PLANTS AS DISEASE PRODUCERS 



311 



Stubs were left which never healed over and through the exposed sur- 

 face the fungi of wood decay gained easy access. 



The injuries produced by meteorologic causes are important. 

 Entire forests have been levelled by tornadoes. Cracks are produced 

 by wind action. Lightning opens a way by cracks to the interior. 

 Snow and ice snap off large limbs and hail stones bruise the bark and 

 leaves of trees so that fungi can readily enter. Chemic substances are 

 rather exceptional destructive agents to which reference has been called 



26. — Black walnut, Juglans nigra. Cold Spring Harbor, L. I. Note large 

 open-branch stub (July, 1914). 



in a previous page. Besides these agents, it occasionally happens, that 

 fungi enter healthy plants through diseased grafts which are inserted. 

 Robert Hartig mentions such a graft union of diseased and healthy 

 roots in the case of the red-rot fungus, Trameies radiciperda. Here 

 contact of the diseased root containing the fungus with the sound one 

 of a neighboring tree and the partial natural graft union of these two 

 roots explains how such infection occurs. An enumeration of the 

 way in which fungi can gain entrance to plants follows: 



