208 VEGETABLE PHYSIOLOGY 
even during an ordinary day, and still more by the alterna- 
tion of day and night; in the case of perennial plants yet 
greater disturbances are caused by the succession of the 
seasons of the year, and the alterations these produce in 
the amount of foliage which the plant preserves ; weather 
and its vicissitudes form a series of disturbing influences. 
We have thus the certainty of failure to survive in the 
struggle for existence unless the initial absorptive and 
constructive processes are supplemented by others, which 
in some way shall make the organism indifferent to these 
changes and intermissions of supply, and capable of carry- 
ing out true nutritive work, when the initial stages of 
such work are checked or suspended. In other words, 
suitable conditions for the construction of food being 
intermittent, the plant must accumulate a reserve store on 
which it can subsist during the periods, short or prolonged, 
when no such manufacture is possible. 
We may view the matter from a slightly different 
standpoint, and yet come to the same conclusion. The 
processes of absorption in a plant depend, as we have seen, 
almost entirely upon physical conditions. Given a certain 
amount of carbon dioxide in the air, and a certain amount 
of water in the plant to which that air has access, the 
carbon dioxide will be dissolved according to the power of 
the water to dissolve it, or—putting it more technically— 
according to its coefficient of solubility. In the presence 
of the chlorophyll apparatus, with the access of sunlight, 
the other subsequent changes which we have discussed 
lead to the continuation of the absorption of the gas. 
This is the case again with the root and its relations to the 
soil. The process of absorption of water with its dissolved 
substances will proceed as long as certain physical condi- 
tions obtain. Thus the plant is, on the whole, rather 
passive than active in the initial stages of its own feeding, 
exercising no inhibitory power, such as that which in an 
animal is attendant upon a failure or cessation of appetite. 
These considerations lead us to the conclusion that when 
