CHLOROPHYLL AND ENERGY 205 



not been possible to show any retarding effect resulting from 

 its accumulation. Although an excess does not appear to be 

 an impediment, a minute quantity, on the contrary, seems to 

 be necessary at the beginning. This necessity has, however, 

 been denied and Gaffron was able to start photosynthesis in 

 an alga in the absence of any trace of oxygen. To ensure 

 that the plant is deprived of oxygen, it has to be kept for a 

 certain time in the shade without this element. Very probably 

 Ufe without oxygen, and consequently without respiration, is 

 unhealthy for the plant and certain internal reductions may 

 be performed in an endeavour to supply the deficiency; a 

 reoxidation would be necessary before activity could be 

 resumed. Reversible oxidations of chlorophyll have been 

 observed and this necessary oxygen might be used for that 

 purpose. 



Among the mineral elements, all those which take part in 

 the synthesis of chlorophyll have a proportional importance 

 for photosynthesis, but this influence is indirect. Certain 

 observations, however, have shown that the presence of 

 potassium considerably increases the rate of assimilation. 

 This effect, which is not explained, might proceed from a 

 better exportation of the glucosides. 



Efficiency 



It is difficult to calculate precisely the efficiency of photo- 

 synthesis. The highest figures obtained from the method 

 previously explained of weighing half a leaf in the morning 

 and the other half in the afternoon are in the neighbourhood 

 of 1 gramme of dry matter per square metre of leaves in one 

 hour. 



This weight of dry matter mostly represents glucosides; 

 it is easy to conclude that 1-54 grammes of carbon dioxide 

 have been fixed and transformed into glucosides in each 

 square metre per hour. 



Much higher figures were pubHshed by Willstater and Stoll, 

 who, for a sunflower leaf subjected to an illumination of 

 48,000 lux, at a temperature of 25° C, observed an increase, 



