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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