THE GINGKO TREE AND GYGADS 115 



deal of the separate sexual 'generation retained. Much 

 of this has been done away with in the process of time 

 in the course of those changes which have given us the 

 great modern group of flowering plants and trees. There 

 is in modern flower-bearing trees and herbs no pit, pro- 

 vided by the female prothallus, containing liquid for the 

 motile sperms to swim about in ; the pollen-prothallus no 

 longer produces motile sperms. It does not even divide 

 into a chain of cells, but becomes a very small thread 

 called the "pollen-tube," and makes its way by growth into 

 contact with the egg-cell embedded in loose tissue, and 

 itself fuses with the egg-cell. Thus, in the modern group 

 of flowering plants, the female prothallus is reduced to a 

 solid particle of tissue in which an egg-cell is set, and the 

 male prothallus does not even arrive at the dignity of 

 forming cells and tissue, let alone sperms ; it is a simple 

 protoplasmic filament which issues from the " small spore " 

 or pollen grain, 



In the living survivors of the great forests of Gingko 

 trees and Cycad-like plants intermediate between ferns 

 and modern flowering plants which flourished in the 

 oolitic period, two Japanese botanists, Hirase and Ikeno, 

 discovered twenty years ago that the pollen-grains the 

 male fertilising spores (familiar in all our modern flowering 

 plants) when carried on to the female flowers, produce by 

 cell-division a growth like the little green " prothallus " of 

 the fern, and that this growth, penetrating the female 

 structures, gives rise to actively swimming sperms (as does 

 the prothallus of a fern), which are received into a liquid- 

 holding cavity of the egg-producing growth of the female 

 spores. These female spores and the little egg-producing 

 plants to which they give rise remain attached to and 

 fixed on the leaf where they originate, instead of being 

 shed, as in the case of ferns ; but the same process of 

 fertilisation by sperms which swim freely in liquid takes 



