GAMETOGENESIS AND SPOROGENESIS 



209 



that their original role has grown less evident as they have been modified 

 in connection with their other function, namely, the bearing of cilia. In 

 forms with non-motile gametes they do not appear. 



Angiosperms. — In its essential features sporogenesis in angiosperms 

 is like that in gymnosperms. In the anther the microsporocytes, usually 

 as they lie in the anther fluid after rounding up, undergo two divisions to 

 form quartets of microspores (Fig. 125). In some cases a separating wall 

 is formed after the first mitosis, but more commonly cytokinesis does not 

 occur until after the second mitosis, when the cell is divided simultane- 





—.- 'y^-s^-i' 



B 



i:s^ 



;^,:::^ 



Fig. 126. — Development of male gametes in angiosperm pollen tube. A-D, division of 

 generative nucleus and delimitation of male cells in Galanthus nivalis. E, male cells in 

 Hemerocallis flava. The vegetative pollen-tube nucleus (p) may either precede or follow 

 the generative nucleus through the tube. {After Trankowsky, 19306.) 



ously by furrows or otherwise into the four spores (p. 167). Each micro- 

 spore develops a thick wall and soon forms a very simple male gametophyte. 

 The microspore nucleus, which has the reduced number of chromosomes, 

 divides to two, around one of which a membrane cuts out a generative cell. 

 The other nucleus and the rest of the microspore cytoplasm constitute 

 the tube cell, which later grows out to form the pollen tube. 



The generative cell divides to form two male gametes. This division 

 sometimes occurs in the pollen grain before it is liberated from the anther, 

 but probably more often it takes place in the growing pollen tube after 



