514 



RESPIRATION 



greatest in relation to the total dry weight of the tissue. In mature tissues, 

 such as photosynthetically active leaves, a larger proportion of the dry weight 

 of the tissue mass is composed of inert cell wall materials, hence the respira- 

 tion rates of such tissues, expressed in the usual terms, are almost invariably 

 less than those of meristems under comparable conditions. Senescent tissues, 

 such as yellowing leaves or ripe fruits in which the proportion of protoplasm 

 to dry weight is still smaller, generally have lower rates of respiration than 

 the same tissues had when in a metabolically active condition. The lowest 

 rates of respiration are found in dormant seeds and spores, a marked increase 

 in the rate of respiration being one of the striking physiological aspects of 

 germination. The relatively slow rate of respiration in such structures is not, 

 however, due primarily to a low proportionate amount of protoplasm, but to 

 other factors, among which deficient hydration of the tissues is one of the 

 most important. 



Representative rates of respiration for a number of plant organs are listed 

 in Table 49. Even for the plant parts tabulated these rates are to be regarded 

 as only approximations, since the rate for any one plant organ or tissue is 

 subject to marked fluctuations due to the influence of various internal and 

 external factors. 



TABLE 49 RESPIRATION RATES OF VARIOUS PLANT TISSUES IN TERMS OF VOLUME OF OXYGEN 



ABSORBED OR VOLUME OF CARBON DIOXIDE RELEASED IN 24 HOURS PER GRAM OF DRY 

 WEIGHT (from DATA COMPILED BY K.OSTYCHEV, I927) 



Kidd, West, and Briggs (1921) have studied the rate of respiration of 

 various organs of the sunflower plant as well as of the entire aerial portion 

 of this plant at different stages of development (Table 50). This is one of 

 the most comprehensive studies which has ever been made on respiration rates 

 in any one species. The principal fact elucidated by this work is that the 

 respiration rate of not only the entire plant, but also the individual organs 



