56 A TEXTBOOK OF THEORETICAL BOTANY 



attached to the anterior end of the cell. They are frequently retracted or 

 shed during periods of rest. Asexual reproduction takes place by the forma- 

 tion of zoospores. Most genera also reproduce sexually, and the simpler 

 members are isogamous. The cell is generally bounded by a cell wall, which 

 may either be delicate or firm and rigid. In the latter case it consists of 

 cellulose. There is usually a single chloroplast situated at the posterior end 

 of the cell and containing a single pyrenoid, or starch- forming centre. There 

 is a central nucleus and one, or more commonly two, contractile vacuoles, 

 which show a rhythmical alternation of rapid contraction (systole) and slow 

 expansion (diastole). They are apparently excretory organs, and in certain 

 forms are known to contain uric acid. 



We shall consider in detail the following members of this group : 

 Chlamydnnintias, Ilaematocucciis, Gonimn, Pandorina, Eudorina, Volvox. 



Chlamydomonas 



This organism is very common in fresh waters, sometimes forming a 

 green scum over the surface, and sometimes colouring whole ponds a turbid 

 green. The vegetative cell (Fig. 30) is usually oval and is enclosed in a 



Fig. 30. — Chlamydomonas. Photograph showing 

 the appearance of the vegetative cells. The 

 prominent central granule is the nucleus. 

 The flagella are not stained and do not show. 



cellulose wall with the protoplast closely applied to it. The cellulose wall 

 distmguishes Chlamydomonas from Euglena. The chloroplast is basin-shaped, 

 with Its base at the hinder end of the cell, the nucleus being situated in the 

 centre. There is a single pyrenoid in the lower part of the chloroplast, 

 which acts as a centre around which starch is formed. The pyrenoid itself 

 stains brown with iodine and is usually more conspicuous than the nucleus. 

 At the anterior end of the cell two long flagella (Fig. 31) pass out through 

 the cell wall, each arising from a basal granule, and between them the cell 

 wall IS often slightly protruded in the form of a beak. Close to where the 

 tlagella arise two contractile vacuoles are formed. 



