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.