242 SECTIONAL ADDRESSES. 
change in the cell walls of the guaid cells lying below the appressorium of 
P. graminis tritici. These guard cells had not been penetrated, the hypha 
passing between them to form the substomatal vesicle, and yet the walls 
of these cells became markedly altered in their reaction to stains. Itmay 
be that both these cases demonstrate the action of enzymes derived from 
the fungus, for diffusibility and non-diffusibility are only questions of 
degree ; it may be, on the other hand, that these cell-wall changes are due 
to changes produced in the host cell as a result of the entry into the cell of 
poisonous fungal products more diffusible than are enzymes. 
The question may now be considered as to what progress has been made 
by plant pathologists in elucidating the quality of natural immunity. As 
has already been stated, the problem of natural immunity is an extremely 
difficult one which animal pathologists on their side have found very 
baffling. It can be said, however, that some success has been achieved 
in a preliminary analysis of some cases of natural disease resistance 
in plants. As is so common in biological work, the difficulty of solution 
is greatly enhanced by the variety in the types of disease resistance. In 
many cases resistance to disease is achieved by keeping the enemy out by 
some physical barrier, or possibly by some special chemical environment 
in the absence of such a barrier. In other cases the parasite achieves 
entry and in a susceptible host makes its way through the tissues 
comparatively unimpeded, while in a resistant the entry calls forth a 
wound reaction leading to the production of cork which hinders or 
sets a complete bar to the progress of the invader. A good example 
of these two types of behaviour is that of Fusarium Lini when 
attacking susceptible or resistant forms of flax. In one case the physio- 
_ logical processes of the resistant host interacting with those of the fungus 
lead to abundant cork-formation; in the other they do not. In what 
manner the physiological processes of the two types of host differ we 
cannot at present say. Nor can we at present explain why the harmonious 
relationship, which in the case of the cereal Smuts is established for most 
of the vegetative life of the host, suddenly breaks down on the development 
of the inflorescence. Is the metabolism of the cells of the developing 
reproductive organs so markedly different from that of the meristematic 
cells that the fungus is stimulated into active development and parasitism ? 
It would seem likely that a further knowledge of the nature of the 
‘ physiological gradients’ between the parts of plants would throw some 
light upon the peculiar relations of host and parasite in the cereal Smuts. 
How elusive may be the factors underlying resistance is exemplified 
by the observations of Walker !? on Onion Smudge due to Colletotrichum 
circinans. He found that onion bulbs with coloured outer scales were 
usually highly resistant, while white varieties were in general susceptible, 
and, furthermore, a watery extract of dry outer scales of the coloured 
onions is a marked toxic to the spores and mycelium of the fungus. On 
further examination it was found that although the internal white scales 
can be infected with ease, yet an extract of these inhibits the germination 
of the conidia and also retards the development of the mycelium. The 
volatile ‘onion oil’ seems responsible for the inhibition and retardation, 
yet when the fungus is growing in the host tissue there is no such action. 
17 J. C. Walker ; ‘ Disease Resistance to Onion Smudge.’ J, Agric. Res., XXIV., 
1019, 1923, 
