50 CARNEGIE INSTITUTION OF WASHINGTON. 



shallow sheet, covering this plain. In the case of the cultivated spe- 

 cies, flooding takes place at fairly frequent intervals through irrigation. 

 Under both conditions, therefore, owing mainly to slow percolation, 

 poor aeration of the soil may occur. Both of the species referred to 

 are relatively but not highly tolerant of poor conditions of soil-aeration, 

 and root-growth may or may not continue in a soil-atmosphere con- 

 taining 0.5 per cent oxygen, remainder nitrogen. It is to be noted 

 that so far as concerns Prosopis the soil may be well aerated during 

 the long dry seasons. 



8. When the amount of oxygen in the soil is diminished to the point 

 where root-growth may or may not take place, the growth of the roots 

 may continue, provided the gaseous mixture is caused to stream slowly 

 through the soil. It is the rate of supply and not the partial pressure 

 of the gas that is of moment. In Stachys, for example, root-growth 

 ceases in a static soil-air containing 0.5 per cent oxygen, while growth 

 will continue if the same gaseous mixture is slowly streaming. A 

 similar condition was observed in several other species. 



9. A deficiency of oxygen may be said, in general, to exist when the 

 oxygen comprises 10 per cent or less of the soil-air, with the remainder 

 nitrogen ; and an excess of carbon dioxid can be said to be 50 per cent, 

 more or less, of the soil-atmosphere when there is normal oxygen partial 

 pressure, and probably less than 50 per cent when the amount of oxy- 

 gen is low. 



10. There is in some species, if not in all, a direct relation between 

 tolerance to a deficient oxygen-supply and the temperature of the 

 soil. In Prosopis, for example, it was observed that when the tem- 

 perature of the soil was about 10° C. below the optimum, growth 

 stopped upon the administration of a gaseous mixture poor in oxygen, 

 but it was renewed with the same soil-air when the temperature was 

 raised to optimum. 



11. Vigorous seedlings are most resistant to deficiency of oxygen. 



12. The size of the cotyledons in Prosopis apparently holds a direct 

 relation to the amount of oxygen in the soil-air. The cotyledons are 

 relatively small when the oxygen partial pressure is low. 



13. The hypocotyls of Prosopis appear to be insensible to a depriva- 

 tion of oxygen. 



14. In certain species, for instance Opuntia, Pisum, and Zea, 

 there may be a differential response to a condition of poor soil-aera- 

 tion by the roots in one and the same plant. Thus, in the case of the 

 two species first named the main roots are less tolerant to poor aera- 

 tion than are the laterals, roots of smaller size or of lower rank, while 

 the opposite appears to be the case in Opuntia. So far as Opwitia 

 is concerned, it is possible that the reaction is in a direction looking 

 toward a superficial placing of the lateral roots, which is a characteris- 

 tic of the mature plant. 



