13 2 
Ingvar Jorgensen and Walter Stiles. 
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 give 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 under 
consideration is not acting as a limiting factor. 
