K.—BOTANY 179 
parasitic fungi as regards their host relationships may be contrasted with 
the views of Marshall Ward ®° who believed in their ‘ educability ’ to live 
upon hosts which they did not normally attack. As an outcome of his 
researches on the Brown Rust of brome grasses he believed that certain 
host species and varieties enabled a particular physiologic form of the 
fungus to pass from one group of species of the genus Bromus to another. 
His hypothesis of the existence of ‘ bridging hosts’ has not, however, 
been substantiated by later work, and the evidence now available tends to 
invalidate the conception that fungi can be easily ‘ educated’ to attack 
new hosts. In N. America barley is readily affected by a form of Puccinia 
graminis Tritici which attacks wheat strongly and rye feebly, and also by a 
form of P. graminis Secalis which attacks rye but not wheat. It might be 
thought, therefore, that barley would act as a ‘ bridging host ’ and would 
enable the form of P. graminis Secalis to pass from rye to wheat. Stakman 
and others,*! however, have shewn that even if the form from rye is 
cultivated constantly on barley for many uredospore generations it does 
not become capable of attacking wheat. Since Marshall Ward enunciated 
his hypothesis of ‘ bridging hosts’ the technique for the investigation of 
physiologic forms has been much improved. Nowadays it is considered 
necessary to establish cultures from single spores because of the known 
occurrence in nature of mixtures of forms, sometimes even in the same 
spore pustule, and special precautions are taken to prevent contamination 
ofthe stock cultures. Mr. P. W. Brian has been carrying out at Cambridge 
a re-investigation of the host relationships of the Brown Rust of brome 
grasses by modern methods. The results which he has obtained so far 
do not support Marshall Ward’s conception of ‘ bridging hosts.’ The 
latter’s results appear to be explicable on the basis of the existence of 
more physiologic forms than had then been identified and by the possible 
intermixture of different forms in the spores used for inoculation. Mar- 
shall Ward’s hypothesis of the ‘ educability ’ of parasitic fungi is neverthe- 
less a fascinating one and evidence for it may yet be forthcoming. 
One of the most striking features of hetercecious fungus parasites is the 
contrast between their frequent extreme specialisation to one or a few 
particular hosts during one phase of the life-cycle and their ability to live 
upon entirely unrelated hosts at different stages in the life-cycle. Heter- 
cecism is, of course, acommon feature of parasitism in general, but we have 
no precise clue as to its origin even though the biological advantages of 
this mode of life are evident. It is remarkable, for instance, that Puccinia 
Pruni-spinose occurs only on certain species of Prunus during one part of 
its life-cycle and only on a few species of Anemone during another. On 
the other hand a rust fungus may have a wide range of hosts during one 
phase of its life, although this may perhaps be wholly or partly due to the 
existence of different physiologic forms, and a single unrelated host for 
the remainder of its life. Further information is needed as to whether an 
apparently wide host range for one phase in the life-history of such fungi 
is often due to a multiplicity of physiologic forms : probably it is not, for 
in Puccinia glumarum certain forms thrive both on wheat and barley. 
30 Annales Mycologici, 1, p. 132 (1903). 
31 Jour. Agr. Res., 15, p. 221 (1918). 
