THE MALE GAMETOPHYTE 



163 



Division of Generative Cell. The generative cell may divide 

 either in the pollen grain (Fig. 95G-H) or in the pollen tube (Fig. 

 95I-J). Formerly the second condition was believed to be the 

 more frequent, but during recent years three-celled pollen grains 



B 



D 



4aV 



G 



H 



E F 



Fig. 101. Stages in development of male gametophyte of Asclepias, showing- 

 protein granules inside cytoplasm of pollen grain and pollen tube. A, microspore 

 mother cells. B, tetrad of microspores. C-F, microspores. G, pollen grain, 

 showing vegetative nucleus and two sperm nuclei. H, terminal portion of pollen 

 tube. (After Guignard, 1922.) 



have been reported in several genera (see Schnarf, 1939) and it 

 seems certain that many of the older records were based on a study 

 of immature pollen. 4 



4 As the pollen grain grows older, the vacuoles become smaller and more evenly 

 distributed and finally they disappear almost entirely so that with the usual methods 

 of fixing and staining the mature pollen grain, like the young microspore, again 

 shows a dense cytoplasm devoid of all conspicuous vacuolation. This is such a 

 constant feature in most angiosperms (excluding some aquatics) that it serves as a 

 useful check for judging whether a pollen grain is fully mature or not (see Schnarf, 

 1937). 



