CHAP, xiii.] Fungoid Diseases of Trees 279 



mechanically by animals or other agencies (as in the case of 

 Phytophthora omnivora on seedlings, new outbreaks of Tra- 

 metes radiciperda at the base of the stem, and the rust on corn 

 occasioned by species of Pucdnia] \ 



The consequences of such infection with fungal hyphae are 

 interference with the normal transpiration and the decompo- 

 sition of the carbonic acid in the part infected, gradual destruc- 

 tion of the cells and consumption of the cellular substance 

 and contents, with chemical disturbance and morphological 

 alteration, generally accompanied by more or less hypertrophy 

 or morbid enlargement, and resulting finally in death. In 

 consequence of the sickly state of the plant, insects are often 

 attracted and assist in the work of destruction. From the 

 mycelium at length proceed the various forms of spore-bearing 

 organs characteristic of the different kinds of fungi, breaking out 

 and appearing sometimes on leaves, twigs, or on the surface of 

 the bark, at old branch-holes, or even from holes through which 

 bark-beetles have previously made their exit from the stem. 

 Millions of reproductive spores are produced, and the cyclus 

 of generation begins anew. Some fungal species of sylvi- 

 cultural importance deviate from this normal type in having 

 frugiferous organs (Rhizomorpha) without any fructification of 

 the mycelium (e.g. in Agaricus melleus, a species closely allied 

 to the common mushroom). 



Many fungi complete their life-cycle within a few weeks or 

 months, whilst in other species the spores hibernate, and in 

 others again the mycelium retains its vital energy for two, 

 three, or more years ; most of the species occasioning diseases 

 of forest trees belong to this last-named category. 



The polymorphic tendency of many fungi, or the change of 



1 Smut on wheat is occasioned by Pucdnia graminis, which in its aecidial 

 form, occurs as Aecidium berberidis on the leaves of the Barberry. This 

 shrub should therefore be treated as a very noxious weed in all hedges, &c., 

 throughout corn-growing districts in which rusts and smuts are in the 

 slightest degree prevalent. 



