383] B. E. Livingston and E. E. Free 185 



plant and the roots are long, thin and little branched, and 

 range widely through the soil. 



When Coleus plants of different sizes are deprived of soil 

 oxygen, the cessation of water intake and the subsequent 

 symptoms of injury appear first and are most severe on 

 the plants which have the larger root systems. Again, a 

 plant with a small root system will tolerate a lesser oxygen 

 content in the soil than will a plant with a large root system. 

 This implies that the crucial matter is the supply of oxygen 

 per unit of root surface (or volume) and this is confirmed 

 by the fact that a low oxygen content in a frequently changed 

 atmosphere is less injurious than a higher oxygen content 

 with less frequent changes. 



The evidence suggests that the cause of injury by exclu- 

 sion of oxygen from the roots is an interference with the 

 respiration of the protoplasm of the root cells, resulting in 

 the death of this protoplasm and the consequent failure 

 of the roots to function as water absorbers for the plant. 

 There is no reason, however, to assume any "vital" func- 

 tion of the root protoplasm in promoting water absorption. 

 The protoplasmic coagulation which is, or accompanies, 

 what is called death would constitute in itself a sufficient 

 change to explain, on a purely physical basis, this effect on 

 water absorption. The fact that the roots of at 'least one 

 plant (Salix) appear to function normally in the absence 

 of free oxygen raises the interesting question whether the 

 respiration of these roots may not be anaerobic. It is im- 

 ppssible to answer this question finally. There is a theoret- 

 ical possibility, in our experiments, of some small access of 

 free oxygen from some source not now suspected. However, 

 the sharp difference in the behavior of Salix and of Coleus 

 under identical treatments suggests some important differ- 

 ence in the respiratory habits of the roots of the two species. 



