24 THE ALGAE 



A characteristic of this genus is the area between the protoplast, 

 which is very similar to that of Chlamydomonas, and the cell wall; 

 this region is filled by a watery jelly and is traversed by cytoplasmic 

 threads passing from the central protoplast to the cell wall. The 

 protoplast contains several contractile vacuoles and one or more 

 pyrenoids, although two is the usual number. Movement is secured 

 by means of two flagella as in Chlamydomonas. Asexual reproduc- 

 tion is by means of two or four zoospores, which may pass through 

 an intermediate non-motile phase before maturing (see Fig. 3). 

 The plants normally occur in ponds and when the water dries up 

 the plant encysts and develops orange-coloured haematochrome. 

 Later, when the ponds fill up again with water, these cysts give rise 

 to isogametes or micro-zoospores. The cysts can be readily trans- 

 ported from pond to pond in mud attached to the feet of birds. 

 Periodic drying appears to be an essential factor if the life cycle of 

 the common species, Sphaerella lacustris is to be maintained. 

 Eight-celled colonies, which behave just like Pandorina (see p. 25), 

 are known in the related genus Stephanosphaera. 



Volvocaceae: Gonium {gonium, angle). Fig. 4 



This genus represents one simple type of development within 

 the colonial forms. The simplest type is probably represented by 

 Pascheriella where three or four chlamydomonad cells are attached 

 to each other laterally. The colony in the different species of Gonium 

 is composed of four, eight or sixteen cells all lying in one plane and 

 forming a flat quadrangular plate, but it has been suggested that 

 the four and eight-celled colonies are merely degenerate forms of 

 the principal species, G. pectorale, where there are four cells in the 

 centre and twelve in the periphery, each cell being surrounded by a 

 gelatinous wall and fused to the neighbouring cells by means of 

 protrusions, whilst the protoplasts of the individual cells are also 

 united by fine protoplasmic threads. The ovoid or pyriform cells 

 contain contractile vacuoles and are provided with a pair of 

 flagella. The centre of the colony is composed of mucus and there 

 is also a firm outer gelatinous layer. The shape of the colony ac- 

 counts for its mode of progression which is by means of a series of 

 somersaults around the horizontal axis. 



At asexual reproduction all the cells in a plate divide simul- 

 taneously to form daughter colonies. If cells should become isolated 

 singly then after a time they will give rise either to a new daughter 



