134 Journal of Agricultural Research voi. xiii, no. 2 



writers' knowledge of sorghum they have reason to believe that its tem- 

 perature requirements resemble those of beans (Fabaceae) and corn 

 {Zea -mays) more closely than they do those of wheat (Triticum spp.) and 

 barley (Hordeum spp.) The optimum temperature for the growth of 

 sorghum is quite likely about 92° or 93°. Above this optimum, growth is 

 retarded by further increases in temperature until the maximum is 

 reached, when growth ceases entirely, and continued exposure to the maxi- 

 mum temperature will cause death. Just how the high temperatures 

 (those above the optimum) effect this retardation of the growth is not 

 well understood. Vines (8, p. 283-284) states that many physiologists 

 believe the fatal effect of high temperatures is due to the coagulation of 

 the coagulable proteids of the cell, but this has not been proved. He goes 

 on to say : 



It is doubtless upon the living protoplasm of the cell that the temperature acts; 

 the effect first manifests itself by a diminution of the metabolic activity of the proto- 

 plasm, and ultimately effects its disorganization. 



Kreusler found that assimilation in species of Ricinus and Rubus be- 

 gins to decrease at 86° and at 113° F. is stopped almost entirely. 



Balls (2), who worked on the "sore shin" fungus of cotton, attributed 

 this decrease of assimilative power and growth in the presence of high 

 temperatures to the accumulation of katabolic products in the cell and 

 not to any inability of the protoplasm to function at temperatures of 98^ 

 to 100° F., which he determined as the practical stopping point of growth 

 in the "sore-shin" fungus. 



Ewart's {4, p. 385-38-7) work seems to confirm Balls's theory. He 

 found that plants of com, beans, pumpkins, etc., subjected for three 

 days to temperatures of 98° to 100° F., if well supplied with oxygen and 

 moisture, showed no inhibitory after effect, but if the supply of air were 

 limited a retardation of assimilation was induced which persisted for 

 sometime after the temperature had been reduced to a point most favora- 

 ble for the plant's growth. Thus, it would appear that it may be a failure 

 properly to dispose of the products of katabolism which interferes with 

 continued assimilation and growth at superoptimal temperatures. 



Carefully controlled tests would have to be made, of course, in order 

 to determine whether sorghum behaves as these other plants have done 

 under similar temperatures; but observ^ations in the field at Bard, Cal., 

 indicate that there is a slowing up of growth in nearly all field crops dur- 

 ing the hottest part of the summer, when air temperatures in the daytime 

 are often consistently above 100° F. 



The manner in which low temperatures act upon the plant to de- 

 crease its rate of growth has been studied less than the effect of high tem- 

 peratures. The minimum temperature for growth has been determined 

 for a number of plants, and several investigators have studied in detail 



