MEASURING PHOTOSYNTHETIC ACTIVITY 255 



to obtain sufficiently large increases in weight it is necessary that the 

 leaves be illuminated at least one to two hours. It is impossible to establish 

 any definite rates of respiration or translocation of materials at the same 

 time. These two factors cannot be disregarded. 



To take the difference in dry weight between leaves removed in the 

 morning and at sunset as a measure of the amount of photosynthetic work 

 done is obviously erroneous. The gain in dry weight of a leaf during a 

 period of time is no more a measure of the amount of synthetic work 

 done than the amount of money which a manufacturer deposits in a bank 

 at the end of a day is a measure of the amount of work which has been 

 accomplished in his plant. We shall probably have a true measure of 

 photosynthesis only when we are capable of making a determination of 

 the total energy changes taking place in the plant. 



