276 TEXTBOOK OF PLANT PHYSIOLOGY 



Intense respiration requires also the presence of a sufficient 

 quantity of respiratory enzymes in the protoplasm. The rate of 

 respiration is, therefore, highest in younger parts of a plant. In 

 the older portions, especially the storage organs, respiration is very 

 low, in spite of an abundance or respiratory material. The 

 amount of respiratory enzymes produced by protoplasm may vary 

 as a result of several causes. Thus, for instance, when potato 

 tubers and other storage organs begin to sprout, they will respire 

 with greater energy than in a dormant state. Various kinds of 

 irritations, as cutting and exposure to poisonous substances and 

 sharp changes in temperature, likewise will cause a considerable 

 increase of the respiratory rate. 



Of the external factors directly influencing respiration the 

 most important is the effect of temperature. Temperature is the 

 general regulator of the rate of chemical processes. Its influence, 

 (see Art. 14), may be represented by Van't Hoff's law, according 

 to which the rate of a reaction almost doubles with an increase of 

 10° C. in temperature. The lowest limit at which respiration may 

 still be observed, at least in hibernating plants, for instance, buds of 

 deciduous trees and needles of conifer, is -20 to -25° C. Differ- 

 ing from assimilation and many other vital processes in which, 

 after a certain optimal point is reached, a further rise in tempera- 

 ture leads to a decrease of the process, the intensity of respiration 

 increases continually until the killing point is reached (about 45 to 

 50° C). This uninterrupted rise of respiration proceeding along 

 with a decrease in the rate of assimilation with an optimum at 30° 

 C. leads to a debilitation of the organism at high temperatures. In 

 cultivated plants, therefore, it is frequently expedient to keep the 

 temperature at a comparatively low level, especially when the 

 light is not sufficiently intense and prolonged, as in greenhouses 

 during winter. Otherwise, respiration will exceed too greatly 

 the process of assimilation, which is very feeble under the condi- 

 tions of insufficient light; the plant will rapidly spend the reserves 

 accumulated in summer and may finally perish from exhaustion. 

 The relation between the rate of assimilation and that of respi- 

 ration plays an important role in the life of plants, since the differ- 

 ence between these two processes determines the amount of 

 accumulated organic substances. Under normal conditions of 

 light in open exposures, the rate of assimilation is several times 

 higher than that of respiration and there is a general positive 



