54 



EVOLUTION OF PLANTS 



9, C, an), and closely resemble the zoospores except in 

 size, and the partial or complete loss of chlorophyll. The 

 spermatozoid has a large nucleus with relatively little 

 cytoplasm, as the nucleus is probably of the most impor- 

 tance in the act of fecundation. 



At maturity the oogonium opens and permits the en- 

 trance of the motile spermatozoid, which at once pene- 

 trates into the egg-cell where its nucleus fuses with 



that of the egg, 

 thus fertilizing it. 

 As the result of 

 fertilization the 

 egg becomes in- 

 vested with a 

 heavy cell-wall and 

 forms a resting- 

 spore which re- 

 mains dormant for 

 a long period, and 

 is capable of re- 

 sisting, unharmed, 

 freezing and drying 

 up. 



In the highest 

 type of all, repre- 

 sented by the pecu- 

 liar genus Coleo- 

 chsete (Fig. 10), the oogonium, with the contained 

 oospore, becomes, after fertilization, invested with a 

 protective covering formed by the growth of adjacent 

 cells, so that the influence of the act of fertilization ex- 

 tends beyond the egg-cell. Coleochsete, as we shall see 



Fig. 10. — A, a plant of Coleochsste scutata, 

 one of the highest of the Confervaceas ; B, 

 fragment of another species, O. pulvinata, 

 ■with an oogonium, og; C, the germinating 

 spore seen in section, showing its division 

 into a nearly globular cell-mass; each cell 

 later gives rise to a single biciliate zoospore. 

 (Figs. B and C after Oltmanns.) 



