526 RESPIRATION 



great effect upon the rate of respiration is observed until a certain water con- 

 tent (which varies according to the tissue) is attained, after which the respira- 

 tion rate increases rapidly. 



Minor variations in the water content of well hj'drated plant cells do not 

 appear to have any very great influence upon the rate of respiration. Walter 

 (1929) found that a reduction in the hydration of the cells of Elodea such as 

 was induced by immersing them in a weight molar sucrose solution had no 

 appreciable effect upon the rate of respiration, although photosynthesis ceased 

 entirely under such conditions. 



7. Light. — As far as available evidence goes light has no direct effect upon 

 the rate of respiration. It does, however, exert certain indirect effects. In 

 chlorophyllous organs, light may affect the rate of respiration because of its 

 influence upon the supply of respiratory substrate resulting from photosyn- 

 thesis. Plant organs exposed to direct illumination almost invariably have a 

 temperature in excess of that of similar organs not so exposed. The heating 

 effect of light is one of its important indirect effects upon respiration. 



8. Injury. — Wounding of plant tissues almost invariably results in a tem- 

 porarily increased rate of respiration. If a potato tuber is cut in half, for 

 example, the loss of carbon dioxide from the two halves will be considerably 

 greater than from the intact tuber. Similar results have been observed for 

 many other plant tissues. The increased respiratory activity of wounded 

 or otherwise injured plant tissues gradually rises to a maximum which is 

 generally attained within a day or two, after which a diminution in rate sets 

 in until approximately the rate which prevailed in the uninjured tissues is 

 re-established. 



Hopkins (1927) and others have shown that this increased respiration of 

 potato tubers following wounding is correlated with an increase in the sugar 

 content of the tuber. This increase, which amounted in Hopkins' experiments 

 to from 53 to 68 per cent of the sugar originally present, occurs gradually, 

 the maximum not being attained until several hours after the injury. The 

 increase in sugar content is greater in the cells close to the cut surface than in 

 those which are more remote from it. This increase in the quantity of the 

 respiratory substrate is apparently an important factor in accounting for the 

 increased loss of carbon dioxide by potato tubers following wounding, and 

 probably of many other tissues as well. 



9. Various Chemical Substances. — Many investigations of the specific in- 

 fluence of various chemical compounds upon the rate of respiration have been 

 made. Studies of the effects of various toxic organic substances such as 

 chloroform, ether, acetone, ethyl alcohol, formaldehyde, morphine, bromine, 

 quinine, etc. have been especially extensive. As a specific example of the 



