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CHAPTER XVIII 
THE ENERGY OF THE PLANT 
Tux 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 
