PLANTS AS DISEASE PRODUCERS 



309 



two branches together. Squirrels in search of food bite off the twigs 

 of trees. Deer and moose browse upon the tender branches and bark 

 of various trees, the moose especially, upon Acer pennsyhanicum and 

 Sorbus americana. Grizzly bears rub their backs against the bark of 

 trees and sometimes in this way decorticate them. Rodents peel off 

 the outer protective layers of roots as food, or as material with which to 

 line their burrows. The mycelia of Rhizoctonia, or the oak-root fungus, 



Fig. 124. — Street tree injured by use as a hitching post. (After Sturgis, W. C, Rep- 

 Conn. Agric. Exper. Stat., pi. Hi, 1900.) 



Rosellinia quercina, which live in the soil, penetrate into roots through 

 wounds produced by field mice and gophers. The honey agaric, 

 Armillaria mellea, forms strands of hyphae known as rhizomorphs, 

 which grow through the soil and find an easy entrance into roots 

 decorticated by rodents. Beavers are active agents in cutting down 

 trees and removing the bark therefrom. Woodpeckers drill holes into 

 trees and in their case it has been definitely proved that they carry the 

 viable summer spqres of the chestnut-bligjit fungus, Endothio para- 



