SPERMATOPHYTES 



267 



MALE GAMETOPHYTE 



Development. In the development of the male gametophyte of 

 angiosperms no vegetative cell appears (except in very rare cases), so 

 that the gametophyte is reduced to an antheridium, and the pollen grain 

 is an antheridium initial 

 (fig. 595). The 'first 

 division of the nucleus 

 of the microspore pro- 

 duces the generative and 

 tube nuclei, and this is 

 the usual condition of 

 the pollen grain at shed- 

 ding (fig. 596). The 

 generative cell is usually 

 organized as a naked 

 cell, which assumes vari- 

 ous forms, generally 

 from spherical to lens- 

 shaped. This cell may FlGg 595 _ 598 ._ Ma le gametophyte of angiosperms, 

 divide before the shed- as shown bySilphium: 595, pollen grain, with its single 

 ding of the pollen grain, nucleus and spiny outer coat; 596, first division, forming 

 , . , , generative and tube nuclei (shedding condition in many 



3 cases); 597, second (and last) division, resulting in the 



nuclei are observed two male cells from the generative cell (shedding con- 

 within the mature grains di tlon * n some cases), a division which often occurs in the 

 /L N ., j. ., pollen tube; 598, the two male cells having elongated. 



(% 597); > or it divides fter ME ^ E 5 L 9 L .' 



after passing into the 



pollen tube. The generative cell is the primary spermatogenous cell, 



and there is only one division, resulting in two equal male cells. 



The gradual reduction in the number of spermatogenous cells pro- 

 duced by the antheridium of a heterosporous plant has reached its ex- 

 treme expression among angiosperms. In Selaginella the generative 

 (primary spermatogenous) cell produces a large number of mother cells, 

 each of which produces a sperm. In Isoetes the generative cell produces 

 four mother cells, each of which develops a sperm. In cycads and 

 Ginkgo the generative cell produces three cells (by two successive divi- 

 sions), two of which are mother cells developing sperms. In conifers 

 the generative cells produce the same number and character of cells as in 

 cycads, but the two mother cells do not develop sperms, functioning 



