1915] 



BURT THELEPHORACEAB OF NORTH AMERICA. IV 639 



Winter^ wrote of Exohasidium Vaccinii in Europe where 

 there is a similar confusion as to species, **der Pilz erzeugt 

 ausnahmslos Formanderungen der verschiedensten Art an 



den von ihm bewohnten Pflanzentheilen Ich finde 



zwischen den einzelnen verschiedene Nahrpflanzen bewohnen- 

 den Formen keine wesentlichen Unterschiede.'' 



The specimens which I have studied show that we have in 

 North America perhaps three species of Exohasidium, two of 

 which are rare and are present in herbaria in so few speci- 

 mens that present conclusions concerning them are somewhat 

 tentative. These species are as follows : 



1. E. Vaccinii (Fuck.) Wor. 



This species is common and wide-spread and is parasitic 

 on many ericaceous host plants. There is as yet no evidence 

 of which I am aware tending to show that so-called physio- 

 logical races or forms with parasitism limited to a particular 

 host exist in this species. This fungus attacks leaves develop- 

 ing leafy shoots, and flowers of susceptible plants, making its 

 most successful infections when these organs are very young. 

 The vegetative hyphae live in the infected organs between 

 the cells, which are stimulated by the presence and activities 

 of the parasitic hyphae to make a more or less marked hyper- 

 trophic growth response, termed a gall. The galls are of 

 varied and sometimes strange form according to the host, the 

 organ, and its age. The distribution of the galls upon the 

 host is dependent upon the susceptibility of its various organs 

 to infection. 



In fruiting, the hyphae push through the epidermis to the 

 surface and produce there a resupinate fructification which is 

 amphigenous in the case of galls from tissues so young that 

 they form galls of wax-like or coralloid structure, and 

 hypophyllous on the more common leaf galls. The fructification 

 is variable in thickness, consisting sometimes of scattered 

 clusters of basidia but usually with hyphae present in vari- 

 able quantity between the basidia so that the fructification may 

 attain a maximum thickness of 60-70 /i, as in the case of col- 



^In Rabenhorst, Krypt. Flora 1^:322. 1884. 



