HYBRIDISATION AND SELECTION. 



79 



the resistance of seeds and seedlings to high and 

 low temperatures, a subject opened out by Sachs, 

 Kny, De Vries, Krasan, Just, Hohnel, Devvar, 

 Dyer, and others ; with the conditions of veeeta- 

 tion which affect the various functions of growth, 

 respiration, assimilation, transpiration, and so 

 forth, on which I cannot even touch in these 

 pages. 



Meanwhile I hope I have succeeded in im- 

 pressing upon you the grand fact that the plant 

 is a living and very complex engine, driven by 

 the radiant energy of the sun, and capable of 

 doing work thereby, and this just as truly as 

 any heat-engine is driven by chemical energy 

 gained by means of the sun's rays, or as a water- 

 mill is driven by power which must be referred 

 to the energy of potential in the head of water 

 placed in position by the sun's work in evapora- 

 tion. Fundamentally the whole of life and work 

 on our planet is to be referred to the one great 

 source of energy which renders possible the 

 establishment of differences of potential. 



This machine, then, doing work in various 

 ways, adapts itself or goes to the wall to the 

 conditions of its work among competing organisms 

 or opposing circumstances. Curiously enough, 

 while in some cases it suffers from the competi- 

 tion, in others it is benefited by its life-actions 

 fitting in between those of other organisms, 

 which in their turn supplement it. In other 

 words new types of this engine, capable of doing 

 the work in various ways, are obtainable; some 



