146 Mineral Nutrition of Plants 



treme differences in critical concentrations of oxygen between species 

 did not appear to be over 1 or 2 per cent.* 



Another factor of considerable significance, reported by Cannon (23, 

 21), Bergman (8), Snow (64), Elliott (■?■?), and Loehwing (47,48), is 

 that many plants have very few or no root hairs under conditions of 

 low oxygen supply. 



There is some evidence that when roots are deprived of oxygen or 

 brought into an environment much lower in oxygen than normal, the 

 roots may die, then be replaced by new, more stubby roots, or the roots 

 may stop growing for a period and then start growing again very 

 slowly. This would seem to indicate that when brought into conditions 

 of poor aeration, some plants may undergo a change which adapts them 

 better to partially anaerobic conditions, providing the demands on the 

 roots are not too great during the period of adjustment. Much addi- 

 tional work needs to be done on this point to determine the behavior 

 of plants when the soil on which they are growing is waterlogged 

 either through heavy rains or improper irrigation. Went (75) reported 

 results with tomatoes which suggest that if the plant can survive until 

 a new root system can be established where the air supply is more favor- 

 able, the plant can continue to grow. He obtained increased growth 

 with tomatoes in nonaerated nutrient solutions which had sent new 

 roots into well-aerated moss (moistened with nutrient solution) tied 

 around the stem above the level of the solution. There is considerable 

 evidence that roots cannot function normally to absorb water and nu- 

 trients in the absence of oxygen. Thus, it does not appear that oxygen 

 supplied to part of the root system of a plant would be sufficient to 

 maintain the normal functions of the remaining nonaerated part of the 

 root system. Instead, the old roots would die and the plant would be- 

 come dependent upon the new root system. Boynton (16) found that 

 few new roots were formed on apple trees when the oxygen level in 

 the soil atmosphere fell below 15 per cent, and when oxygen was at 

 10 per cent or below and carbon dioxide was from 5-10 per cent, both 



* Cannon designated the lower critical pressure as that partial pressure at which 

 growth ceases, and the upper critical pressure as that at which growth proceeds 

 normally. Between these two values growth continues, but at a rate and in char- 

 acter which are subnormal. 



