CHLOROPHYLL AND ENERGY 195 



is no more rapid than in the middle of the night? Photo- 

 synthesis is then much more predominant than respiration, 

 but it does not give off more than ten times the quantity of 

 oxygen absorbed by respiration. 



The true photosynthesis is measured if everything which 

 does not proceed from it is eliminated, while taking only the 

 final result gives the apparent photosynthesis, whatever the 

 factors may be that have entered into it. 



All the methods used measure the apparent photosynthesis, 

 either by the increase of dry weight, by the oxygen given off or 

 the carbon dioxide absorbed. 



Measurement of the Dry Weight 



Sachs inaugurated the technique of measuring dry weight. 

 To find out what a leaf can absorb in a day, he weighed it in 

 the morning and in the evening, or, more exactly, he dried 

 and weighed half of a leaf in the morning. The other similar 

 half passed the day in the sun and in the evening was dried 

 and weighed. The difference in weight enabled the balance of 

 assimilation and the quantity of glucosides synthesized per 

 unit area to be determined. 



The method has been modified by various investigators 

 who have used fragments of a leaf, but a simpler and more 

 reUable method consists in making the measurements on a 

 tree with pinnate leaves like the ash; one foHole is taken in 

 the morning and the opposite one in the evening. 



The principal defect of this method is that it fails to take 

 into account the transport of the glucosides which continues 

 throughout the day with a speed and rhythm which are almost 

 unknown. Moreover, it is still difficult to estimate on the 

 whole of the leaf the repercussions of this traumatism which 

 probably modifies the respiration and even the assimilation. 



These objections are less important when the plant is not 

 dissected and a long-term measurement is made by estimating 

 the result of photosynthesis according, to the total dry weight. 

 But who knows what the plant has expended and what dry 

 weight would truly represent all that it has assimilated? The 



