THE MALE GAMETOPHYTE 



155 



pronounced asymmetry, the wallward pole being blunt and the free 

 pole acute. More recent studies (Brumfield, 1941) seem to indicate 

 that this asymmetry is associated with the form of the prophase 

 nucleus. In Allium, where the nucleus is strongly flattened on the 



Fig. 95. Diagram to illustrate the more important stages in development of male 

 gametophyte. A, newly formed microspore. B, older stage, showing vacuolation 

 and wallward position of microspore nucleus. C, microspore nucleus dividing. 



D, division completed; two-celled stage, showing vegetative and generative cells. 



E, generative cell losing contact with wall. F, generative cell lying free in cyto- 

 plasm of vegetative cell. G, H, division of generative cell in pollen grain. /, J, 

 division of generative cell in pollen tube. (After Maheshwari, 19/ f 9.) 



wallward side, the asymmetry is extreme (Fig. 96); in Pancratium, 

 where it is only slightly flattened, the asymmetry is much less pro- 

 nounced; and Tradescantia shows an intermediate condition. The 

 direct cause of the asymmetry has been attributed to a difference in 



