Feb., 1914.] 
Transpiration in Relation to Growth. 
247 
successfully reproduce themselves and constantly extend 
their range of distribution largely because the various responses 
in vegetative characters or reproduction, in differences in 
abundance, in effectiveness of competition, are more frequently 
matters concerned with inherent vitality, with endurance and 
acclimatization, with the physico-chemical complex of the plant 
itself, rather than with favorable habitat conditions. As one 
travels into the interior of a continent the increasingly con¬ 
tinental character of the climate is accompanied by the appearance, 
on the whole, of open and woody plant associations which do 
not show growth or the strong development of woody tissue 
as a response to the influence of greater amounts of transpiration 
water. As has been pointed out elsewhere by the writer, the 
scattered types of geographical distribution and the trend of 
the migratory movement of individual species and of associations 
tend more frequently to indicate the importance of functional 
plasticity and the nature of the invasion level, i. e., whether the 
plants become dominant, or enter as dependent species and either 
become assimilated with the vegetation type or are slowly 
exterminated. At all events the facts cannot be related merely 
by taking into account the transpiration current or the quantity 
of water evaporated. The rate and character of growth, the 
demand for materials and the destination of the migratory 
materials of various kind are conditioned usually upon the char¬ 
acter of the constructive metabolism. 
That no direct relation exists between growth, green and 
dry weight of plants and transpiration even under the same 
conditions of experiment is further illustrated by an examination 
of the quantity of water associated with metabolism. Water, 
in addition to its important physical influence in imbibition 
and turgor phenomena, has various other roles. In the living 
plant organisms are going on many chemical reactions within 
limited conditions of temperature and moderate concentrations 
of solutions. All these energy transformations take place in the 
presence of an excess of water within the plant. They come to 
an equilibrium point or to an end by the dilution or removal of 
the products of the reaction; the velocity of these reactions is 
regulated by the general physical factors governing such changes 
within a colloidal system. The most important reactions 
upon which the life and the growth of plants depend are those 
by which water is held and fixed in organic combinations (1) 
in the synthesis of food and body material, and (2) in hydrolytic 
reactions whereby water unites with insoluble carbohydrates as 
well as with fats and proteins to form diffusible products for 
translocation to active cells and to the growing region. 
The quantity of water combined in synthetic reactions is fairly 
well known. Assuming that as much water is set free in the breaking 
