.\n Effect of CEMnxT Dust ox Orange Trees. 285 



green and glossy, Init Uial they are gray and dull. The green 

 is concealed. On the other side of the train one sees a cement 

 works from which, by night as well as by day, a cloud of dust is 

 rising. This cloud drifts in one direction or another, according 

 to the pri'vaih'ng wind, the greatest amount of dust faliiag from 

 this cloud nearest the works, the finest particles being carried 

 farthest awav. Further along the same line of railroad, the 

 foliage in the orange groves is fair and green. The contrast be- 

 tween the uncovered, undusted foliage of orange, lemon, and 

 grape-fruit, and that which is coated with a crust, much of which 

 is composed of cement, is very great. 



The particles of dust falling on the smooth and glossy upper 

 surface of the shiny leaves of these citrus fruit trees, accumulate 

 after a time into a layer of considerable thickness; these particles 

 cohere and set in dew or fog or gentle rain, adhering to the sur- 

 face of the leaf in rather a surprising degree. In fact, the adhesion 

 is often so great that it is not possible to remove the crust com- 

 pletely from the upper surface of even such smooth leaves as 

 these. I have repeatedly tried to remove the crust completely 

 from the thickly coated leaf, and have rarely been able to do so 

 without damage to the leaf itself. 



Even a thin crust of cement is fairly opaque. To prove 

 this, it is only necessary to rub off as completely as possible, 

 the cement from one half of an orange leaf and then to hold the 

 leaf up to the sun. One will at once notice how plainly the oil 

 glands show' in the clean half of the leaf, and how indistinctly 

 they can be seen, if seen at all, in the coated half. Such an opa- 

 que crust interposed as a screen over the upper surface of the 

 leaf can not fail to reduce the amount of light entering the leaf. 

 When one realizes that the manufacture of a non-nitrogenous 

 food by green plants is dependent upon the absorption of energy 

 from sunlight, one can realize also the possibility, if not the prob- 

 ability, of the interference of such a crust with the manufacture 

 of sugar in the leaves. If the manufacture of non-nitrogenous 

 food is interfered with, all the other activities of the plant are 

 correspondingly affected; for upon the manufacture of non- 

 nitrogenous food depends the construction of cell-wall, the sup- 

 ply of material to which nitrogen is added in the manufacture 

 of proteid, and in the further elaboration of the complex constit- 

 uents of protoplasm. Interference, then, with the manufacture 



