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 fairh" approxiniate criterion. But much 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 geniiination 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 inaximum 



