Feb., 1914.] Transpiration in Relation to Growth. 243 



It is necessary either to abandon the tenn, because investigators 

 are no longer certain of denoting consistent results obtained by 

 means of the value, or to change the meaning of the term so that 

 it may carry with it the implication which appears in the experi- 

 mental results of various workers. 



In most of the research that has been made on the water 

 recjuirement of plants the investigators have not fully considered 

 the relation of transpiration to growth. Transpiration is un- 

 doubtedly of value as an indicator of different soil and climate 

 conditions and in exhibiting differences which exist between 

 different species and varieties of plants. The general literature 

 bearing upon transpiration has been so well brought together 

 and summarized that a review of it need not be given here. 

 Among the different factors which are directly related to the 

 problem and which affect physically the transpiration value of 

 plants may be mentioned the water content of the soil, the satura- 

 tion deficiency of the air, and the character of the plant, length 

 of active period, relative size, root and leaf area, morphological 

 stnicture, etc. Of these factors the soil water content is con- 

 sidered to be the most important and more complex variable. 

 Its value is a function of the structure, type and amount of soil, 

 tillage, the per cent of humus and clay content, and the quantity 

 of mineral salts (here considered merely as affecting the vapor 

 pressure of water). These conditions modify also the rate of 

 water movement. The value of the transpirational water loss 

 may be determined aj:'d expressed as a ratio in terms of any one 

 condition affecting it directly, but which of these is the better 

 criterion may be left in abeyance for the present. The transpira- 

 tion ratio may fittingly be called the ecological water require- 

 ment. As a criterion for comparing the available water of 

 agricultural soils; as a measure of the quantity, the permanence 

 or the fiuctation of the water relation of plants in their habitat, 

 transpiration under these conditions is very important, and 

 an adequate and simple index of habitat conditions. It is greatly 

 increased with the higher soil water content and decreases within 

 limits as the soil moisture is lowered; where the range in soil 

 water is small the effect is not marked. The loss of water from 

 plants is inappreciable in saturated air, is greater in dry than 

 in moist atmospheric conditions, and less for plants nearer the 

 ground stratum. Under these conditions (assuming in all 

 cases that secondary injurious conditions are eliminated) it 

 indicates the continuity of the water relation between the soil, 

 the plant and the air — the water is absorbed without greatly 

 altering or expending the energy of cell constituents. The 

 transpiration ratio indicates the magnitude of the water factor 

 within the zone of shoot and root activity which controls the 

 individual plant or the association ; it further indicates the limiting 



