5: DISEASES OF TREES 



amongst the cell-contents, as, for instance, in the case of attack 

 by PhytophtJiora omnivora, it also not unfrequently occurs that 

 starch resists the destructive influence of wood-parasites longer 

 than the thick lignified walls of the cells in which it is contained. 

 In fact the manner of decomposition -of the starch-grains varies 

 exceedingly according to the species of fungus that attacks them. 

 Similarly as regards the cell-walls. The solvent action of living 

 hyphae is manifested in two distinct ways. Where a hypha 

 touches a cell-wall it dissolves the particles of calcium oxalate 

 contained therein, exactly as a root-hair, by means of the 

 acid solution which it exudes, dissolves the particles of cal- 

 cium carbonate with which it comes into immediate contact 

 This action is confined to the surface of the cell-wall which is 

 actually in contact with the fungus-filament. But even* para- 

 sitic fungus that lives in the wood of growing trees destroys 

 the wood in a manner peculiar to itself. When one and 

 the same species of fungus, eg. Polyporus sulpJiureiis^ vegetates 

 in trees of such different species as oak, willow, and larch, it 

 changes the wood so peculiarly in a short time that at first 

 sight it is difficult to distinguish these timbers from each other, 

 although, in a sound state, they are so strikingly different This 

 can be satisfactorily accounted for only by assuming that each 

 species of fungus exudes an extremely powerful and charac- 

 teristic ferment, which permeates the walls for long distances, and, 

 to begin with, frequently dissolves only the incrusting substances, 

 more especially the lignin. 



In the accompanying figure (10) the upper part of the wall is still 

 lignified, whereas the lower part consists of pure cellulose. After 

 the removal of the lignin the middle lamella, which is most ligni- 

 fied, is the first to disappear, the result being that the various 

 organs become completely isolated, as happens when sound wood 

 is treated with potassium chlorate and nitric acid. The hyphas 

 which pierce the walls with their apices disappear later on. when 

 they themselves are dissolved by the ferment In Fig. n is 

 shown how the elements of the wood of the oak have been com- 

 pletely isolated and dissolved by the action of a ferment 



In the case of other wood-parasites the decomposition takes 

 place in the following manner. By the extraction of the incrust- 

 ing substances a zone bordering the lumen is first converted into 



