46 



PLANT MORPHOLOGY 



depending on the species. The sperms escape into the water and, like 

 the zoospores, swim by means of a crown of cilia. An oogonium also 

 commonly arises from the smaller upper cell produced by the division of 

 an ordinary vegetative cell, but this cell subsequently enlarges by the 

 accumulation of food (Fig. 34D). The oogonia may occur separately or 



wmm^ 









mmm\ 



Fig. 33. Nuclear and cell division in Oerfogo/iiMm gra?ic?e, X320. A, elongation of nucleus 

 and appearance of young ring; B, metaphase; C, anaphase; D, formation of cross wall and 

 separation of nuclei; E, broken outer layer of cell wall and stretching of ring; F, straighten- 

 ing of ring and migration of cross wall upward to unite with inner layer of cell wall. {After 

 Ohashi.) 



several may be cut off in a series. The entire protoplast of the oogonium 

 becomes a large nonmotile egg. 



A sperm enters an oogonium through a pore in its wall and unites with 

 the egg. The zygote becomes a heavy-walled resting cell that later 

 produces four zoospores (Fig. ME, F). When liberated, these are 

 enclosed by a common membrane that soon disappears. From each of 

 the zoospores a new filament is developed. The reduction of chromo- 

 somes occurs in connection with the germination of the zygote, and so the 

 four zoospores are haploid. 



