549 



Discussion of Data. 



It is very obvious from the foregoing tests that a surface 

 coating does not affect leaf functions in the same way that a 

 screen does, for in the latter case most or all of the heat energy 

 of the light is excluded at the same time that the illumination 

 is reduced. It must appear from the foregoing data that the 

 presence of a dust coat or even of a black coating, as lamp- 

 black or a smut growth, is not injurious to the leaf in so far 

 as the function of carbon assimilation is concerned. 



Orange leaves coated for two or three years are just as 

 bright and green when the dust is removed as a normally clean 

 leaf of the same age is. Qilorophyll solutions of equal area 

 of tissue of cleaned and of coated leaves have been repeatedly 

 compared and no diminution of chlorophyll has been detected 

 by the writer in leaves coated with dust. Similar tests of 

 leaves coated with lampblack for four or five months have been 

 made with the same results. These latter leaves appeared as 

 normal and thrifty and green on the black-coated half as on 

 the other, although the coat had been applied for four months. 



This has an important bearing on several problems in horti- 

 cultural science. First, spraying trees does not impair the func- 

 tion of food manufacture in the leaves, provided the spray is 

 not caustic. Second, dust coats on leaves, so long as the 

 stomata are not clogged, do not impair the food-making 

 functions, assertions to the contrary notwithstanding. Third, 

 fungus smut on scale-infested trees does not reduce carbon 

 assimilation. Two injuries may be discovered in such trees. 

 The scale insects themselves are sucking out a very large 

 amount of sap and starving the trees ; and, second, the black 

 coat may increase transpiration to a dangerous degree. 



It appears from certain of these tests that more radiant 

 energy is absorbed by a coated leaf, and hence, the internal 

 temperature would be increased. No attempt has been made in 

 these experiments to determine the leaf temperature, but it 

 seems probable that any increased temperature would be equal- 

 ized by the increase of transpiration, so that the actual tempera- 

 ture would be about the same as in an uncoated leaf, but the 

 number of heat units in the leaf would be much greater. 



