FUNGUS RESISTING PLANTS. 381 
B. hordeaceus, B. velutinus, and B. secalinus, and remain perfectly free 
from the mildew in question; but if plants of B. racemosus already 
infected are placed in their neighbourhood, and spores from these reach 
them, the “immunity” would completely disappear, and the disease be 
virulent. It is very probable that such cases of partial immunity, i.e. 
immunity against all but one of the numerous specialised forms of the 
fungus, which appear inexplicable until the specialisation of parasitism 
shown by the fungus is known, are common among cultivated plants and 
their fungus diseases. 
Now, as a rule—in the case of the fungus we are considering—each 
species of Bromus shows “ constitutional ’’ characters which hold good 
for all examples of the species obtained from different localities. But 
there are exceptions to this rule, and we find that the problem becomes 
complicated by the existence of “ biologic forms” of the host-plant: that 
is, forms or races which are morphologically identical, but which differ 
“constitutionally” or physiologically as shown by their different 
behaviour to the same fungus. Such a case is illustrated by Diagram II. 
Gon Con Com Oo 
aiducunensts commulatiis becalinus adoens15 
Dracram II, 
Diagram illustrating the different constitutional characters of the two plants 
B. mollis and Bb. “ hordeaceus,”’ which are morphologically identical. 
B. “hordeaceus”’ is infected by the four forms of H. Graminis on 
B. arduennensis, &e., while B. mollis is immune against their attacks. 
The plant here called hordeacews has been grown at the Cambridge 
Botanic Garden from seed originally received (under this name) from the 
St. Petersburg Botanic Garden. On the plants arriving at maturity, they 
were identified by both Professor Marshall Ward and Dr. Stapf as 
being identical morphologically with B. mollis. Nevertheless this race 
of B. mollis which has been called hordeaceus possesses different 
“constitutional ’’ characters from those shown generally by the species. 
In the comparative inoculation experiments which were made, B. mollis 
proved completely immune against the attacks of the fungus on B. 
arduennensis, B. commutatus, and B. adoénsis, and practically so against 
the fungus on B. secalinus, while B. hordeaceus in every case proved 
susceptible. We must conclude, therefore, that the morphological species 
B. mollis includes two “races,” or sets of individuals, possessing 
