729 



from the neighborhood of a copper mill likewise injured by SOo. The com- 

 mon characteristic consists of more or less sharply defined brown specks in 

 the intercostal fields. The spots are usually surrounded by a brown zone 

 which may vary in, tone. In many trees (for example, the red beech) a 

 transparent yellowish green band of diseased but not dead tissue is found 

 around this peripheral zone. 



Figures 165, 166 and 167 illustrate leaves from a rose plant, a beech 

 and a birch, which have been artificially injured by hydrochloric acid. They 

 have the dry periphery, which may usually be observed after the action of 

 pure chlorin vapor. Nevertheless, it should be emphasized that in testing 

 smoke effect no definite conclusion may be drawn from such structural 

 pictures showing the habit of growth, because, on the one hand, the forms 

 of injury vary according to 

 the individual habitat and de- 

 velopment of the tree and, on 

 the other, dift'erent factors 

 may produce similar injuries. 



Hydrofluoric Acid. 



More often than was for- 

 merly supposed, hydrofluoric 

 acid produced by the opera- 

 tion of superphosphate, glass 

 and chemical factories has 

 proved injurious to vegeta- 

 tion. The fact, at first so 

 puzzling, that smoke from 

 kilns and terra cotta factories 

 is very injurious in many 

 cases and in others non-inju- 

 rious has been explained by 

 this action of the acid. The 

 difference in effect depends 



upon the presence and amount of fluorin compounds to be found in the clay 

 and raw phosphates. According to Ost, action manifests itself in small, 

 brown, corroded spots which in many plants are surrounded by a yellowish 

 zone. Smoke experiments carried on by other investigators produced in oak 

 leaves narrow, yellowish brown, sharply defined peripheral discolorations. 

 The Norway maple showed similar tracery along the edges of the leaves and 

 the leaf surface and later also turned brown. Lindau^ describes the ana- 

 tomical condition in the oak. He found both of the epidermal layers to be 

 intact and the contents of the mesophyll cells slightly browned. The indi- 

 vidual chloroplasts were still recognizable, "but the rest of the cell contents 

 had an oily appearance." 



ch leaves injured by hydrochloric 

 n finne.s. (After v. Schriider and 

 Reuss.) 



1 Loc. cit., p. 250. 



