[Chap. XVIII RESPIRATION AND PLANT DEVELOPMENT 163 



containing 16 per cent of water may be 2.5 times as rapid as it is when 

 the seeds contain 15 per cent of water; and when they contain 17 per 

 cent of water it may be 10 times as rapid. This fact illustrates the effect 

 water may have on the rate of respiration in seeds during germination 

 and also when they are in storage, where an injurious amount of the 

 released heat energy may accumulate. 



Temperature effects. When no other factors are deficient, the rate of 

 respiration is doubled for every increase of 10° C. up to a temperature 

 that becomes detrimental to the further survival of the plant. This fact is 

 recognized in refrigeration, which preserves the food value of fruits in 

 two ways : ( 1 ) by decreasing the rate of oxidation of the food within the 

 cells of the fruit, and ( 2 ) by decreasing the respiration and rate of 

 growth of bacteria and fungi that may destroy the fruit. 



The increase in rate of respiration with increase in temperature is 

 often an important factor when the plant is growing, because the maxi- 

 mum rate of photosynthesis occurs at much lower temperatures than the 

 maximum rate of respiration. During the hot days and nights of summer 

 the rate of respiration may exceed the rate at which food is being made. 

 This fact may be illustrated by curves showing the relative rates of 

 photosynthesis and respiration in leaves of potato during a 10-minute 

 exposure (Fig. 62). 



14 



12 



< 



§ 10 

 O ^ 



< ® 



cL 

 O 



_l 4 



_J 



2 - 



50< 



59 « 



68< 



77» 



66* 



95« 



I04<= 



122 < 



TEMPERATURE IN DEGREES FAHRENHEIT 

 Fig. 62. Relative rates of photosynthesis and respiration in potato leaves during 

 10-minute exposures to different temperatures in shade and in full sunlight. Recal- 

 culated from data by H. G. Lundegardh. 



