94 PLANT-LIFE ON LAND [CH. 



originating from an Algal source, have adapted 

 themselves to life upon the land. But their sexual 

 propagation is still through the medium of external 

 fluid water, to which they have occasional access in 

 case of rain or copious dew (compare Figs. 8 and 9). 

 Their spermatozoids are capable of active transit 

 through water, and their course has been shown not 

 to be entirely at haphazard, but a chemically directive 

 influence is exerted by the diffusion of soluble 

 substance from the neighbourhood of the egg-cell, 

 which attracts the motile sperm. In such cases the 

 obstacles to intercrossing are not such as to require 

 extraneous aid, and there are analogies with what 

 occurs among ambulatory animals. 



But it is otherwise with those organisms which 

 have taken the further step in specialisation to life 

 on the land involved in the adoption of the Seed- 

 habit. In these a condition for fertilisation to take 

 place is that by some means the pollen-grain shall be 

 transferred bodily from the stamen where it is pro- 

 duced to the stigma, or in Gymnosperms to the ovule, 

 or future seed itself. Since the stamen may be on a 

 dififerent branch, or even on a different plant from the 

 ovule, a real transfer, often through a considerable 

 space, is a necessary feature of propagation. The 

 pollen-grain is itself incapable of spontaneous move- 

 ment. And so some external agency must of necessity 

 be called in if pollination is to be carried out at all. 



