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CHAPTER 2.

The Nature of Immunity In Plants.

The author distinguishes two kinds of natural immunity; m.e c h o-

n i c a 1 or p a s s i v e immunity dependent on mechanical peculiarities

in structure and growth of plant organs, which prevent the penetration of

parasites into the plant tissues, ;;and p h y s i o l o g i o a l . or a c t i v e

immunity which depends on active resistance of host-plant cells, usually

accompanied by a complicated physiological reaction in response to the

penetration of a parasite. Externally this reaction manifests itself through

the death of plant-cells and fungus at the place of inoculation, or by forma

tion under the

the parasite of new tissues, which isolate the

action of bacteria or fungi.

•

The mechanical immunity depends on morphological and anatomical

peculiarities in external tissues, on peculiarities in the habitus of the plant,

on the rapidity of cicatrizing of wounds and,cracks in outer tissues, on pecu

liarities in flowering, namely on closed flowering (o. g. in .relation to smuts

and ergot), on closed grain (in relation to. smut), oh the overgrowth of fungus

by the plant tissues, etc.

In some cases passive immunity depends-'on the secretion by the outer

tissues of different ethers and resins, impeding the penetration of infection.

The nature' of this kind of immunity is comparatively clear, and one

of the first theories of immunity of plants was the m e c h a n i с a 1

t h c o r y , proposed by C o b b .

This theory is undoubtedly applicable to some cases of immunity, but

it is far from being universal. As it was shown by ‘ E r i к s s о n, H e n

n i n g , M a r s h a l l W a r d and by the author (1. c.) it is unable to

explain for instance the immunity of cereals to rusts and mildew.

The peculiarity'of the mechanical immunity, besides its passiveness,

is its relativeness. If we remove these mechanical obstacles, if we bring

infection, for instance into the flowers of varieties, immune, thanks to their

closed flowering, we can infect them just as if they were not immune.

Perhaps it would be more correct to consider varieties, possessing me

chanical immunity not as imhuine, but as disease-escaping plants only.

>A ' broader region of phenomena is related to the p h y s i o l o g i

c a l or a c t i v e immunity.'

The study of the nature of physiological immunity is based on histo

logical and cytological investigations of the process of infection in. immune

and susceptible varieties; (M a r s h a 11 W a r d, S a l m о n, G i b s o n ,

E v a n s , M a r r y a t, S ta ^ k ra an ,;. W i l t s h i r e , T i s d a l e ) .

These investigations show that parasitic fungi penetrate non only into sus

ceptible varieties, but also into immune ones and that immunity depends

not on anatomical peculiarities of plants, but on internal enzymatic rcac-

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