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This book includes a critical examination of our present knowledge

of natural immunity in plants wiih addition of the author’s experimental

work in this direction, as well as an exposition of some regularities in the

distribution of immunity to fungous diseases among plants.

CHAPTER 1.

The Extension of the Phenomena of Immunity among Higher Plants.

The majority of parasitic fungi—the chief agent of infection in plants—

are by nature strictly limited in the choice of their hosts and attached to

definite genera and species of plants. In some cases the fungi are limited

to a few or even to one host-species only. In most cases they are limited to

one plant genus.

As limitation,in parasitism to a definite plant genus means at the same

time immunity of other plant-genera to the same fungus, so the most usual

form of immunity in plants is immunity of genera.

But practically, the phenomena of immunity are usually connected

with immunity of races and varieties, rarely with that of species.

. . One of the usual peculiarities of immunity of varieties is that very

often it is not absolute, but apparent only in a greater or lesser degree.

..Jo distinguish the difference in susceptibility to diseases the

author has used the scale of degrees of susceptibility of Prof, E r i k s-

s on, denoting the highest degree of susceptibility. by mark—4. the

smallest degree by mark— i . Nought (0)—signifies a complete absence

of pustules of fungus. This scale is completed by indications of morpho

logical changes in tissues of host-plants under the influence of parasitic

fungi in connection with greater or lesser resistance. The coloured table

added to the book may serve as an illustration of the scale of marks used

hv the author *).

The following is the detailed explanation of marks for leaf rust and

mildew (applied under optimal conditions of infection).

Plants very susceptible. Comparatively large pustules of fungus

densly cover the upper part of leaves. There are no yellow spots of decaying

tissue of leaves around the pustules.

Plants feebly resistant. The upper leaves are partly free from fungi.

Many pustules on middle leaves, but smaller than in the first case, more

scattered and surrounded by yellow spots.

*) For black, brown and crown rust of cereals (Puccinia graminis, P. triticina and

P. coronifera) coloured tables are given in another work: N. 1. V a v i l o v (N .W a -

w i l o w ) . Beitrago zur Frage iiber die verschiedene Widerstandsfahigkeit der Getreidc

gegen parasitische Pilze. Arbeiten der Versuchsstation fill- Pflanzenziichtung am Moskaner

Undvrirtschaftlichen Institnt. 1 Folgc, 1913.

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