THE COMPENSATION POINT 



515 



such as leaves, stems, and stem tips decreases consistently with increasing age. 

 The rate of respiration of the stem apices is consistently higher than that of 

 the leaves, which in turn exhibit a consistently higher rate than the stems. 



TABLE 50 CHANGES IN THE RATE OF RESPIRATION OF SUNFLOWER PLANTS WITH AGE 



(data of KIDD, WEST, AND BRIGGS, I921) 



• From this date the stem ape.x was the inflorescence only. 



The Compensation Point. — In the leaves or other chlorophyllous tissues 

 the rate of photosynthesis usually exceeds the rate of respiration during the 

 daylight hours. The carbon dioxide produced in respiration is re-utilized 

 by the cells in photosynthesis, but since the latter process is occurring more 

 rapidly than the former, additional carbon dioxide is continuously diffusing 

 into the plant from the outside environment. Similarly photosynthesis pro- 

 duces more oxygen than is used in respiration, the surplus diffusing out of 

 the plant. Hence during the daylight hours, as long as conditions favorable 

 for photosynthesis prevail, there is a net movement of carbon dioxide into the 

 green parts of plants, and a net loss of oxygen from them. Under such con- 

 ditions the occurrence of the gaseous exchanges accompanying respiration in 

 leaves is completely masked. 



At night or in the dark the reverse condition obtains, oxygen diffusing 

 into the green parts of a plant and carbon dioxide diffusing out of them. 

 Similar gaseous exchanges are characteristic of the non-green organs of a 

 plant, whether in the light or in the dark. The magnitude of the gaseous 



