CHLOROPHYLL AND ENERGY 197 



Factors Influencing Assimilation 



Assimilation depends on a number of different and inter- 

 dependent factors. For each of these factors there is an 

 optimum value; for instance, photosynthesis is best effected 

 when there are no glucosides in the leaf and it is checked 

 when they are too abundant. Heat, on the contrary, is indis- 

 pensable, but it can be restrictive in excess; there is an 

 optimum for it, as well as a maximum when it becomes 

 inhibitory or fatal, and a minimum when it becomes Hmiting. 

 Also the optimum value of a factor may vary and have its 

 limits displaced according to the quantity of some other 

 factor, since these factors are not independent of one another. 



The measurement of their influence will therefore be 

 rather delicate, not only because they are interdependent, but 

 also because, durinp the time of the measurement, the plant is 

 actively developing. It is useless to keep the external con- 

 ditions constant if the internal conditions are changing and 

 being modified ; glucosides, for example, may accumulate and 

 their accumulation will change the optimum value of another 

 factor. 



Moreover, how is the influence of these factors to be 

 measured if not by making several measurements of photo- 

 synthesis before, during and after the variations introduced? 

 Optimum conditions as estimated by the maximum of carbon 

 dioxide assimilated, for example, may be measured. But here 

 a new difficulty arises. This factor greatly promotes photo- 

 synthesis in the first instants, but, too near its maximum, its 

 restraining or even injurious effect is soon manifest and the 

 rate of assimilation diminishes more or less rapidly until 

 sometimes it becomes very low indeed. 



If the measurement is made only for the first few moments, 

 the result will seem amazingly good, while it may prove 

 disastrous if the experiment is continued for several hours. 

 So many precautions have to be taken that it is not surprising 

 to find very different figures given by^ different investigators 

 for the same factor. 



Liebig, one of the first physiologists to approach these 



