150 THE BIOLOGY OF FLOWERING PLANTS 



, assimilation at 22*3° C. (with light and carbon dioxide in 

 excess) to be 26*2 mg. carbon dioxide ; taking this 

 figure as applying also to the sunflower, he draws the con- 

 clusion that in bright sun the assimilation of the sunflower 

 leaf is limited by temperature up to 23° C, and that below 

 this the normal supply of carbon dioxide is more than 

 sufficient to cover the possible assimilation, Willstatter, 

 however, has obtained a value of 80 mg. for the sun- 

 flower with temperature at 25° C. limiting, and this would 

 indicate that temperature is limiting only below 11° C. If 

 Willstatter's result is correct, and it is probably too high, 

 we have the very important fact that the carbon dioxide 

 supply, small though it is, is yet sufficient to maintain 

 assimilation at the highest possible value for all temperatures 

 below 11° C. (or 52° F.) as long as the light is sufficiently 

 strong ; while, if Blackman's figure is taken, the carbon 

 dioxide of the atmosphere can maintain maximal assimila- 

 tion up to 23° C. (or 73° F.). It will be noted that the 

 sunflower is one of the plants with high assimilating capacity. 

 The more numerous plants of the first group in Table 

 XXVII will be even better situated as regards sufliciency 

 for their needs of the carbon dioxide supply, especially in 

 temperate cHmates. 



Looking over these results, we have a vivid view of the 



way in which the different controlling factors cross each 



other as the day waxes and wanes and the weather changes. 



In the cool hours of the morning the supply of carbon 



dioxide is ample, but temperature or dull light is limiting. 



The sun mounts the heavens, and temperature rises with 



light intensity ; assimilation also rises, limited from moment 



to moment by one or the other of these two factors. The 



sun passes behind a cloud, and the heated leaf no longer 



receives sufficient light to maintain its full efficiency ; it 



shines for a period, and the carbon dioxide supply is no 



longer sufficient to supply the leaf's capacity in brilliant 



light and high temperature. Evaporation increases, the 



leaves flag, and the stomata close, carbon dioxide supply is 



cut off, and, it may be, assimilation ceases for that day. 



