228 D. R. Hoagland 



ties of root cells, but also by those of soil microorganisms. The 

 latter aspect of soil-plant interrelations, although it is of utmost 

 importance, cannot be discussed in this brief review. 



Energy Supply for Accumulation of Solutes 



Since energy exchanges are involved in the processes of accumu- 

 lation of solutes, a supply of available carbohydrate must be fur- 

 nished to root cells and synthesis and translocation of sugars are 

 therefore closely interrelated with mineral nutrition. Preliminary 

 evidence is available that depletion of the supply of sugar or 

 other available carbohydrate in the root system is accompanied 

 by a retardation or inhibition of the absorption of mineral 

 solutes and that feeding sugar to the roots, under otherwise 

 suitable conditions, restores, at least in part, the accumulating 

 power of the root cells, lost as a result of sugar depletion. 



The mineral elements absorbed from the culture medium are, 

 directly or indirectly, indispensable to the photosynthetic activi- 

 ties of green cells, and beneficent or vicious cycles in the soil- 

 plant system may be established. For example, a limitation in 

 the supplying power of a soil for potassium may lead to an 

 impairment of leaf function (but not necessarily because potas- 

 sium has a direct role in the process of photosynthesis) and con- 

 sequently to a diminution in the rate of supply of carbohydrate 

 to the root system. A retardation of root growth or metabolic 

 activity may follow with a corresponding retardation of the rate 

 of accumulation of potassium from the soil solution. A further 

 starvation of the shoot for potassium ensues, and so the cycle 

 proceeds, with a trend toward some sort of equilibrium between 

 shoot and root. Injury or death of the plant may occur, however, 

 before such an equilibrium is attained. 



The amount of fruit or seed production, which is determined 



