GREEN ALGAE 



393 



cell-wall. After a period of rest it germinates, the contents dividing, 

 and escaping as zoospores, which grow into new filaments. 



Ulothrix takes a low place both as regards structure and propagative 

 method. The differentiation of its sexual cells is imperfect. Not 

 only is there no distinction of sex in the form of the gametes, but 

 occasionally the gametes may themselves germinate without fusion. 

 They are strikingly similar in form and origin to the zoospores. The 

 facts are in accord with the theory that fusion of gametes (syngamy) 

 is a means of strengthening otherwise weak cells, which were originally 

 organs of vegetative propagation. 



Oedogonium and Bulbochaete are also filamentous Algae, but with 

 more elaborate structure of their cells. Various species are very 



FIG. 331. 



Oedogonium. A = escaping zoospores. fi = free zoospores. C=sexual organs 

 before fertilisation. D = iu process of fertilisation. O=oogonia ; a=dwarf-inales. 

 S=spermatozoid. (After Pringsheiin, from Strasburger.) ( x 350.) 



commonly found attached to stones or submerged parts of plants 

 in quiet fresh water. Their cells are uninucleate, and contain a 

 single reticulate chromatophore. They may be propagated by 

 zoospores, which are formed from the whole content of a cell ; 

 it escapes through a transverse slit in the wall into water, having 

 an oval form, with a fringe of cilia round the colourless anterior 

 end (Fig. 331). After a period of movement the zoospore settles, 

 forms a cell-wall, and grows directly into a new individual. The 

 plants are readily distributed by this means. The sexual organs 

 of Oedogonium are antheridia and oogonia, which differ in size. The 

 oogonium is a large barrel-shaped cell, containing a single egg. It opens 

 at maturity by a transverse slit, as in the liberation of the zoospores ; 

 but the ovum remains in silu, and is motionless (Fig. 331, C, D). At 

 the same time cells, of the same or of a separate filament, undergo 



