700 PROTECTION AOAIXST BISKASES. 



first ground for determination is whether the damage to 

 exposed woods is caused solely by the fumes or is due to other 

 causes (frost, heat, dry winds, infection, or fungi). 



Tiiere are only two methods for determining the cause of 

 the damage : — 



(a) Chemical analysis of the injured tree-parts (leaves, 

 flowers, etc.). 



(h) Microscopic examination of the marks on the injured 

 needles or leaves. 



On this subject there has been since 1895 a keen literary 

 dispute, in which Borggreve, Schroder, Hartig, Eamann, Yater 

 and Wieler have contended. Most of these disputants prefer 

 chemical analysis, to determine the amount of sulphuric acid 

 in the ashes of the leaves. This must obvio.usly be done by 

 a chemical expert. The excess of sulphuric acid in affected 

 leaves over the amount in leaves from woods unexposed to 

 fumes decides the question. As, however, the quantity of SO3 

 in perfectly healthy leaves varies much, Yater considers the 

 following conditions necessary to prove damage by fumes : — 



1. All injured and sound trees experimented on must have 

 been growing on similar soil and at various distances from the 

 smelting furnaces. 



2. As a comparison, the average quantity of SO3 in sound 

 trees must not be taken, but sound trees must be found and 

 the quantity in them measured. 



3. A sufficient number of sample trees must be chosen in 

 order that reliable results may be obtained. 



Pi. Hartig prefers the microscopic method, at any rate for 

 spruce. He considers the foxy red colour of the contents and 

 walls of the cells bounding stomata, and, when the fumes are 

 very strong, the red colour of the prosenchymatous bundles, 

 as sufficient proof of the poisoning. Such needles cannot 

 assimilate CO2. They remain apparently healthy for several 

 years on the trees, and die when the woody bundles become 

 affected. 



Piamann and Soraner both disagree with Hartig's statement 

 tliat the change in the contents of the cells points exclusively 

 to damage by fumes, as it may l)e due to damage by other 

 factoi-s. 



