LABORATORY FOR PLANT PHYSIOLOGY. 59 



on the respiration of the roots of representative species are required, however, 

 to give it exact expression. 



Growth of Roots at Different Temperatures of the Soil and in a Constant but 



Deficient Supply of Oxygen. 



Within the range of soil temperature favorable for the growth of roots are 

 well-marked cardinal temperatures for growth, by which is meant the lowest 

 temperature at which root-growth of a species begins (the minimum temper- 

 ature for root-growth), the temperature at which in given time the growth 

 is most rapid (the optimum temperature for root-growth), and the highest 

 temperature at which growth continues (the maximum temperature for root- 

 growth). In any event an appropriate substratum, suitable supply of food 

 material in solution and of water, and adequate aeration are postulated. 

 An absence or great alteration of one essential factor of the soil environment 

 causes especial adjustments and reactions which may materially alter reac- 

 tions of the root, as shown by disturbance in the rate of growth. Such 

 appears to be strikingly the case when roots are grown in small amounts of 

 oxygen. The results of preliminary experiments appear to indicate, for 

 example, that not only may the rate of growth be greatly modified by the 

 partial pressure of oxygen, as outlined in the preceding paragraphs, but that 

 the cardinal temperatures for root-growth may possibly also be somewhat 

 (perhaps occasionally greatly) changed by the same cause. 



In Prosopis velutina the optimum temperature for root-growth is about 

 32.5° and the maximum temperature about 42° C.^ Under such conditions 

 the aeration of the soil is normal. When, however, the supply of oxygen is 

 much depleted there is apparently a marked alteration of these cardinal 

 temperatures, which is associated with the degree of the oxygen deficiency. 

 Thus, 30° is probably the maximum for root-growth in a soil atmosphere con- 

 taining 0.5 per cent oxygen, but a fair rate of growth (possibly the most rapid 

 for this oxygen partial pressure) occurs at 20° C. 



The optimum soil temperature for root-growth in Potentilla anserina is 

 27° to 30° C. in normal soil atmosphere. When the amount of oxygen is 

 reduced to 1.2 per cent, 27°, or possibly less, is the maximum, while a fair 

 growth-rate, possibly the most rapid in this amount of oxj'-gen, goes on at 

 18° C. The optimum temperature for growth of roots of Potentilla in 2 per 

 cent oxygen appears to be about 20° C, above which the rate rapidly falls. 



In Zea mays the optimum temperature for growth of the shoot is 33.7° and 

 the maximum temperature is 46.5° C. The cardinal growth temperatures of 

 the root of the species used, Golden Bantam corn, have not been determined. 

 It is apparent, however, that in a soil atmosphere deficient in oxygen, although 

 containing a relatively good supply, the maximum and optimum temperatures 

 for growth are both much lower than when the supply of oxygen is normal. 



The optimum temperature for root-growth in Opuntia versicolor, with normal 

 aeration conditions, is about 32° C. When 2.5 per cent oxygen is given the 

 roots, the growth-rate becomes less at all temperatures, but the optimum 

 appears to be about as under normal oxygen relations. In a soil atmosphere 

 containing 1 per cent oxygen, however, 32° is the maximum or above the 

 maximum for root-growth, while active growth occurs about 10° below this, 



^ W. A. Cannon, Relation of the rate of root-growth in seedlings of Prosopis velutina to the 

 temperature of the soil. Plant World, vol. 20, p. 320, 1917. 



