chlorophyll and are either brown or nearly white in colour. There is no special growing region and any cell of the filament may divide as elongation takes place. Vegetative multiplication is often effected by fragmentation of the filaments. 

Asexual Reproduction in Ulothrix Zonata

This consists in the formation of zoospores. They arise from the ordinary cells of the filament, which divide up into a number of separate parts, generally from two to sixteen, though occasionally only a single zoospore may be produced from the mother cell. They are liberated into a delicate mucilage vesicle through a small aperture on one side of the cell. 

The zoospores are of two kinds, macrozoospores, which are slightly flattened and have four flagella, and microzoospores, which are ovoid and have either four or two flagella. The latter type form an intermediate group between the macrozoospores and the gametes, and are not clearly distinguish­able from the latter in appearance. 

Each zoospore contains a single nucleus; the chloroplast is situated in the broader part of the spore, while the apical end consists of clear protoplasm, attached to which are the flagella. There is a single contractile vacuole which pulsates about every ten seconds. Each zoospore is provided with an eye spot which is apparently sensitive to light. In light of moderate intensity they swim towards the source of light, but in very brilliant illumina­tion they swim in the opposite direction. Finally, after swimming about for several hours, they come to rest, attach themselves and grow out directly into new filaments. On germination the first cell produced is a colourless rhizoidal cell attached to the substratum. 

Sexual Reproduction in Ulothrix Zonata

The gametes are produced either in cells of the same filaments as the zoospores or in different ones. They resemble the microzoospores osely, except that each possesses only two flagella. Each mother cell indes to produce from eight to thirty-two gametes. Conjugation normally takes place only between gametes from different filaments. Fusion is similar that described in Chlamydomonas, forming a zygote with four flagella. After fusion movement does not continue for long and soon the flagella are retracted and the zygote comes to rest, develops a thick wall and is trans­formed into a zygospore. It divides into a number of non-flagellated cells (aplanospores) which are liberated and form new filaments. The first division of the zygote is the reduction division, so that the cells of the filaments are monoploid. 

We see, therefore, that in Ulothrix we have an example of perfect isogamy, in which the two gametes are of identical size and structure, though from the fact that gametes from the same mother cell will not fuse together we realize that there must be some physiological difference between them, which may explain why gametes of different origin are at­tracted to one another. 

Asexual reproduction in Ulothrix takes place chiefly in the winter, and it is at this time of the year that the growth of this Alga is most active. During the summer re­production is by sexual means, and it is interesting to note that after the forma­tion of the zygospore ger­mination may proceed very slowly, and it may rest for a considerable period whilst still unicell ular. I t is only when conditions become more favourable that the new filament is produced. We see, there­fore, that here, as in many other lower organ­isms, sexual reproduction unfavourable to vegetative may be associated with conditions which are growth. 

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The Algae - Chlorophycae - Chaetophorales - Coleochaete pulvinata

Phyllum Thallophyta - The Algae - Chaetophorales - Coleochaete pulvinata 

This Alga occurs not uncommonly in this country attached to the sub­merged leaves of water plants such as Sparganium and Callitriche, or to the fronds of Lemna. The thallus is green and heterotrichous, con­sisting of a flat prostrate part from which vertically growing filaments arise forming a cushion. Growth of the vertical filaments is apical, and of the prostrate part marginal. 

The cells are all uninucleate and possess a single, large parietal chloroplast of irregular shape, "vith one or two prominent pyrenoids. Usually every cell bears a characteristically sheathed bristle which begins with the development of a pore in the outer cell wall, which is followed by the secretion of a membrane forming a tubular sheath extended outwards through the pore and attached to its margin. This sheath becomes impregnated with wall material and is soon indistinguishable from the wall itself. Its upper end becomes perforated, and a stream of cytoplasm passes out through the sheath, becoming homogeneous and solid, and forming the hair. Usually these hairs project at right angles to the surface of the thallus: later they may break off leaving only the sheath behind. 

