200 LIGHT, VEGETATION AND CHLOROPHYLL 



Others, especially when the weather is cloudy, may find light 

 a limiting factor for their rate of assimilation. 



The rate of assimilation is difficult to measure when the 

 oxygen given off is less than the oxygen absorbed by respiration. 

 The condition when the two rates are equal is called the point 

 of compensation and then the plant is breaking down by 

 respiration as much as it is assimilating. Every morning as 

 day breaks all plants pass the point of compensation, but not 

 all at the same time ; shade plants pass it much earlier without 

 waiting for the light intensity needed by sun plants. They 

 also suffer more than the others from an excess of light, for 

 too much of it disturbs the plant; the optimum for assimi- 

 lation, even for sun plants, is below full sunlight. 



In very strong light, assimilation diminishes and the leaf 

 suffers from insolation. Insolation occurs more quickly if 

 carbon dioxide is lacking but more slowly if glucosides are 

 abundant. A high temperature also promotes it and the 

 effects seem to be cumulative, for it is much more rapid 

 following great heat. 



Insolation appears to have several effects and, when it 

 has been intense, the plant takes several days to recover. 

 Sometimes it causes a diminution of the chlorophyll and the 

 leaves that have just been exposed to very strong light are a 

 little less green than the others ; considerable oxidation of the 

 tissues, apparently auto-oxidation, accompanies this excess of 

 hght. 



Another effect observed is the orientation of the chloro- 

 plasts inside the cells and the manner in which they protect 

 themselves from excessively strong light by sheltering in the 

 shade of one another. This would also explain the diminution 

 of the green colour. 



But the most important effect, and the one which would 

 seem best to account for all these symptoms, is the inactivation 

 of the diastases which are sensitive to excesses of heat or 

 light. 



It may therefore be concluded that light, which is essential 

 to photosynthesis, can be a very variable limiting factor for 



