134 



Chlorophyceae 



lateral position. The rapid vibratile action of the cilia causes the zoogonidium 

 to swim quickly through the water, but the movements of some zoogonidia 

 are much more rapid than others. After a time (of variable length, but 

 rarely more than an hour or two) the zoogonidium comes to rest, the 

 cilia disappear, the protoplast secretes a cellulose wall, and the quiescent zoo- 

 gonidium develops into a new plant (consult figs. 185 H J; 189 G). Many 

 zoogonidia on coming to rest attach themselves by their anterior colourless 

 extremity, which often develops into a hold-fast of greater or less complexity. 



Pascher ('09) has found that in some of the Ulotrichales the swarm-spores 

 may soon lose their cilia and then creep about in an amoeboid manner 

 for half to three-quarters of an hour, after which they settle down and 

 germinate in the ordinary way. 



All zoogonidia must be regarded as homologous structures, and as will be 

 emphasized in subsequent paragraphs they must also be considered as the 



Fig. 91. A and B, Ulothrix idiospora G. S. West, showing scrobiculated akinetes (ak). C, 

 a form of Zygnema ericetorum Kiitz., showing aplanospore (apt). D, chain of akinetes (A - ) 

 of Microspora floccosa (Vauch.) Thur. E, aplanospores (apl) of Microspora sp. [probably 

 M. abbreviata (Rabenh.) Lagerh.]. All x about 500. 



direct representatives of the vegetative individuals of the primitive motile 

 forms which were derived from the Flagellata. 



In some groups (Protococcales, Ulotrichales, various Heterokonta j ), single 

 non-mobile gonidia are frequently produced from the vegetative cells. If the 

 gonidium is formed by the rejuvenescence of the cell and the original cell- 

 wall is retained as part of the final thick wall, the gonidium is said to be 

 an akinete (fig. 91 A, B, and D). If, on the other hand, the rejuvenescence 

 results in the formation of an entirely new cell-wall around the protoplast, so 

 that the gonidium is only set free by the rupture of the original cell-wall, 

 the reproductive cell is known as an aplanospore (fig. 91 C and E). Some 

 of these non-motile asexual gonidia germinate at once, but others undergo a 

 variable period of rest before germination and are often termed ' hypnospores.' 



