60 CARNEGIE INSTITUTION OF WASHINGTON. 



which may be the optimum for the oxygen partial pressure. In 1.5 per cent 

 oxygen the actual rate of growth of the roots is not far different at 20° than at 

 30°, although the relative growth-rate at the latter temperature is much less 

 than at the former soil temperature. 



The observations as above outlined on the relation between oxygen partial 

 pressure and the maximum and optimum temperatures for root-growth appear 

 to indicate that where the oxygen deficiency is critical these cardinal tem- 

 peratures are greatly lowered, but where the amount of available oxygen is 

 above this the optimum temperature at least is not greatly affected. 



The Growth Ratio, or Index op Growth, -5-; The Relative Growth op Roots 



IN A Depiciency of Oxygen. 



In studying the reaction of roots to a small amount of oxygen in the soil it 

 has been found advisable, owing to individual variation, to use each specimen 

 experimented with as its own control by first observing the rate of growth, at 

 known and constant soil temperature and for a given period of time, and 

 immediately afterward by measuring the growth for the same time and tem- 

 perature but while in any desired gaseous mixture. The root-growth in the 

 experimental atmosphere divided by the growth under conditions of normal 



T 



aeration, -^, is the growth ratio, or growth index, for the temperature and 



oxygen supply used. It is the relative root-growth, expressing not the actual 

 amount of growth, but merely the capacity of the root for growth under the 

 temperature and aeration conditions of the experiment. 



The growth ratio, being usually less than unity, reveals the depressing 

 effect on growth of a deficiency of oxygen. This varies greatly in different 

 species, different temperatures, as well as different percentages of oxygen. 

 In Allium cepa, in 1.6 per cent oxygen, ratios of 0.5 to 0.9 were obtained at 

 17° and 0.12 to 0.25 at 22° C. In 2.8 to 3 per cent oxygen and at 27° the ratio 

 was found to be 0.8 to 1 . The roots of Opuntia versicolor in 1 .6 per cent oxygen 

 had a ratio of 0.6 at 22° and 0.3 at 30°. In the rough lemon, in 2.8 per cent 

 oxygen, a ratio of 1 was obtained at 22°, but at 27° it was only 0.4 to 0.7. The 

 ratio in the Brazilian sour orange at 27° was 1 with 1.2 per cent oxygen and 

 0.6 with 8 per cent oxygen, while at 22° a ratio of 0.5 was obtained in 

 0.8 per cent oxygen. At 27° and in 2.8 per cent oxygen the growth ratio 

 in Prosopis velutina was 0.7, and at 20° it was 0.14 in 0.6 per cent oxygen, 

 0.3 in 0.8 per cent oxygen, and 0.7 in 2.2 per cent oxygen. In Potentilla 

 anserina and in 2 per cent oxygen the ratio was 1 at 18°, 0.3 at 27°, and 0.2 

 at 30° C. At a temperature of 26° in Salix lasiolepis the ratio was 1 in 1.6 per 

 cent oxygen, but at 30° it was only about 0.2. In 3 per cent oxygen Zea mays 

 gave a ratio of 0.28 to 0.39 at 17°, 0.2 at 22°, 0.08 at 27°, and 0.06 at 30° C. 

 The same variety of corn (Golden Bantam) in 3.6 per cent oxygen had a ratio 

 of 0.6 at 17° and 0.4 at 30°, while in 10 per cent oxygen the ratios for these 

 temperatures were 1 and 0.89, respectively. 



From the summary given above it will appear that although different species 

 may have unlike growth ratios, yet the ratios have this feature in common, 

 namely, they decrease in value with the mounting temperature of the soil for 

 the temperature range employed. In the face of a constant but deficient 

 supply of oxygen such would be expected, inasmuch as the oxygen require- 



