GINKGO ALES 



207 



becoming brown and forming a resistant beak that long persists as a 

 cap on the sac. The pollen grains, carried well below the beak by 

 the deepening of the chamber, send out their tubes in every direction 

 into the adjacent nucellar tissue, but chiefly away from the embryo 



240 239 



Figs. 235-240. — Ginkgo biloba: the male gametophyte; fig. 235, pollen grain, 

 showing evanescent prothallial ceU and persistent prothallial cell; the mitosis in the 

 antheridium initial will form the generative and tube cells; April 24; X500; fig. 236, 

 poUen at the shedding stage, with prothallial, generative, and tube cells; X500; fig. 237, 

 part of poUen tube July 10; X500; fig. 238, the nucleus of the generative cell has 

 divided, the stalk and body nuclei lying side by side; July 11; Xsoo; fig. 239, the body 

 cell has divided; blepharoplasts are faintly visible; X120; fig. 240, the two sperms 

 produced by the body cell; September 11; X226. — After Hirase (16). 



sac, and often directly toward the beak. Into these young tubes the 

 tube nuclei pass, and remain there so long as the tube system 

 is developing. 



About the beginning of July, the tubes have penetrated the nucel- 

 lar tissue deeply and have branched freely, the tube nucleus remaining 

 where a tube begins to branch. This extensive system of branching 



