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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