H. D. Young 



97 



was first studied. Clear glass plates about 2" xß" were placed in a 

 tray and covered with water having cement-dust in Suspension. 

 After allowing the dust to settle for 24 hr., the plates were carefuUy 

 removed and dried. Measurements were taken, with a Luinmer- 

 Brodhun photometer, of the amount of Hght excluded. The plates 

 were then cleaned and the dust weighed. The data in this con- 

 nection are summarized in Table 2. 



TABLE 2 



Data pertaining to the amount of Hght cut off by a given amount of cement-dust 



on glass-plates 



From the figures previously given it will be seen that numbers 

 7 and 8 (in Table 2) have almost exactly the same weight of dust, 

 per sq. cm., as the more heavily covered leaves. It is very probable, 

 however, that the amount of light excluded by the dust-covered 

 glass-plates is somwhat in excess of the amount excluded by the 

 dust-covering of the leaves in the field, since the dust deposited 

 from Suspension in water is probably more coherent than that de- 

 posited in a natural way. The degree of exclusion of light by the 

 natural dust-coating is very difficult to determine directly, since the 

 Opacity of the leaf itself and its coloration interfere. Considerable 

 practice in such determinations, however, minimizes the importance 

 of these sources of error. 



A series of determinations, using the leaves themselves, was 

 made. A number of rather large, very heavily coated, leaves were 

 taken, and the percent of light transmitted, before and after clean- 

 ing was determined, the difTerence being the amount of exclusion 

 due to the dust. See Table 3. 



Effect of shading on carbohydrate synthesis. Carbo- 

 hydrate synthesis in the leaf seemed to offer the easiest measure of 



