ENVIRONMENTAL FACTORS INFLUENCING GROWTH 587 



(i) Temperature (soil and air) 



(2) Radiant energy 



(3) Humidity 



(4) Soil water 



(5) Soil aeration 



(6) Concentration of solutes in the soil solution 



(7) Concentration of gases in the atmosphere 



(8) Gravity 



(9) Atmospheric pressure 



The environment to which the roots are exposed is usually very different 

 from that which the aerial organs of plants encounter. Because of reciprocal 

 influences between the roots and tops of a plant, however, effects of any en- 

 vironmental factor upon the development or physiological processes of the roots 

 almost invariably will be indirectly reflected in the behavior of the aerial 

 organs, and vice versa (Chap. XXXIV). 



Some important environmental factors such as precipitation (rain, snow, 

 and hail), and wind are usually indirect in their influence on plants, operat- 

 ing through their effects on one or more of the direct factors listed above. 

 Precipitation, for example, influences not only the soil water content, but 

 also soil aeration and atmospheric humidity. 



Many of the environmental conditions to which plants growing under 

 out-of-door conditions are subjected are in turn influenced by more remote 

 factors. The intensity and quality of impinging sunlight, for example, are 

 functions of the angle of the earth's inclination to the sun (which varies with 

 the latitude and season), the pitch of the slope upon which the light falls, and 

 the direction towards which the slope faces. The soil water content is con- 

 trolled not only by the precipitation, but by the surface run-off (which in turn 

 is largely a function of the slope and the porosity of the soil), and by factors 

 which influence the rate of evaporation such as air temperature, humidity, 

 wind, and insolation. Similarly, with increase in altitude differences in such 

 physical factors as the intensity, quality, and duration of radiant energy, soil 

 and air temperature, atmospheric pressure, etc. are encountered. 



Furthermore, complex inter-relationships exist among the medley of en- 

 vironmental factors which exert direct effects upon plants. Changes in the 

 magnitude or duration of one factor seldom occur without inducing subsidiary 

 changes in other factors. Increase in the intensity of radiant energy in any 

 habitat results in an increase in soil and air temperatures; increase in soil 

 water content diminishes soil aeration, etc. 



Environmental factors influence plant development only because of their 



