545 



Another means of comparing the quantity of carbon fixation 

 in coated and uncoated leaves was tried. One-half of the sur- 

 face of many leaves was cleaned, as before, and the leaves 

 allowed to remain on the tree for several days. At about the 

 middle of the afternoon these leaves were cut from the tree, 

 quickly brought into the laboratory and carefully cleaned, and 

 then a number of discs were cut from each half. These were 

 dried thoroughly and their dry weights compared. As a check 

 on this method, I first cut a number of ordinary orange leaves, 

 cleaned them carefully, and from each side of the midrib of 

 each leaf cut an equal number of discs. The two lots of discs 

 taken from opposite halves of each leaf were then dried and 

 weighed. The dry weights of these two lots of discs were 

 almost identical, showing that when a considerable number of 

 leaves are used the dry weight is practically the same on each 

 side of the midrib, per unit area. A number of tests were then 

 made to determine any difference which might exist between 

 the clean and coated halves of leaves. This does not determine 

 the quantity of carbohydrates made nor does it do more than 

 simply give a comparison between the two. If unit areas on 

 each side of the leaf normally weigh the same, but when one 

 side is coated with a dust film it should be found that the dry 

 weight per unit area diminishes, it mig-ht then be assumed that 

 less carbon was being fixed beneath the coating on the surface. 

 If, however, the dry weight of the coated side remains prac- 

 tically the same as that of the other side, then we may conclude 

 that it corroborates the data of the tests just preceding. This 

 is the case, as the following tables show. The differences in 

 dry weights of the two halves are practically identical, the 

 slight differences being probably only experimental error. This 

 method, alone, of determining carbon assimilation is wholly 

 inadequate, but as a corroborative test with that in which the 

 hourly rate of fixation was made, it is quite worth while. 



