K.—BOTANY. 243 
It is only in the Erysiphacee and Uredinee that we have knowledge 
of any cell reactions (though not of any general reaction of the plant body) 
comparable with those occurring in the infectious diseases of the higher 
animals. In these two groups the phenomenon of so-called specialisa- 
tion of parasitism is well marked, and it is a comparative study of the 
behaviour of the biologic forms of the parasite on susceptible and resistant 
hosts that has been most fruitful. As has been known for some time, the 
normal relation of host and parasite in the mildews and rusts is, in the 
early stages of infection, one in which the fungus develops at the expense of 
the host cells; these, however, are not killed but stimulated to active de- 
velopment. De Bary observed long ago that the mildewed leaf may retain 
its green colour longer than the uninfected one. Salmon observed some 
years ago that the conidia of Hrysipha graminis when growing on other than 
their normal host might send down haustoria into the epidermal cells, but 
such absorbing organs were short-lived. Neger!® has recently investigated 
more closely the result of sowing upon the leaves of Hieraciwm of the 
conidia of £. Cichoracearum from Sonchus asper. The germ-tubes send 
into the epidermal cells outgrowths which start to produce haustoria. 
In contrast with infection of the normal host, the cells react markedly ; 
they become filled with a gum-like mass which encapsules the haustoria. 
The epidermal cells then lose their turgor, die, and the development of 
the fungus is stayed. A leaf sprayed with suspension of such conidia 
_ appears as if it had been sprinkled with minute drops of a corrosive fluid. 
However, it is in relation to the cereal rusts that we have the clearest 
picture—in its purely superficial aspects at least—of the nature of resist- 
ance. With the discovery by Professor Biffen that resistance to the attack 
of Puccinia glumarum was associated with a single Mendelian factor, atten- 
_tion was naturally turned to the question of the nature of this resistance. 
Miss Marryat, comparing in Professor Biffen’s laboratory the susceptible 
Hinkorn and the resistant Michigan Bronze wheats, made the surprising 
discovery that the resistance was in one sense no resistance at all.*° 
The variety Einkorn was not able to keep the parasite out, for the 
hyphe attacked the mesophyll cells, but the invaded leaf-cells—instead 
of establishing an harmonious working relationship with the mycelium 
as with Michigan Bronze—react very strongly, with the result that both 
they and the invading hyphe are killed. The course of infection is thus 
Stayed as a result of this hypersensitiveness of the host. The result with 
. glumarum was later extended to P. graminis. The striking and assiduous 
work of Stakman and his co-workers has revealed to us that even 
P. graminis forma tritici consists of twenty or thirty different strains with a 
widely varying range of susceptibility and resistance among the different 
Varieties of wheats. Stakman* in 1915 was able to confirm the violence 
of the reaction when strains of this form are sown on a resistant 
host ; hypersensitiveness here also is the key to resistance. Last year 
7° FP. W. Neger: ‘ Mehltaupilze—eine Art von gedultete Svmbiose.’ Flora, CXVI., 
6 3 
dy . d 
“D.C. E. Marryat; ‘Notes on the Infection and Histology of Two Wheats immune 
to the Attack of Puccinia glumarum.’ J. Agric. Science, I1., 129, 1907. 
_ *#. C. Stakman: ‘Relation between Puccinia graminis and Plants highly 
sistant to its attack.’ J. Agric, Res., V., 193, 19165. 
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