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The same has been noticed in relation to yellow rust by crossing two

varieties of

T. vulgare

immune to this rust.

The author lays stress on the great role of the individuality of varieties

in the determination of numeric relations in the process of segregation and

in the dominance or recessiveness of immunity to diseases. So for example, in

crossings of «Persian Wheat» with other varieties, immunity to mildew acts

as a dominant character. By crossing

T. dicoccum

var.

picnurum

immune

to mildew with

T. vulgare

susceptible to this fungus the immunity proved

to be recessive *).

■>

The idea of independence of immunity in segregation on other

morphological and physiological characters in plants ought not to be taken

as absolute (see В i f f e n) but it is .very likely that it is often con

nected with other characters of varieties, ' especially physiological ones.

This connection'can be seen in the dependence of immunity and suscepti

bility on the genetical place of variety among other varieties (see Chapt. 5).

The author draws attention, to certain difficulties met with in using

hybridisation as a means of obtaining new and valuable varieties. In many

cases varieties markedly, different in their relation to parasites cannot be

crossed, on account of their genetical distinction, e. g.

Arena brevis

immune

to smut and mildew cannot be crossed with

A . saliva

susceptible to these

fungi;

Trilicum monococtam

Strongly immune to brown, black and yellow

rusts cannot be crossed with

T. vulgare,

susceptible to these rusts, etc. Very

often such sharply different varieties give by crossing a great percentage of

plants completely or partly sterile. To be sure of success in such a crossing

it is necessary to take as parents nearly allied varieties. The second difficulty

is the complicacy of the process of segregation and a small numeric proba

bility of separation of a homozygotic plant with all desirable characteristics.

CHAPTER 7.

Selection of Immune Varieties and its Limits.

High specialisation of parasitic fungi, the so called phenomenon of

«bridging species» (Ward, Salmon, Evans, Freeman, Johnson, Steiner) as

well as the experiments of K l e b a h n with

Piiccinia Smilacamm-Dig-

raphidis,

where he succeded in changing the specialisation of this fungus, in

') The statement of Prof. Biffen in his «Studies on the Inheritance of -Disease-resis

tance» (II,

Joar.of

Agr. Sc. Vol. 4. Part 4, 1912), that the susceptibility of wheat to ergot

depends on two separato Mendelinn factors is certainly wrong. Prof. Biffen’s statement is

basedonthofact oftheappearance in the Fa of his crossing of Triticum vulgare with T. tur-

gidum of some plants which were infected by ergot, when at the same time both parents

were free from ergot. Bnt the ergotised plants of Fa were certainly the result of the appea

rance of some sterile plants in this crossing as an inter-species crossing. The very fact

of appearance of some sterile plants in Fa of this crossing was already noticed by Rimpau

in his «Kreusungsprodukto landwirtschaftlichcr’Kulturpflnnzen» ppl 11— 12). The sterile

plants of cereals, as is known, flower usually with open glumes, remain many days in

this state and commonly are badly attacked by ergot. The Mendelian factors or suscepti

bility to orgot have in no wise, any place in this crossing.

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