232 Wisconsin Academy of Sciences^ ArtSy and Letters, 
walls to mechanical puncture by the fungus hyphae. Tisdale (79) 
describes the formation of a layer of suberized cell walls in the 
cortex of resistant flax plants which checks further advance of the 
wilt fungus, Fusarium Uni. He points out, however, that the 
actual resistant quality may possibly be contained in certain prop¬ 
erties of the protoplasm of the normal cortical cells which retard 
the progress of the parasite during the suberization of the under¬ 
lying cells, the latter being a natural host reaction. Valleau (80) 
considers it very possible that the slow development of Sclerotinia 
cinerea in resistant plums is due to a slight difference in the com¬ 
position of the middle lamella which makes the latter less easily 
soluble in the secretions of the fungus. Further work by Willaman 
(93, 94) shows a higher crude fiber content in resistant than in 
susceptible plums. 
The Nature of Rust Resistance 
The nature of resistance to rusts has been studied by a number 
of investigators. The work of Cobb (17), stressing the importance 
of the anatomical features of the host, has already been cited. 
After a careful and comprehensive study of the question, Ward 
(88, 89) failed to find any correlation between external host fea¬ 
tures and infection. He showed (89) that in the case of wheat 
susceptible to Puccinia glumarum the hyphae of the latter “are 
typically stout, branched, and contain hundreds of nuclei. They 
also form numerous haustoria, and the attacked cells show no 
evident signs of injury to the chlorophyll-corpuscles or nuclei 
until a late stage of growth.’’ In the case of highly resistant 
wheat, however, the substomatal chamber is invaded normally, but 
when the hyphae come to invade the cells they “show evident 
signs of degeneration in all respects and we conclude, from com¬ 
parison with experimentally starved hyphae, that they are under¬ 
going death-changes owing to one of two events, viz., they are either 
starving for want of food supplies or they are being poisoned.” 
The same investigator (88) described earlier a similar condition 
existing between Puccinia dispersa and susceptible and highly 
resistant varieties of bromes. He (89) sums up his conclusions 
on the subject as follows: 
In other words, infection, and resistance to infection depend on the 
power of the fungus-protoplasm to overcome the resistance of the cells of 
the host by means of enzymes or toxins; and, reciprocally, on that of the 
