Factors Influencing the Intake of Carbon Dioxide. 65 



Thus the rate of carbon assimilation in the leaf may depend on 

 five obvious factors : — 



1. Carbon dioxide supply. 



2. Water supply. 



3. Intensity of illumination. 



4. The quantity of chlorophyll. 



5. Temperature. 



Any one of these might act as a limiting factor in assimilation. 



We may quote with advantage an illustration of the operation 

 of limiting factors given by Blackman. A leaf is supposed to have 

 so much light falling on it as would givi^ energy sufficient to 

 decompose 5 c.c. of carbon dioxide per hour. If, now, the leaf is 

 subjected to such a pressure of carbon dioxide that 1 c.c. of carbon 

 dioxide is assimilated by the leaf per hour, there is sufficient 

 energy provided to enable the whole of this carbon dioxide to be 

 assimilated. When the pressure is raised to double the amount, 

 so that 2 c.c. diffuses into the leaf per hour, the energy is 

 sufficient to bring about the assimilation of the whole of the carbon 

 dioxide, and so on until the pressure has been increased to five 

 times its original value. But if the carbon dioxide supply is further 

 increased, no further increase in carbon assimilation will take place 

 as the energy is only supplied at a rate sufficient to allow 5 c.c. of 

 carbon dioxide to be assimilated in an hour. Whatever the value 

 of carbon dioxide supply above this value, the amount of assimilation 

 will always be the same, i.e., the maximum possible for the value 

 of light intensity. The curve connecting assimilation and carbon 

 dioxide supply will therefore be of the form ABC (Fig. 5). On the 

 other hand, if the light intensity be now increased to double its 

 value, it will be sufficient to allow 10 c.c. of carbon dioxide to be 

 assimilated in an hour, and increases in carbon dioxide supply, will 

 result in a steadily increasing carbon assimilation with increasing 

 carbon dioxide supply, until this latter gives an assimilation of 

 10 c.c. an hour, when illumination will again put a limit on assimil- 

 ation, and a curve of the form ADE will be obtained. With still 

 stronger light, the curve AFG would be produced. Thus it is 

 impossible to investigate the relation between carbon dioxide 

 supplied and the amount of assimilation without considering the 

 factor of light. Similarly, other factors must be taken into account 

 and care must be taken that a factor other than the one imder 

 consideration is not acting as a limiting factor. 



