The Plant as a Metabolic Unit 229 



in part by climatic and biological factors, will affect the require- 

 ment of the plant for potassium or other nutrients and therefore 

 affect estimates of the adequacy of soil conditions. Heavy fruit- 

 ing will tend to diminish the amount of carbohydrate available 

 for root activities, and, according to the views already presented, 

 lessen the capacity of the root system for absorbing nutrients. 

 The soil does not possess a fixed power for producing plant 

 growth, but has instead a physiological value which cannot be 

 adequately appraised without reference to the whole soil-plant 

 system, and in relation to internal plant factors and to climatic 

 environment. A basis for these statements has been established 

 primarily through experiments with plants of agricultural inter- 

 est, but the general principle is of wide application. 



Certain Factors of the Climatic Environment 

 AND Root Growth 



Other interrelations of root and shoot exist which are much 

 more obscure. In growing young barley plants in the greenhouse 

 at different times of the year, under a given uniform nutrient 

 supply, it has been observed that both the quantitative relation of 

 root to shoot and the nature of the root system, as reflected by 

 development of fine lateral roots, vary in a highly significant 

 manner, depending on seasonal conditions. The relationship is 

 illustrated in figure 2. The proportionate yield of roots is far 

 higher in summer than in winter. Without discussing details of 

 the experiments at this time, it may be said that not temperature, 

 but some factor of illumination, seems to determine the extent 

 and nature of the root system. It is not a question of ordinary 

 photoperiodism, but of the quality or intensity of the light. 



Recent investigations on plant hormones^' ^ suggest that 

 growth determinants of this type, the production of which de- 



