BIOLOGIC FORMS 



153 



conidia were now sown on B. commutatus, and proved 

 capable of infecting this species. That is to say, B. hor- 

 deaceus here acted as a 'bridge,' enabling the fungus on 

 B. racemosus to pass over to B. commutatus, a species 

 which is safe against its direct attack. 



FIG. 45. 



Thus we see that in certain cases the infection-powers 

 of a biologic form may be considerably influenced by the 

 effect of its residence on a new host-plant. 



This leads us to the interesting question whether the 

 immunity shown by certain host-plants to certain biologic 



