546 



Comparison of Dry Weight per Square Meter of Leaf 

 Tissue of Clean and Coated Leaves. 



Average : 



Check: — 114 discs cut from each side of midrib of 40 

 normal leaves: — Lot 1, 101.316 grams 



Lot 2, 101.623 " 



Difference — 0.307 grams 



It appears very certain from these data that the presence of 

 a coating- of cement dust is not diminishing in the least the 

 rate of carbon assimilation. The differences in weights of 

 coated and clean parts of the leaves by both methods of test- 

 ing, above outlined, are very small and to be accounted for 

 simply as experimental error. These same tests were repeated 

 in Claremont, using leaves which were heavily coated on one- 

 half of the upper surface with a paste made of cement dust. 

 This coating was much thicker than that found on the leaves 

 in the dustfall zone about the cement plant. By weighing discs 

 cut in the early morning and in the afternoon, as before, it 

 was found that even this thick coating did not reduce the 

 amount of carbon assimilation in the least. The average of 

 several tests made in Claremont shows a difference in favor of 

 the clean side of the leaves of 0.047 grams. 



It seemed evident from the latter tests that not much illu- 

 mination could enter the leaf through such a thick coating, and 

 this led to making similar tests with orange leaves coated on 

 half of the upper surface with lampblack paste. Obviously this 

 would intercept practically all the luminous rays of sunlight. 

 Proceeding as before, it was determined by the weighing of 

 many discs that fixation of carbon progressed very normally 

 beneath this heavy coating of lampblack, the gain in dry matter 

 being quite equal to that of the unblackened half of the leaves. 



