Hoagland — 116 — Plant Nutrition 



tion is not unwarranted, in view also of evidence 

 from the field, that frequently soil aeration constitutes 

 a limiting factor for the growth of some types of 

 crops and that it may be an ecological factor of sig- 

 nificance. 



Not only is the absorption of nutrients accelerated 

 by the vigorous aeration of the culture solution in 

 which tomato plants are grown, but there also exists 

 a relation between aeration of the solution — that is, 

 the introduction of oxygen and the removal of carbon 

 dioxide — and the absorption of water. On hot days 

 we and others have observed that tomato plants 

 quickly show evidence of wilting if the aeration of 

 the culture solution is stopped, or even markedly 

 diminished. Whether this is merely a matter of per- 

 meability changes in root cells or whether some active 

 transport of water is possible is not a finally settled 

 question, but I argued in an earlier lecture that 

 permeability is closely associated with metabolism. 

 (See plates 2U and 25). 



Many of these remarks have as their basis the 

 results of experiments conducted with tomato plants. 

 Valuable as the tomato is as a test plant, to draw too 

 broad conclusions from this one species would be 

 most unwise. For the same type of aeration that is 

 so beneficial to the growth of the tomato plant is in- 

 jurious to some other kinds of plants. The rice plant 

 is an example of a plant that may be injured by too 

 high aeration of the root system. The willow is an- 

 other. Of course, knowing the conditions under which 

 these plants can grow out-doors this response is not 

 surprising. The question merits some additional com- 

 ments. Vlamis (1941) carried on an investigation 

 in Berkeley of salt absorption by the rice plant. In 

 the first place, he learned that the excised roots of 

 barley, tomato and rice plants did not behave very 

 differently with respect to effects of oxygen on salt 



