544 



remainder left clean. Narrow strips were cut from these 

 leaves in the afternoon and after fixation in chrome-acetic 

 solution and infiltration with paraffine, they were sectioned with 

 a microtome. Sections were in all cases made 10 micro-mm. 

 thick. They were stained in iodine dissolved in xylol, mounted 

 in balsam, and as soon as possible a typical spot was photo- 

 graphed. Study of photographing of these sections must be 

 done quickly, for the iodine is soluble in balsam and the color 

 soon fades out. By this method one can gain a good idea of 

 the comparative amounts of starch in sections, but by no 

 means is it a quantitative test. Unless the sections are of uni- 

 form thickness the comparison is valueless, for it is obvious 

 that the mass color efifect of the stained starch in two layers 

 of cells w^ould be much greater than that of one laA-er of cells. 

 A great many leaves were sectioned and photographed in this 

 manner. 



Increased Dry ^Matter. 



In determining the efifect of a cement dust coating on orange 

 leaves the studies were made in two localities — in the region 

 about the Riverside Portland Cement Company's plant where 

 the leaves were coated by dust blown into the atmosphere, and 

 also in Claremont. In the latter place leaves were partially 

 covered with a paste made from "treator dust," which is the 

 same as that blown into the atmosphere at the cement plant. 

 In the Riverside region the procedure was as follows : Alany 

 of the most heavily coated leaves were very carefully cleaned 

 with a dampened cloth on one-half of the surface. Usually 

 the midrib was taken as the dividing line, but in some series 

 the basal or apical half was cleaned. Thus cleaned, the leaves 

 were allowed to remain on the tree for several days, and then 

 discs were cut out before daybreak and again in the afternoon, 

 as above set forth. Records were kept of the average tem- 

 peratures during the experiments and of the conditions of the 

 sky, weather, etc., whether bright or cloudy. These experi- 

 ments were repeated on several days throughout the late sum- 

 mer, fall, winter and spring. 



