24 THE SMOKE INVESTIGATION 



Conclusions 



1. It is a matter of general opinion that smoke from 

 mills injures nearby vegetation. That this opinion is jus- 

 tified is evidenced not only by the general external appear- 

 ance of many of the constituent plants, but also by their 

 internal appearance, as shown by the size of annual rings 

 and by lesions in leaves. 



2. The experiments described in this paper show that, 

 especially in the case of pines, when soot is applied in 

 small quantities over a considerable interval of time, meas- 

 urable injury follows. Soot is therefore poisonous to 

 vegetation. 



3. Soot is a mixture of pure, finely divided carbon, 

 ash, tar and gases. Since, however, pure carbon can not 

 be regarded as poisonous, its effect on plants, if indeed it 

 has any, must be charged to the mechanical clogging 

 of stomata, or to the ash, which may, in the presence 

 of water (rain) provide solutions injurious to leaves. 

 Aside from this latter possibility, which has not been in- 

 vestigated in this study, actual observation reveals the 

 fact, that only a small percentage of the stomata were 

 clogged, from which it is inferred that little or no inter- 

 ference with interchange of gases occurs. It is true that 

 a certain number had the appearance of being clogged, 

 nevertheless it is doubtful, in the opinion of the writer, 

 that there was an efficient hindrance to the diffusion of 

 gases in view of the studies of Brown and Escombe (1) 

 and Lloyd ( 12) . It is of course obvious from results of such 

 studies that the effect of the interference by small particles 

 on diffusion through a minute opening can not be inferred 

 but must be made the subject of specific investigation. 



4. The injury done by the soot of smoke to vegeta- 

 tion, is therefore, probably due chiefly, if not entirely, to 

 the accompaning ash, tar and gases. 



