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pilus).
No change of reaction in immune plants in relation to specialised
fungi was noticed in these experiments.
On the whole it is possible to say, in reference to physiological immu
nity of plants, that the specific hereditary nature is stronger than envi
ronment.
CHAPTER 4.
Laws o( Distribution of Immunity among Varieties of Plants.
To find an immune variety among plants, according to the opinion
of plant-breeders is a matter of accident and good fortune. The first impres
sion made by a study of distribution of immunity to fungous and bacterial
diseases is disorder and lack of regularity. Why is it that some varieties
are so very susceptible to disease and others so resistant? Why is a va
riety, immune to a certain disease susceptible to another? Why are there
many varieties immune to one fungus species and none immune to another?
Nevertehless the investigation of many hundreds of varieties of cereals
iii relation to all their chief fungous diseases—to mildew, rust, ergot and
smut, as well as of many varieties of roses, flax and clover, has persuaded
the author that- there exist definite regularities in the distribution of
immunity among varieties of plants, in many cases even allowing a pre
diction of the behaviour of a variety to this or that disease, and if we
know its relation to one or two diseaseesa foresight of its behaviour to
other parasitic fungi.
The data for cereals on which these regularities were established are
given on pp. 119—139; for wheat in relation to its different fungous diseases
on pp. 119—127, for varieties of oats on pp. 129—134, and for barley on
pp. 135— 139. These data arc the results of observations and experimerts
of infection made by the author during 1911— 1918. More varieties were
investigated than are enclosed in the tables. (For varieties of wheat and
oats the data on their relation to mildew and rusts are given in detail and
for a greater number in «Beitrage», 1. c.). In these tables the typical varie
ties only were taken, differing physiologically and morphologically l).
’ ) Varieties of wheat were studied by the author in relation to brown rust (Puccinia
Iriticina), black rust (P. graminis) and mildew {Erysiphe graminis) in Russia, in relation
to yellow rust (P. glurnarum) in England (Cambridge,'Merton, Reading) and Russia,
in field conditions. Experiments of artificial infection by loose and stinking smut (Usti-
lago tritici. U. avenae and Tilletia Iritici) were done in Moscow.
For infection of wheat by loose smut, fresh spores of fungus were brought into
the flowers at the time of the beginning of flowering. To infect the seeds of wheat and
oats by Tilletia tritici and Ustilago avenae, grains were moistened in water, well covered
by spores of smut and, in order not to wipe out the spores, infected grains were carefully
put, with the aid of pincers, into holes prepared in the soil. Usually by such a method
about 30—50% of the plants were infected.
For barley all data were obtained in field conditions, without artificial infection.
In order to mark the differences in susceptibility of. varieties to mildew and rust
Hie abovementioned scale of marks was used (Chapter 1), signifiing by 4—the highest
susceptibility, by I— the smallest degree, by 0—absolute immunity.
For smuts, the greatest susceptibility was noted by two crosses Ч-+; the medium
infection and simple susept b lity without denoting exactly the % of infected plants—
one cross -h; and by nought (0) the complete immunity.
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