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the minds of some authors (N a v a s h i n, M a g n u s). all testify
to a great plasticity and adaptability of parasitic fungi and oblige,
them to be rather sceptical about immune varieties. Plants now immune
to this or that fungus may succumb to it in the next few years—this is the
view of these scientists.
But there are much more data on an extreme constancy of immunity.
Immune varieties as
Tviticum monococcum
, many immune varieties of
T. dicoccum
, varieties of naked barley immune to mildew,existed thousands
of years ago and have not lost their immunity.
Arena slrigosa
and
A . brevis
now immune to smut, mildew and rust were cultivated at least two or three
hundred years ago. A great number of varieties of common wheat, durum
wheat, varieties of oats, barley, roses, apples, pears,'potatoes,immune to
different fungous and bacterial diseases, the list of which is given in . the
Chapter 1, have been cultivarted not less than 50—100 'years. These
facts, about which there can be no doubt, prove the insolvency of a scepti
cism to constancy of immune varieties.
Moreover, the facts mentioned above of a great changcableness and ada-
ptibility of parasitic fungi are not unquestionable. К I e b a h n did not
experiment with pure lines(clons)of
Puccinia Smllacearum-Digraphidis.
It may
be on the «pure Iint»principle of .Johanssen there was no change of parasite
but simply a selection of separate races of rust different in their specialisa
tion, through different host-plants.
As to «bridging species», there are firstly not many species of fungi,
where they are found. They have not been found in many species of cereal rusts
notwithstanding the attempts to find thorn. Against a broad generalisation
of them there is secondly , the. possibility, proved by many observations, of
cultivating immune and susceptible varieties side by side without the lo
sing of immunity by the first; thirdly the possibility of obtaining immune
varieties by crossing.
As to phenomena of «bridging species» in
Puccinia graminis
, it is ne
cessary not to forget in generalisation the great physiological differentiation
of this species into races. In general, experiments with «bridging species»,
in order to he convincing, should be made with pure lines (cions)1).
There is no reason to deny completely the possibility of change in
parasites. In the long perspective of ages it may happen that even varieties
immune at present will become susceptible. It may be that «Persian Wheat»
will sometimes be infected by a new race of mildew. But it is possible that
this will not happen, as it has not with the old immune
Triticnm monococcum
and
T. dicoccum.
In any case, there can be no doubt of the applicabiliiy
4 Evans’ experiments with the infection by Puccinia graminis of hybrids of wheat,
in whirh the hybrids acted as a bridge between immune and susceptible parent varieties
may be explained differently. There are at first no varieties of wheat strongly immune
to black rust, secondly the spores of rust taken from the first generarion of hybrids (Fi)
can have being of a better germinating capacity, as in this case Fi was more suscep
tible to the parasite in comparison even with susceptible parent varieties and consequent
ly the fungus grew on it in very favourable conditions.
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