GYMNOSPERMS. 



'43 



The mature microspore of Cycas revoluta, according to Ikeno, 

 consists of a large tube cell the so-called vegetative cell, which gives 

 rise to the pollen tube, and two smaller prothallial cells (Fig. 57, A, 

 A, A)- The nucleus of the tube-cell is large, and contains a loose 

 thread-work and a nucleolus. The nuclei of the prothallial cells are 

 smaller, and flattened to conform with the shape of those cells. The 



kz. 



'st 



D 



FIG. 57. Microspore and development of male gametophyte in Cycas revoluta.(Mter Ikeno.) 



A, mature microspore. > t , outer,/,, inner prothallial cells ; ez, tube cell. 



B, proximal end of pollen tube capped by exineof spore; two prothallial cells,/, and/,, have rounded 



off and increased in size. 



C, same at later stage of development; the inner prothallial, or antheridial, cell has divided into the 



generative cell *nd stalk cell (st) ; > x , first prothallial cell; c, c, primordia of blepharoplasts; 

 r, nucleolus of generative cell nucleus. 



D, later than C ; the blepharoplast primordia (c) have moved away from nucleus. 



E, proximal end of pollen tube shortly before division of generative cell (kz) which has increased 



greatly in size ; the large blepharoplasts are provided with beautiful radiations ; the tube nucleus 

 (ezk) has migrated back into proximal end of tube. 



walls cutting off the prothallial cells,' according to Ikeno, are straight, 

 meeting the wall of the pollen spore, while in Zamia Webber finds 

 that these walls, which are only plasma membranes, are arched out 

 into the tube cell. The inner cell (A) gives rise to the antheridium, 

 and may be known as the antheridial cell. 



A period of about three months elapses between pollination, which 

 takes place early in July, and fecundation in October. Immediately 



