} 



PRIMARY FACTORS IN PLANT GROWTH. 13 



Heat occurs, so far as its relation to plants is concerned, under two 

 important phases — air heat and soil heat. The temperature of the soil, 

 although in general conforming to that of the air above it, neverthe- 

 less always presents variations that have a marked effect in determin- 

 ing the habitat of plants. The effect of soil temperature is especially 

 conspicuous in the growth of cultivated vegetation. 



The influence of light upon plant growth is as fundamental as that 

 of heat. It enables the plant to carry on certain chemical operations . 

 necessary to its life. Little attention need be paid to it, however, 

 in considering geographic distribution, as it is a scarcely variable 

 factor. Light is always present during the day in nearly constant 

 amount, and even when somewhat variable is so intimately associated 

 with coexisting variations in heat that the exact effects of each are 

 difficult to distinguish. The most important local variations in amount 

 of light are those due to the shade of trees and other plants and to 

 cloudiness. 



Water might, perhaps, in a strict physiological classification, better 

 be included under the head of food. Only a comparatively small 

 amount, however, is utilized by the plant as food proper, the remain- 

 der being devoted to mechanical purposes. It has seemed better, 

 therefore, to treat the subject independently. Water acts as a medium 

 for the absorption of food materials by the roots of a plant, serves as a 

 vehicle for the transfer of juices, and by keeping the tissues moistened 

 and distended makes possible the performance of other vital functions. 

 It is with soil moisture that a plant has principally to deal, but the 

 amount of moisture in the air has an important influence upon the plant, 

 a greater degree of humidity tending to reduce transpiration. 



The food of plants needs no discussion here, except the bare state- 

 ment that certain quantities and certain kinds of food are necessaries of 

 plant existence. The word food is here used as a general term for raw 

 food-materials ready for absorption by the roots of the plant. 



Like water, air strictly should be included among the food materials 

 of plants, for from the carbon dioxid which it carries is obtained the 

 carbon which goes to make up the elaborated food of plants, while its 

 oxygen is consumed in plant respiration. Yet air does not act the part 

 of a nutritive material merely, and in general it is such an important 

 item in plant life that it is best treated separately. One conspicuous 

 relation which air holds to vegetable life, is that of a medium through 

 which heat is transmitted to the plant and moisture evaporated from 

 it. In addition, most plants demand that, to properly carry out their 

 functions, their roots have access to a certain amount of air in the soil. 

 So universal a substance is air, however, and so nearly constant its 

 composition, that its variations play but little part in governing the 

 distribution of plants. 



Under the head f mechanique are included a variety of physical or 

 mechanical necessaries of plant life. For instance, a plant might have 



