CHAPTER V 

 GEOWTH 



The processes so far discussed supply the plant with the 

 materials and with the energy to carry on other processes 

 popularly regarded as functions peculiar to living beings, 

 and at all events performed with a greater degree of inde- 

 pendence and self-control by them than by lifeless objects. 

 In this respect, however, these processes do not differ from 

 those already examined, for we have seen that respiration 

 is oxidation controlled and, to a certain extent, carried on 

 by the living organism ; that nutrition depends upon chemi- 

 cal syntheses accomplished by living protoplasm, which uses 

 simple substances obtained by itself, through its application 

 of the laws of diffusion and osmosis ; that the absorption 

 and transfer of food-materials and of foods are physical 

 processes made possible and regulated by the living organ- 

 ism. The processes which we have still to study — growth, 

 irritability, and reproduction — involve and consist in chemi- 

 cal changes not confined to living organisms but controlled 

 by them. 



Growth cannot be understood unless the sensibility of the 

 growing substance is constantly borne in mind. A crystal 

 of copper sulphate in a concentrated and slowly evaporating 

 solution of this salt could not grow if the molecules of cop- 

 per sulphate were not mutually attractive, if those which 

 were still moving freely in the solution did not respond to 

 the attraction exerted upon them by those already settled. 

 The limits of form and size, of rate and direction of growth, 

 of copper sulphate crystals, are fixed by physical laws, but 

 though these laws are universal, the balance in any one 

 spot may be very different from that elsewhere, and the 



