THE EFFECT OF SOOT ON VEGETATION. 23 



amounts of soot used did in large measure compensate for 

 the shorter periods of exposure. In the determined soot 

 fall, computed for one year, it was found that the tar and 

 "fixed carbon" amounted to 10.8 gm. per square meter in 

 one instance and 11.52 gm. per square meter in the other. 

 In the experiments tar and "fixed carbon" were used in 

 amounts varying from .21 gm. to 17. gm. per square meter. 

 The amounts of soot generally used in the experiments 

 were therefore atxmt equivalent to an entire year's soot 

 fall, and in consequence may be regarded as more toxic 

 because ( 1 ) under experimental conditions the soot falling 

 on the plants was more likely to remain, wind and rain 

 being absent, and because (2) the amount used fell on the 

 plants in a much shorter period of time. In each instance 

 the plants were completely covered with soot during the 

 experiments. Therefore, it appears that so far as the 

 amount of soot is concerned, the obtaining of equivalent 

 conditions has been approximately secured. 



It may be maintained that the injury is purely or 

 partly a mechanical one brought about by the plugging 

 of the stomata, the plant consequently suffering from the 

 suppression of the free exchange of gases. But inasmuch 

 as a very small percentage of the stomata in the leaves 

 collected in the field and from the plants experimented 

 with in the nursery were found to be clogged, this opinion 

 finds little or no support. Nor did soots of varying per- 

 centages of tar and "fixed carbon" display any differences 

 in this respect. 



In localities where smoke and gases are a nuisance 

 there is a general though vague impression that soot and 

 tars are toxic or otherwise injurious to plants. If this 

 be the case, the writer would have expected to have ob- 

 tained much more pronounced results in the above experi- 

 ments, that is to say with solids and semi-solids. The ques- 

 tion of the effect of gases has not been considered in this 

 paper. 



