The Algae - Chlorophycae - Chaetoporales - Pleurococcus naegelii

Phylum Thallophyta - The Algae - Chaetophorales 
The Chaetophorales are Chlorophyceae in which the thallus is divided into a flat, prostrate system of branched filaments attached to a substratum, and a projecting system of filaments which branch and grow upwards from the prostrate part. Such a thallus is said to be heterotrichous. The upright branches may develop hairs which are either composed of single cells or of rows of narrow elongated cells with scanty colouring matter. In Coleochaete they are merely outgrowths of the walls of the cells of the filaments and may be regarded as setae or bristles.
Reproduction is very variable. In many forms sexual reproduction is isogamous, with small motile gametes, in others it is oogamous, while in the most advanced members there is an elaborate alternation of sexual and asexual generations resembling more closely the Rhodophyceae than the Chlorophyceae. Asexual reproduction is either by means of zoospores or non-motile aplanospores.
The two types selected for special study may be regarded as representing the two extremes of the group. Coleochaete is the most elaborate, in fact its life history is in some ways more complex than that of any other Green Alga. Pleurococcus, on the other hand, is a very specialized but reduced form which has not only largely lost its filamentous character but also its reproductive mechanism, and relies on vegetative multiplication of cells by simple division. It would be difficult to select any genus which might be said to be typical of the order as a whole.
Pleurococcus naegelii 
Pleurococcus is one of the commonest of the Algae found in terrestrial situations, occur­ring as a thin incrustation on the windward side of trees, stones, walls and palings. Probably no Alga has been subjected to more discussion as to its correct name. Frequently it is referred to as Protococcus viridis and also as Chlorococcum vulgare, while ac­cording to some workers its correct name is Pleurococcus vulgaris, though most authorities seem now agreed that it should be named Pleurococcus naegeli£.
The structure is extremely
simple. The mature cells are sometimes isolated and more or less spherical, but they are more frequently found in groups of two, three, four or more, owing to their slow separation after division. In moist conditions there is a tendency for the cells to remain attached to one another and to elongate, with the result that short branched filaments are formed and such filaments readily arise when the Alga is grown in culture flasks.
Each cell is surrounded by a firm cell wall which is usually unthickened. The protoplast does not possess any obvious vacuoles and contains a hollow, spherical chloroplast with, on one side, an opening with a lobed margin. So large is this chloroplast that it seems to fill the cell, and it is only when the cell is in a suitable position that the lobing of the chloroplast can be seen. There is no pyrenoid present.
Reproduction in Pleurococcus Naegelii
Reproduction is effected entirely by vegetative means; by simple division followed by the separation of the cells. There is no evidence that this species possesses any special resting stage for the ordinary, cells are able to withstand long periods of desiccation without injurry. The Algae can obtain what water is required directly from atmospheric moisture.
Accounts have not infrequently been given of a motile stage closely resembling Chlamydomonas in appearance, with two apical flagella. Recent work indicates that such organisms are not part of the life-history of Pleuro­coccus but belong to other genera.

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