58 CARNEGIE INSTITUTION OF WASHINGTON. 



keep pace with the evolution of carbon dioxide, even if the actual amount of 

 oxygen absorbed is greater at higher than at lower temperatures. When, 

 therefore, as will be shown in this report, the amount of oxygen present in 

 the atmosphere of the soil is much below normal, the rate of growth, which is 

 usually related to respiration, is of necessity less than normal. Under such 

 conditions there is a very close relation between the rate of root-growth and 

 the amount of oxygen available to the roots, and the oxygen deficiency 

 becomes a factor limiting growth-rate. 



As shown in another paragraph, under conditions of a constant supply 

 of oxygen, especially if the partial pressure is low, the relative growth-rate 

 varies with the temperature in a very definite way. When, however, the 

 temperature of the soil is maintained constant, a variation in the rate of 

 growth of roots can be induced by varying the amount of available oxygen, 

 provided always it is small. This can be shown to be the case by citing 

 a few typical observations. 



Potentilla anserina was studied in this regard at 18° and 30° C. and in 

 1.2 and 2 per cent oxygen. At 18° soil temperature and in normal soil 

 atmosphere a total root-growth in 24 hours of 3.5 mm. was recorded. During 

 the following 24 hours, however, when 1.2 per cent oxygen was given, the 

 growth was 1 mm. only. At the same temperature but in 2 per cent oxygen, 

 the growth during 24 hours was found to equal that under normal conditions 

 of aeration. With the temperature of the soil 30° it was found that, under 

 normal conditions of soil aeration, the growth of the root in 24 hours was 7.5 

 mm., but in 1.2 per cent oxygen growth ceased. In a soil atmosphere con- 

 taining 2 per cent oxygen, on the other hand, and at the same soil temperature, 

 the growth during a 24-hour period was 3 mm., or nearly half that to be 

 expected under normal conditions of soil aeration. 



Root-growth of Opuntia versicolor was studied at 20° and at 30° C, and in 

 0.5, 1.2, 1.6, and 2.2 per cent oxygen. At a soil temperature of 30° and 

 in normal soil atmosphere a growth of 9 mm. during 24 hours occurred, but 

 in 2.5 per cent oxygen and at the same temperature the growth was reduced 

 to 3 mm. Upon being given 1.2 per cent oxygen for the same length of time 

 and at the same temperature of the soil, however, growth ceased. At a soil 

 temperature of 20° root-growth continued at a slow rate in 1.2 and in 1.6 

 per cent oxygen, and somewhat more rapidly in the soil containing the latter 

 amount. In another experiment, in which the experimental atmosphere 

 employed contained 0.5, 1, and 2.2 per cent oxygen, the growth for 24 hours 

 had the ratio 1 : 2 : 4 to 5, or it was approximately proportional to the amount 

 of oxygen present. 



Root-growth in Zea mays (Golden Bantam corn) was observed in 3.6 and 

 10 per cent oxygen and at 18° and 30° C. At a soil temperature of 18° and 

 with normal supply of oxygen, the root of the corn increased 12 mm. in 

 length in 24 hours. During a similar period, but in 3.6 per cent oxygen, 

 the growth was 8 mm. and in 10 per cent oxygen it was 10 mm. At a soil 

 temperature of 30° and with normal aeration a growth of 45 mm. was ob- 

 served. In 3.6 per cent oxygen the growth fell to 19 mm., but in 10 per cent 

 oxygen it became 37.5 mm. 



Analogous results were obtained with Prosopis velutina, and with the 

 Brazilian sour orange and rough lemon, all of which suggest the possible 

 general applicability of the principle. More intensive quantitative studies 



