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CHAPTER XVIII 
THE ENERGY OF THE PLANT 
The various operations which we have seen to be con- 
tinually going on in the body of the plant involve the execu- 
tion 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 processes 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 
