164 TEXTBOOK OF BOTANY 



The rate of respiration in this experiment increased with the increase 

 of temperature until the heat became injurious. If the experiment 

 had continued for a whole day instead of ten minutes, the first evi- 

 dences of this injurious effect would have become visible somewhere 

 between 100°-110° F. The maximum rate of photosynthesis, however, 

 occurred at a much lower temperature, 68° F. In connection with these 

 curves it is interesting to know that the maximum )^ield of potatoes is 

 obtained in areas that have cool summers, where the mean temperature 

 during the growing season is less than 65° F. Under experimental con- 

 ditions where the temperature is kept constant both day and night 

 throughout the period of growth, maximum yields of potatoes are ob- 

 tained at about 62° F. If the plant is exposed continuously to a tempera- 

 ture of 85° F., no tubers are formed. The interrelations of the different 

 organs of a plant with respect to environments will be discussed in 

 Chapter XXI. 



The compensation point. Another interesting point in curve No. 3 is 

 at 104° F. where the rate of respiration is equal to the rate of photo- 

 synthesis. For convenience of reference this point is often called the 

 compensation point. In curve No. 2, which represents the rate of photo- 

 synthesis in potato at 1/25 of full sunlight, the compensation point 

 occurs at 90° F. This fact is of interest even when the plant is in full 

 sunlight, for many of its leaves are in the shade of others. These facts 

 illustrate the general principle that the compensation point occurs at 

 lower temperatures when the light intensity is low. It also occurs at lower 

 temperatures when the concentration of carbon dioxide is decreased. 



It is perhaps evident that a plant kept throughout the day at the 

 compensation point would not gain in dry weight, except for the very 

 small amount of mineral salts that may enter it from the soil; and that 

 during each night there would be a loss in dry weight due to respiration 

 in the absence of photosynthesis. A plant kept continuously under 

 such conditions would soon starve to death. If the nights were cool, 

 it would survive longer than it would if the nights were hot. 



It must also be evident that the yield of plants depends upon the rela- 

 tive rates of photosynthesis and respiration. Under excellent growing 

 conditions photosynthesis may exceed respiration in com fourfold dur- 

 ing the growing season. This would not occur unless photosynthesis ex- 

 ceeded respiration on the average about eightfold during the day. In 

 plants that are merely surviving in the shade of others, growth is limited 



