204 



ORIGIN OF SEX AND OF THE SOMA 



The zoospores of such a filamentous alga may in 

 certain cases conjugate instead of germinating at once, 

 thus acting as isogametes. Such zoospores are called 

 facultative gametes. But in most cases the gametes are 



Fig, 30. — Filamentous green algae : a, vegetative thread of Ulothrix 

 with band-shaped chloroplasts {chl.) ; b, thread whose cells 

 are forming zoospores (like the individuals of Chlamydomonas 

 but without walls) ; c. Free-swimming zoospore ; d, germination 

 of zoospore, which has lost its flagella, is attached by the front 

 end to a solid object and has secreted a wall ; e, the cell has 

 grown in length ; /, young filament derived from e by further 

 growth in length and transverse cell division (this grows into 

 a filament like a) ; g, branched filamentous alga (Stigeoclonium), 

 some of whose cells are vegetative, others have formed zoospores ; 

 h, free zoospores. 



smaller than the zoospores, though quite similar in 

 structure, i.e. more of the gametes, often double as 

 many, are produced from the mother cell. Some 



