547 



At the same time similar tests were made to determine the effect 

 of smut growth on scale-infested trees, and it was found that 

 here again the black covering was not reducing in the least 

 the amount of carbon assimilation. It appears from this latter 

 that the real injury to the orange tree is not from the smut 

 (except as it aft'ects the salability of the fruit), but from the 

 sucking out of sap by the scale insects. 



As a check on the foregoing tests a great many microscopic 

 examinations of leaf sections have been made, as previously 

 explained. Leaves in the Riverside district coated by the falling 

 cement dust were partially cleaned and left on the trees for 

 several days. Then sections were cut from the coated and the 

 clean portions of the leaf and stained with iodine. In no case, 

 among the many hundred sections made, was there any indi- 

 cations of there being substantiall_\- more starch on either of 

 the two sides or portions of the leaf. Alany leaves at different 

 seasons of the year and on different kinds of days, cloudy 

 and sunny, have been examined and no differences have been 

 discovered. Peirce * asserts that leaves partially cleaned, as I 

 have explained above, and sectioned and stained with iodine, 

 show four or five times as much starch in the cells of the clean 

 side as of the coated side. Certainly nothing in the tests re- 

 ported above would bear Peirce out in this statement even to 

 a slight degree. Peirce used hand sections decolorized and 

 stained in iodine, and it is (juite probable that there is a source 

 of error, for it is obvious that a slightly thicker section would 

 appear to have more starch in it than a thinner one ; and 

 furthermore, in a freehand section the chloroplastids are cjuite 

 apt to fall out and leave the cells empty, and thus increase the 

 effect of starch shortage. 



In the same way a great many sections have been made of 

 leaves partially coated with lampblack, thickly. These examina- 

 tions confirm the conclusions derived from the comparison of 

 dry weights of discs cut from blackened and clean parts of 

 leaves, for in no case did there appear to be any less starch 

 in the coated portion than in the clean. In fact, not a few 



* Peirce, C. J., Plant World, Vol. 13, p. 286, 1910. 



