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.
