FILAMENTOUS GEEEN ALG^ 203 



Besides the motile green algse, some of which have 

 been described in the preceding pages, there are many 

 immotile unicellular forms, such for instance a? Proto- 

 coccus (Chapter IV). Some of these hve on damp 

 earth, tree-trunks, or in similar damp situations, 

 while many float in water. Some are ccenobiate, like 

 the Volvocines, and others form irregular loose colonies 

 with an indefinite number of cells. 



But there are also a large number of filamentous 

 (thread-Uke) forms, the body commonly consisting of 

 simple (Fig. 30, a) or branched {g) threads composed of 

 cyhndrical cells placed end to end and containing one 

 or more chloroplasts of various shapes. Many of the 

 filamentous forms which hve in water are reproduced 

 by the division of the contents of their cells into 

 motile flagellate cells called zoospores (Fig. 30, h), of the 

 same type of structure as the Chlamydomonas cell, 

 but smaller and without a cell wall — cells in fact 

 closely similar to CUlamydomonas gametes. These 

 zoospores escape from the mother cell in which they 

 were formed, swim about for a tmie (c), and then settle 

 down on some solid object and germinate, secreting a 

 cell wall {d), growing in length (e) and dividing to 

 form a thread of cells of the type characteristic of 

 the species (Fig. 2,0, f). We may consider that in this 

 form of reproduction the plant reverts to the condi- 

 tion of a Chlamydomonas-Yike ancestor for the purpose 

 of reproduction : or to put the matter another way a 

 Chlamydomonas-\i\^e^ ancestor settled down and divided 

 without separation of the daughter cells from the 

 cell wall, and then by growth and further division a 

 thread of cells was produced, any of which at a later 

 stage can produce a brood of Chlamydomonas-Vike 

 cells which escape, and each of which reproduces the 

 filament. 



