K.— BOTANY. 24.5 
marked dissimilarities between the two. As has already been insisted 
upon, the immunity which has to be explained in plants is natwral, while 
that resistance which animal pathologists have explained, at least in part, 
is acquired. The explanation of the difference in the behaviour of the 
mesophyll cells of the susceptible and resistant wheat must le in the 
difference in the normal physiological processes of the two. This demon- 
strates how dependent is plant pathology for its advance on plant physio- 
logy. The differences do not seem to be merely differences specific to 
the wheat varieties, differences such as would be common to all the cells 
of the plant—or if there are such differences they are easily masked by 
other factors—for Miss Allen observed that while the mesophyll cells of 
the resistant wheat reacted violently when invaded, yet if an epidermal 
cell was attacked the haustorium developed might attain its full size and 
function for some time. It is evident that we must await fuller knowledge 
of the normal physiological processes of the cells of the two varieties, and 
of the physiological differences between the cells of different tissues, before 
much light will be thrown on the nature of such immunity as is met with 
in the Erysiphacee and Uredinee. 
A consideration of the nature of disease resistance in plants thus leaves 
us with no expectation of finding means for endowing plants with artificial 
disease resistance. Apart from the protection of plants from infection 
by the use of fungicides, etce., our chief hope of combating disease lies in 
two directions—one, that of breeding disease-resistant forms of plants, and 
the other that of the enhancement of the natural resistance of the plant. 
In breeding for disease resistance, marked successes have been obtained 
since Biffen’s fundamental work on Mendelian inheritance of resistance to 
Puccinia glumarum. In a number of cases of rust resistance in cereals 
since examined, immunity has been found to be dominant over sus- 
ceptibility. The question of breeding wheats resistant to P. graminis, 
which is, of course, one of great economic importance, has been much 
complicated by the discovery, to which reference has already been made, 
that a very large number of biologic forms or strains of P. graminis tritici 
exist ; high resistance to attack by some of the strain may be associated 
with marked susceptibility to attack by other strains. Aamodt, however, 
claims to have demonstrated that it is possible to build up synthetically 
a wheat which will be resistant to a large number of biologic forms of 
P. graminis tritici.® 
Although we find that the field of control of plant diseases by substances 
lethal to fungi and by the breeding of disease-resistant host plants is being 
actively cultivated at the present time, yet the field of inquiry as to the 
effect of environment on the liability of plants to diseases is comparatively 
unworked. The view that immune plants, such as cereals immune to rust, 
might suddenly lose their resistance under new conditions is now no longer 
held ; the apparent loss of resistance is probably in part explicable by the 
fact that the host in its new environment has been subjected to attack by 
another biologic form of the fungus than that to which it is resistant. In 
spite of this, however, it is perfectly clear that with numerous diseases the 
degree of natural resistance is markedly affected by the conditions of 
230.8. Aamodt: ‘The Inheritance of Growth Habit and Resistance to Stem Rust 
in a Cross between Two Varieties of Common Wheat.’ J. Agric. Res., XXIV., 457, 1923. 
