GYMNOSPERMS. 



I 49 



Ginkgo, the spermatozoid of Cycas, as has been pointed out for the 

 fern, is a transformation of the entire mother-cell. 



The development of the spermatozoid in both Ginkgo and Zamia 

 closely resembles that in Cycas. That in Zamia differs, however, 

 according to Webber, in certain important details, and because of this 

 fact the process in Zamia will be given also in some detail. Webber 

 investigated two species growing in Florida Zamia floridiana and 

 Z. pumila. 



As a rule the mature microspore of Zamia consists of the tube cell 

 and two prothallial cells (Fig. 60, A). Only in exceptional cases were 

 evidences of a third cell observed, but if three prothallial cells are 

 formed in the development of the pollen spore as is claimed for Cycas, 

 the first is generally absorbed before the spore is mature, leaving only 

 a trace in the form of a dark line. The two prothallial cells are pro- 

 vided with only a plasma membrane. The first prothallial cell is shaped 

 like a plano-convex lens and arches out into the second prothallial 

 cell. The second prothallial cell is attached to the first and arches out 

 into the tube cell (Fig. 60, A, B). This is especially marked during 

 the growth of the pollen tube. The nucleus of the tube cell is larger 

 than those of the prothallial cells, and of the latter the nucleus of the 

 first is larger than that of the second. Very soon in the growth of the 

 pollen tube the second or antheridial cell, together with its nucleus, 

 greatly exceeds the first. 



The process of pollination, which occurs in Florida in January, 

 brings the pollen grains into the pollen chamber, a cavity in the apex 

 of the nucellus, formed by the disorganization of the tissue of the 

 latter. Webber ('01) states that the passage of the pollen grain 

 through the micropyle is evidently accomplished by suction. 



A somewhat mucilaginous fluid is secreted by the cells which sur- 

 round the micropyle, and a drop of this fluid is probably protruded 

 at the time of pollination. The fluid disappears later, and during the 

 formation of the pollen chamber a suction is formed by the breaking 

 down of the cells in its formation, so that the fluid, together with 

 'the pollen grains that may be held in it, is brought down into the 

 pollen chamber. 



In a short time after the pollen grains have been brought into the 

 pollen chamber they germinate, the tube bursting out of the exine of 

 the grain at a point opposite the prothallial cells (Fig. 60, B). No 

 matter what the position of the grain may be, the tube always pene- 

 trates the tissue of the nucellus adjacent to the chamber. The tube in 

 Zamia does not branch before entering the nucellar tissue, and only 



