The Plant as a Metabolic Unit 227 



physiologically available to the plant unless oxygen is supplied 

 to the roots, and carbon dioxide removed, at the necessary rates. 



Our general knowledge of the oxygen requirements of roots 

 leads to the supposition that in this regard great differences exist 

 among plants of different types. Investigations of this laboratory 

 suggest that barley plants can maintain active accumulation of 

 mineral solutes at relatively low oxygen tensions, considerably 

 lower than that of the atmosphere. But similar oxygen tensions 

 may be inadequate for certain plants, and some probably have 

 oxygen requirements much lower than that of barley. 



The supply of oxygen to the roots may not be derived exclu- 

 sively from the soil atmosphere. Cannon^ has concluded that 

 translocation of oxygen from the leaves to the root system may 

 occur during periods of photosynthetic activity. If this is an 

 important source of oxygen for root activity, then illumination 

 has a role in the absorption of mineral elements not previously 

 suggested, and anatomical adaptations facilitating the transfer of 

 oxygen assume significance in the study of the mineral nutrition 

 of plants. The importance of this transfer would depend upon 

 the physical structure of the soil, rainfall, activities of microor- 

 ganisms, and all other factors which determine the power of the 

 soil to supply oxygen to the roots. 



In connection with root activities, the effects of carbon dioxide 

 concentrations in the soil atmosphere are likewise of interest. 

 Such concentrations may rise to relatively high levels at certain 

 times. It has not been shown that absorption of mineral solutes 

 is necessarily depressed by carbon dioxide concentrations likely 

 to be present in the soil atmosphere, but the whole relation of 

 carbon dioxide-bicarbonate equilibrium to root growth and 

 solute absorption requires further careful study. It is obvious 

 that the soil system is influenced not only by the metabolic activi- 