Asexual Reproduction in Coleochaete Pulinata

Asexual reproduction occurs chiefly in spring and early summer. Any cell of the thallus is capable of functioning as a zoosporangium, but most frequently it is the terminal cell of a filament which is involved. The contents round up to produce a single large, ovoid, biflagellate zoospore which frequently has a single laterally placed chloroplast; it has no eye spot. After swimming for a while it settles down and divides. If the first division is horizontal the upper cell forms a hair, while the lower cell forms an embryo thallus; if the division is vertical each segment grows out laterally as a thallus cell, but in either case the hair formation takes place at a very early stage. 

Aplanospores, i.e., non-motile spores, with fairly thick walls, may be developed from any cell of the thallus, one being produced from each cell. 

Sexual Reproduction in Coleochaete Pulvinata

As has already been stated the sexual reproduction in Coleochaete is oogamous and shows an alternation of generations. In fact it is 

the only example of a non-homologous alternation of generations in the Chlorophyceae, that is to say, an alternation in which the two generations are morphologically different. Sexual reproduction usually occurs from May to July. 

The antheridia are developed in clusters at the ends of the vertically growing branches, frequently on the same branches that bear the oogonia. 

They arise as small colourless outgrowths which become cut off from the parent cell. These antheridia are flask-shaped cells, each of which gives rise to a single, colourless, oval antherozoid, provided with a pair of apical flagella. The antherozoids are set free by the breakdown of the apex of the wall of the antheridium. 

The oogonium is formed terminally on a short lateral branch from a vertical filament, but it is usually pushed into a lateral position by the up­growth of a branch arising from the underlying cell. The oogonium is flask­shaped. The basal part is swollen and contains the chloroplast, while the neck is prolonged into a tube or trichogyne, which contains only colourless cytoplasm. 'When mature the tip of the trichogyne breaks down and some of the cytoplasm is extruded, while the basal part of the oogonium rounds off and forms a single oosphere. 

Fertilization takes place by the antherozoid entering the trichogyne and passing down to the oosphere. The nucleus of the male gamete is con­siderably smaller than that of the female, but as the male gamete migrates towards the female nucleus, its nucleus increases in volume so that at the time of actual fusion the two nuclei are of approximately equal size. 

After fertilization the oogonium enlarges, while the neck is cut off by a septum and a' wall is laid dovvn around the oospore so formed. This wall is thick and brown in colour and may be partly derived from the wall of the oogonium. Meanwhile new branches arise from the underlying cells of the branch bearing the oogonium, and grow around and envelop the oospore forming a pseudo-parenchymatous investment. These cells subsequently die and the oospore is shed enclosed both in its own wall and in that derived from the pseudo-parenchyma, which becomes fused to it. In this state it remains over the winter. 

The following spring the contents gradually assume a bright green colour and divide, first into two by a wall perpendicular to the long axis, followed by a second division in the same plane and then by two walls at right angles to the others, cutting up the -body into eight cells. Meiosis is said to take place during the first two divisions in this process so that this tissue formed within the germinating oospore is monoploid. By subsequent further cell division a wedge-shaped mass of sixteen or thirty-two cells is formed, by which time the oospore bursts open. From each of these cells a single zoospore develops and is liberated through the opening in the oospore wall. These zoospores are quite similar in shape to the zoospores formed in asexual reproduction. According to some workers they only give rise to asexual plants, and several asexual generations may be produced before the sexual plants reappear. 

In the life-cycle of Coleochaete pulvinata, therefore, there is an alternation of sexual and asexual generations which is independent of the cytological alternations. All the vegetative individuals are monoploid, and the diploid condition is only maintained in the oospore, \yhile the monoploid condition is again re-established during the germination of the oospore. We shall see later that this type of alternation of generations is remarkably similar to that in Batrachospermum. In fact there are a number of points of similarity between the sexual reproduction in Coleochaete and in members of the Nemalionales, though whether there is any justification for considering this to be indicative of any phylogenetic relationship is highly doubtful. It is generally regarded as an example of parallel evolution. 

It will be readily seen from this account that there is little or no obvious similarity between Coleochaete and Pleurococcus, and it is only on the grounds that Pleurococcus may occasionally form a filament of cells with an elongated terminal cell and the fact that it can produce a fiat, prostrate cell mass, that a relationship is postulated. It will be realized, however, that Pleurococcus is highly reduced, as is indicated by the loss of sexual reproduction and motile zoospores. 

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