Feb., 1914.] 
Transpiration in Relation to Growth. 
243 
It is necessary either to abandon the term, 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 
requirement 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 
structure, 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 and 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 fluctation 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 
