144 FOREST PROTECTION 



An anatomic — microscopic proof of injury due to SOj or SOs can- 

 not be given (Haselhoff and Lindau, p. 93 and p. 37). 



A number of injurious influences (frost, heat, desiccation of soil, 

 insects, fungi (Schroeder and Reuss, p. 110) fire, etc.) bring 

 about, within the leaves and needles, identical or similar al- 

 terations of the cell-structure (Haselhoff and Lindau, p. 12 ff). 



The consensus of opinion, amongst scientific specialists (R. Har- 

 tig, p. 6; Winkler, p. 379; Schroeder and Reuss, p. 126) is to 

 the effect that excessive contents of SOs within the leaves 

 are not necessarily injurious. 



Injury due to sulphurfumes can be assumed only when there are 

 at hand 



A. death visible to the naked eye; 



B. no other plausible cause of such death; 



C. contents of SOj in the leaves which are unmistak- 



ably increased by the reaction of the leaves 

 and needles on sulphur fumes. 



Unmistakably increased contents of SOi 

 proven chemically within the leaves are 



a. not identical with abnormal con- 



tents; 



b. not such contents as exceed the av- 



erage contents of leaves within 

 territories acknowledged to be 

 beyond the reach of sulphur 

 fumes; in other words, 



c. not particularly high percentages of 



SO 3 found within the leaves. 

 General averages holding good 

 for the contents of SOj within 

 the leaves of healthy trees do 

 not exist (Haselhoff and L ndau, 

 p. 67). 

 If the contents of SOj found within the in- 

 jured or uninjured leaves and needles of 

 a given tree exceed those obtained by 

 averaging a large number of analytic re- 

 sults obtained from the tests of healthy 

 leaves and needles, then and in such case 

 ^ the excess is frequently due to any one, 



or to a combination of the following 

 causes : 



(a) Soil: A soil naturally rich in 

 SO 8 or irrigated with water 

 containing S0», produces 



