Feb., 1914.] 
Transpiration in Relation to Growth. 
249 
correlated with the various functions of a plant or cf different 
plants. On the other hand, plant temperature follows very close¬ 
ly that of the environment; hence, it seems likely that the effec¬ 
tiveness of temperature conditions upon the general development 
and growth of plants, from the time of germination to that of seed 
maturation, and the limits of temperature requirement (for 
morphogenesis and for plant distribution as well) may be meas¬ 
ured. Indeed, values have been obtained in various ways and 
used as a fairly approximate criterion. But rrmch needs yet to be 
determined empirically. Inquiries of the highest importance 
concern the relations between reaction activities and the regulative 
functions, and their degree of interdependence. A compact 
sturdy growth and a greater yield in seed can be obtained in 
most plants only over a comparatively restricted range of tem¬ 
perature, and hence only over a limited geographical range, 
if the water supply increases. Differences in the ability of 
species or of associations of plants to grow under conditions 
widely different from those of their typical habitat and thus 
the increase of their areal range, again point to the limit finally 
set by the relative ability of the protoplasmic functions in accli¬ 
matization or competition. 
It seems to be known only in a general way that the greater 
the proportion of the water component in the plant, the nearer 
is the equilibrium point to the position of complete hydrolysis, 
thus affecting the concentration and the character of the food 
materials. It is well enough known that an increased water 
supply prolongs the vegetative period of growth and increases 
the forage value of the crop rather than the yield in grain, and that 
the less water used in growing grain, the greater is the percentage 
of gluten in the seed and the higher the food value. In recent 
years the tendency all over the western United States is toward 
a more economical use of water, even in localities where water for 
irrigation is still reasonably low in price. In the east correlation 
studies have been made between rainfall and the yield for a number 
of agricultural crops. The data indicate a general relation be¬ 
tween yield and the water supply during the months of July and 
August,—during the intermediate period of active growth when the 
plants are undergoing hydrolytic changes in metabolism prepara¬ 
tory to building up seeds and fruit. A greater water content 
within the plant is required during this period for such purposes 
than is needed during germination or ripening or at any other 
stage in growth, and the danger of impairing the vitality of the 
plant is greater at this time if it lacks this physiological water 
requirement. It must be present in a certain minimum quantity 
before maturity and ripening can take place; otherwise the ripen¬ 
ing processes are retarded and growth results in a small yield, in 
dwarfing of the whole plant, and in injuries when the maximum 
