150 ARCHEGONIATES. 



occasionally afterward (Fig. 65, A). During the early development 

 of the tube, the prothallial cells increase in size, becoming bi'oader 

 and longer. The first prothallial cell pushes out into the second, 

 which becomes shaped like a concavo-convex lens, and is crescent- 

 shaped in cross-section (Fig. 60, B, C). As stated in a preceding 

 paragraph, the behavior of the tube nucleus is similar to that in Cycas. 



G 



FIG. 60. Microspcre and development of male gametophyte in Zamia. (After Webber.) 



A, mature pollen grain ; at point of attachment of the two prothallial cells, on left, a dark crescent- 



shaped line represents a layer in wall of spore, which may be the remains of a third resorbed pro- 

 thallial cell. 



B, germinating pollen grain, early stage. The two prothallial cells have not yet begun to increase 



in size. 



C, later stage of germinating pollen grain ; the tube nucleus has increased in size and passed out into 



tube ; prothallial cells unchanged. 



D, proximal end of pollen tube ; the two prothallial cells have increased in size, the first having crowded 



out into the second in a marked degree. 



E, proximal end of pollen tube ; nucleus of second prothallial cell, antheridial cell, in telophase of divi- 



sion, lower end of mitotic figure being crowded to one side by the encroaching first prothallial cell. 



F, prothalliumin proximal end of tube, after division of antheridial cell into stalk and generative cell. 



G, prothallium in later stage of development after appearance of blepharoplasts ; the double plasma 



membrane, separating first prothallial cell and stalk cell, shows that there are two distinct and inde- 

 pendent cells of separate origin. 



A little later the second cell has arched out very greatly, and the 

 increase in size of the first prothallial cell has brought the second, or 

 antheridial cell, out beyond the limits of the pollen grain and into 

 the tube (Fig. 60, D). However, the prothallium remains in con- 

 nection with the wall of the pollen spore until the spermatozoids are 

 mature. 



The next important step in the development is marked by the 

 division of the second prothallial cell into the stalk cell and generative 



