198 LIGHT, VEGETATION AND CHLOROPHYLL 



problems of nutrition from the chemical aspect, discovered 

 the law of the minimum ; when several substances are indis- 

 pensable to growth, it will be checked as soon as one of them 

 is lacking and will be resumed, in proportion as the missing 

 substance is suppHed — so long as it remains below its optimum. 

 As the optimum is approached, the increase of growth will 

 no longer be proportional to the increase of the substance 

 the influence of which will tend to be nullified. Only so long 

 as its quantity is insufficient for the needs of the plant will 

 the substance be Hmiting by its deficiency. Blackman very 

 fittingly applied these laws to the factors influencing photo- 

 synthesis, but the interference of the factors must never be 

 forgotten and it is impossible to trace precisely the curve of 

 influence of any one of them, for this curve will be diff'erent 

 if some other factor varies. More intense illumination, for 

 example, will increase the utilization of carbon dioxide and 

 consequently the former requirement, or even the former 

 optimum value, will very probably be insufficient. 



In brief, photosynthesis, like other vital functions, allows 

 a certain margin of variation of the necessary factors. Inside 

 this margin none of them has any influence, but they become 

 limiting as soon as they pass beyond it, either by excess or by 

 deficiency, unless the variation of some other factor causes a 

 modification or displacement of the margin proper to each. 

 The further a factor moves from its optimum, the more 

 limiting it becomes. 



Since photosynthesis is a utilization of carbon dioxide by 

 chlorophyll under the influence of light, the abundance of 

 chlorophyll, of fight and of carbon dioxide will obviously 

 influence it, as well as heat and water and the accumulation of 

 glucosides synthesized. 



The Influence of Chlorophyll 



Contrary to what might be expected, the quantity of 

 chlorophyll has relatively little influence on the rate of assimi- 

 lation. A study of leaves belonging to diff'erent species discloses 

 no relationship between this rate and the amount of chloro- 



