Parasitism in the Erysiphaceae. 
57 
characters in a plant. As a rule each species of Browns shows 
physiological characters which hold good for all examples of the 
species derived from different localities. But there are exceptions 
to this rule, and we find that the inter-relations of the “ biologic 
forms ” of the fungus with certain of their host-plants become 
complicated by the existence of “ biologic forms ” of the host- 
plants. Such a case is represented in the adjacent diagram 
(Fig. 63). 
B. arducnnensis, &c., while B. mollis is immune against their attacks. 
Here we have represented the relations which exist between 
four forms of the fungus (viz. the Oidium on B. arduennensis, B. 
commutatus, B. secalinus, and B. adoensis), and the species B. 
mollis and a plant called B. “ hordeaceus." x The latter plant has 
been grown at the Cambridge Botanic Gardens from seed originally 
obtained from the St. Petersburg Botanic Gardens, and is identical 
morphologically with B. mollis. Now let us observe the different 
1 In this and in the succeeding diagrams the number of inocu¬ 
lations made and their results are expressed in the form of 
a fraction, in which the numerator expresses the number 
of times in wdiich infection resulted, and the denomi¬ 
nator the number of times a leaf was inoculated. 
