98 



Relation of Cement Dust to Citrus Vegetation 



TABLE 3 



Data pertatni»g to the aniount of Hght cut off by cement-dust on orange leaves 



any injury that might be caused by the presence of cement-dust. 

 Investigation showed that approximately 90 percent of the stomata 

 were on the under side of the leaf, which remained free from dust. 

 Very httle interference could, therefore, occur in the gaseous inter- 

 change between the leaf and the air. If, however, dust excludes so 

 much light from the leaf that photosynthesis is impaired, it should 

 be possible to ascertain the extent of this injury by determining the 

 difference between the total amounts of carbohydrates formed. 



Some preliminary work was done on the starch content, by the 

 well known Sachs method. Leaves which, in the morning, had been 

 freed from dust on one side of the midrib, were cut off in the even- 

 ing, and the halves treated separately, The amount of starch was so 

 great in each half that no satis facto ry conclusion could be drawn 

 as to any possible difference, either macroscopically or micro- 

 scopically. 



After a number of attempts this method was discarded in favor 

 of chemical ones. After cleaning the leaves as before, they were 

 examined according to the methods used by Brown and Morris^ in 

 their study of the chemistry of leaves. A composite of 25 or 30 

 leaves was taken, the midrib having been discarded. The leaves were 

 thoroughly dried the same evening they were cut, ground to pass 

 through a 0.25 mm. sieve and extracted with ether. The sugars were 

 extracted with alcohol and determined gravimetrically by the Fehl- 

 ing method. The residue from the alcoholic extract was " inverted " 

 by saliva and determined in the same way. Table 4 shows the 

 results obtained, as expressed in percentage of absolute dry weight. 



While the figures show great irregularities, the averages are in 



2 Brown and Morris: The chemistry and physiology of foliage leaves. 

 Journal Chemical Society, 1893, 63, 604-677. 



