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CHAPTER XVIII 



THE ENERGY OF THE PLANT 



THE various operations which we have seen are continually 

 going on in the body of the plant involve the execution of 

 a considerable amount of work. This is very evident when 

 we observe only the enormous development of a large tree, 

 and compare it with the relatively small seed from which 

 it has sprung. Such a process of construction has involved 

 the preparation of a vast quantity of highly complex 

 material from very simple chemical substances. The pro- 

 cesses incident to life also, though they may not lead directly 

 to the formation of such substances, cannot be conducted 

 without involving a considerable amount of work, whether 

 the plant is a minute body consisting of a single protoplast, 

 or an organism of a much higher degree of complexity. 



We must therefore turn our attention to the question 

 of the supply and utilisation of the energy at the expense 

 of which the various processes of life are carried out. At 

 the outset it will be well to consider what demands for 

 energy we find presented by the plant, or what are the 

 ways in which energy is expended or lost. 



Some of these have been incidentally alluded to in the 

 preceding chapters, though we have not specially regarded 

 them from this point of view. We may refer especially to 

 the very great evaporation of water from the living cells 

 into the intercellular spaces, which we have seen is in 

 some cases supplemented by an evaporation from the 

 general external surface, when this is not covered by any 

 very distinct cuticle. It is evident that the great quantity 

 of water which is given off by the leaves of a sunflower, to 



