LIGHT AND VEGETATION 99 



in which the plant was previously cultivated and to which it 

 has become accustomed and decreases if it is higher. 



The stronger the illumination, the more intense is the 

 activity at the beginning ; then the plant progressively modifies 

 the efficiency of the operations involved so that the speed 

 tends to be the same value in spite of the change of illumi- 

 nation. The period of adaptation depends on the temperature; 

 it lasts some hours when assimilation is being accelerated in 

 poor Hght and is much shorter when it is being slowed down 

 in unusually strong light. 



In the course of the day, the natural variations of sunHght 

 have very marked effects on some plants. Kostytschew reported 

 examples in which assimilation is balanced, and even more 

 than compensated, by respiration at midday, while it passes 

 through two maxima, one in the middle of the morning and 

 the other in the middle of the afternoon. 



If, after an interruption, a plant is again placed under the 

 illumination to which it is accustomed, the rate of assimi- 

 lation begins by increasing, then returns to normal after a 

 few oscillations. Every sudden variation of illumination 

 immediately provokes a corresponding variation of the rate 

 of photosynthesis, followed by a new adaptation which tends 

 to minimize the influence of the disturbing cause. 



It is apparent from these observations that, even for one 

 and the same plant, a given rate of assimilation does not 

 correspond to a given illumination. This rate depends on the 

 time for which the plant is illuminated and especially on its 

 previous history and on the illumination to which it has 

 adapted itself. 



Great caution is therefore needed before a quantitative 

 correlation between illumination and photosynthesis can be 

 stated. For example, the point of compensation, i.e., the 

 illumination at which assimilation exactly balances respiration, 

 depends to a large extent on the adaptation to Ught; when 

 one part of the same plant has been put in the sun and the 

 other in the shade, the first has its point of compensation at 

 170 lux, the second at 37 lux. 



