Minnesota Plant Diseases. 



57 



though some weak points in the plant organization may be re- 

 sponsible for the successful attack of the parasite, as has been 

 experimentally proven within the last year. In other words, the 

 most successful parasitic fungi are those which can stimulate 

 the affected parts of host plants to extraordinary effort, or at 

 least do not immediately injure those parts. 



FIG. 26. "Birds-nest" witches'-broom on red cedar caused by the birds-nest rust fungus 

 (Gymnosporangium nidus-avis). The bush-like broom stands vertical on the horizontal 

 branch of the host. The difference between the diseased and healthy leaves is very 

 marked. The former are very similar to those of the common juniper. Original. 



Degrees of proficiency in parasitism. The simplest modes 

 of parasitism are undoubtedly to be met with in those half- 

 saprophytes which are just learning the methods of parasites. 

 Some of these have already been described in certain mold fruit- 

 rots. Such a beginner can only obtain its food from living 



