38 



GYMNOSPERMS 



the drop not only brings the pollen into the pollen chamber, but 

 seals the chamber, so that the male gametophyte is })rotected during 

 its further development. 



Figs. 32, 33. — Hypothetical sections of megasporangia of a member of the Cycado- 

 filicales after it has passed beyond the heterosporous fern stage and become a seed 

 plant. Fig. 32, a megasporangium at the time of fertilization. Three pollen grains in the 

 pollen chamber. No pollen tubes. The megaspore coat is thinner than in the heteros- 

 porous fern stage. Fig. 33, the seedling is developing while the megaspore is permanent- 

 ly retained within the megasporangium. The seed stage has been reached. 



Fig. 34. — Hypothetical development of the male gametophyte in a member of the 

 Cycadofilicales, showing, in order, a microspore; a prothallial cell and a fertile cell; both 

 vegetative and fertile portions developing; differentiation into spermatogenous cells and 

 wall cells; mature sperms; sperms escaping. 



In early stages, in phylogeny, the male gametophyte, by the 

 breaking down of a few cells at the top of the sporangium, came into 

 contact with the female gametophyte, and there was no formation 

 of a pollen tube. When the sperms were shed, they were in as close 



