THE MALE GAMETOPHYTE 



157 



produces a pollen tube, the other presumably serving as a source of 

 food material. 



It may be noted that unlike the reduction divisions, which occur 

 more or less simultaneously in all the microspore mother cells of an 

 anther, the microspores usually divide without any such synchron- 

 ization, and the same loculus may show different although not 

 widely separated stages of division and development. In those 

 plants in which the microspores remain together in a tetrad, all 



C 



E 



H 



1 



K 



L 



Fig. 97. A-I, Zostera marina, division of microspore to form vegetative and 

 generative cells. Pollen grains are so long that only a part of each is shown. (After 

 Rosenberg, 1901 .) J, Vaccinium vitis idaea, pollen tetrad showing generative cell 

 cut off toward outer side of each microspore. (After Samuelsson, 1918.) K, Xyris 

 indica, pollen tetrad, showing generative cell cut off toward inner side of each 

 microspore. (After Weinzieher, 1914.) L, Acacia baileyana, pollinium, showing 

 various stages in division of microspore. (After Newman, 1934.) 



four cells in a tetrad are usually in the same stage of division, but not 

 all the tetrads of an anther. A complete synchronization may 

 perhaps be expected only where the microspores are united into 

 pollinia (Mimosaceae, Asclepiadaceae, and Orchidaceae), for here 

 the cells probably exercise some influence over one another through 

 the uncuticularized walls which lie between them (Barber, 1942). 

 Exceptions do occur, however, even in such cases. Figure 97 L 

 of the pollinium of Acacia baileyana (Newman, 1934) shows one of 

 the microspores in prophase, another with the tube and generative 

 cells already formed, and the rest in various intermediate stages. 



