ЭлБиб

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species of

Linuni.

Hence the existence of varieties of flax immune to

this rust.

The second moment which determines the behaviour of varieties to

parasites is their genetical place among other varieties of the same plant,

the genetical differentiation among species.

If we look for distribution of immunity to narrowly specialised fungi

among wheat, oats, and barley, we find an evident connection between

immunity and genetical differentiation of varieties. Among the 8 species of

wheat, for example, all varieties belonging to

T. monococcnm

are extremely

resistant to brown, yellow and black rusts, and this markedly distin

guishes this from other species of wheat. Genetically

T. monococcnm

occu

pies a separate place among wheat and does not give fertile hybrids with

the other 7 species.

T. polonicum, T. turgidutn

and

T. durum,

including hundreds of

varieties comparatively resistant to brown and yellow rusts and to mildew,

are genetically nearly allied, can be easily crossed, give quite fertile hybrids,

and occupy altogether a separate phvlogenetical place among wheats.

At the same time they are genetically distinct from the susceptible group

T. compacium

and

T. vulgare

(by crossing with the latter they give sterile

and partly fertile hybrids).

Among oats the most immune varieties belong to quite distinctive

species like

Arena brevis

A . strigosa,

which cannot be crossed with ordinary

susceptible oats A.

saliva.

Varieties of

A . byzanlina

immune to rust and

smut are geographically and phylogenetically distinctive from European

ordinary susceptible oats.

One variety of

Arena strigosa

(from Kew) which morphologically be

longs to this species,proved unexpectedly susceptible to smut, mildew and

rust. But, as it was shown by the author it " is different genetically

from ordinary races of this species, and can be crossed, with susceptible

A. saliva.

A. falua

and

A. Ludoviciana

which are genetically nearly allied to

saliva

and cross easily with it, are equally susceptible to all its parasites.

Naked barleys are distinctive in their phylogcnv from ordinary varie

ties of barley. This genetical individualisation evidently explains its lesser

susceptibility to mildew and rust. In general, cultivated barleys are not so

much differentiated genetically as wheats and oats. All varieties of barley

can be easily crossed. Hence there are no sharp differences in immunity

to rust and mildew. This circumstance probably explains a complete absence

of immune varieties to the narrowly specialised

Uslilago hordei

Jens.

Triticum dicoccum

represents a genetically differentiated group, and,

correspondingly, it includes strongly immune varieties as well as susceptible.

«Persian Wheat», genetically quite different from

T. vulgare

var.

fuliginosum

AI. to which it was referred by svstematists, accordingly distin-

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