140 



MORPHOLOGY OF GYMNOSPERMS 



tozamia longifolia. His material, which was secured by growing 

 the spores upon juicy pears, showed four cells in the spore, and no 

 further divisions seem to have occurred during such early stages in 

 the formation of the pollen tube as could be obtained in this way. 

 In 1896, in a preliminary paper (19), followed by the full' paper (27) 

 in 1898, Ikeno presented a detailed account for Cycas revoluta; 

 while in 1897 (26) and 1901 (32) Webber 

 published his results for Zamia integrifolia. 

 More recently Caldwell (52) has described 

 the male gametophyte of Microcycas calo- 

 coma, and Chamberlain (63, 70) that of 

 Dioon edtiie. 



The microspore has two sharply differ- 

 entiated coats, the exine and the intine. 

 The exine is thickest in the basal region of 

 the spore, and becomes very thin toward 

 the apex, where the pollen tube is to emerge; 

 while the intine is thinnest in the basal 

 region, where it is in contact with the thickest 

 portion of the exine, and thickest along the 

 sides of the spore (figs. 155-159). 



The microspore germinates while still 

 in the microsporangium, the nuclear divi- 

 sion forming two unequal cells, a small 

 persistent prothallial cell, and a larger cell, 

 the antheridium initial. Immediately after- 

 ward the nucleus of the larger cell divides, 

 forming two unequal cells, one a small cell 

 so closely applied to the prothallial cell that it looks as if it had 

 arisen by the division of the prothallial cell itself, and the other the 

 tube cell. This second small cell has been called the generative 

 cell, although it is probable that it cannot be regarded as the primary 

 spermatogenous cell, since one of its daughter cells (the stalk cell) 

 has proved to be persistently sterile throughout gymnosperms. In 

 all the genera, the pollen is shed in this three-celled condition 

 (fig. 156). The nuclei of the generative and prothallial cells are 

 comparatively large, making the pollen grain look like a three- 

 nucleate ceU. 



Fig. 154. — Zamia muri- 

 cata: three stages in the 

 development of microspores 

 from the mother cell, the 

 lowest figure showing the 

 four microspores within the 

 thick-walled structure; 

 lowest figure, X466; other 

 two, XS96. — After TmUB 

 (II). 



