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 



