222 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, though 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 coemcient 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 



