144 FOREST PROTECTION 



An anatomic — microscopic proof of injury due to SO 2 or SO 3 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 Heuss, p. 126) is to 

 the effect that excessive contents of S0 3 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 S0 3 in the leaves which are unmistak- 



ably increased by the reaction of the leaves 

 and needles on sulphur fumes. 



Unmistakably increased contents of SO a 

 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 s found within the leaves. 

 General averages holding good 

 for the contents of SO* within 

 the leaves of healthy trees do 

 not exist (Haselhoff and L ndau, 

 p. 67). 

 If the contents of S0 3 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 » or irrigated with water 

 containing S0«, produces 



