92 PLANT PHYSIOLOGY 



velocity, the break of the curve displaying a sharp angle; and 

 finally at a temperature of about 40 to 50°C., the process ceases 

 completely. In the temperature curve of vital processes, there 

 are found the three basic, or, as they have been termed by 

 Sachs, cardinal points: the minimum, at which the process 

 just starts; the optimum, when it proceeds with the highest 

 velocity; and the maximum, in excess of which the process 

 ceases. 



The lower temperature limit of respiration of plants lies 

 beneath — 10°C. and is apparently conditioned chiefly by freezing 

 of the tissues. In hibernating green parts of plants, e.g., in the 

 buds of deciduous trees and in needles of conifers, it is much 

 lower. Perceptible respiration may be observed at tempera- 

 tures about 20 to 25° below zero. With an increase in tempera- 

 ture, respiration increases rapidly and up to 40°C. follows the 

 rule of van't HofT, the coefficient Qio varying from 1.9 to 3.9. 

 Above 40°, there may frequently be observed a rapid jump 

 upward, then at about 50° an abrupt fall, accompanied by the 

 simultaneous death of the plant. 



Investigations (Kujiper, 1910; Fernandez, 1921) show that 

 essential corrections must be introduced in determining the 

 optimum point, i.e., the temperature of the greatest intensity of 

 respiration. If the intensity of respiration of a plant placed 

 for a prolonged period in definite temperature conditions is 

 determined, it will be noticed that with temperatures in excess 

 of 35°C., the process of respiration, initially very intense, soon 

 begins to decrease and after some hours becomes considerably 

 weaker than at temperatures about 25 to 20°C. and lower. This 

 phenomenon may be explained by the fact that at temperatures 

 in excess of 35°C., the protoplasm undergoes changes that very 

 soon become manifest in a decrease of respiration. The tem- 

 perature at which respiration first shows an increase and then 

 is followed by a decrease must not be taken for the optimum. 

 At the actual optimum temperature, the intensity of respiration 

 remains continually at a high level. This optimum usually is 

 between 35 and 40°C., at least for germinating seeds of common 

 crop plants. It is interesting to note that this is the temperature 

 that is found in man and in other warm-blooded animals and 

 that is absolutely indispensable for the normal course of their vital 

 processes. 



