Evening star., 1938-10-25

and survival OF the fittest and because
of mutations" which take place oc-
casionally In all races. A mutant has
different, hereditarily transmitted
characters than its parent. Such an
organism usually perishes but occa-
race. The greater the number Of or
ganisms the greater the possibility of
variations through natural selection
and the greater the chance of mutants
which will survive. The number Of in
dividual organisms Of the corn smut
is essentially infinite compared with
the numbers in any race of higher Or
ganisms. and consequently the speed
Of evolution among them may be vastly
accelerated, DR.. Stakman pointed out
These particular corn smut fungi
with which the most critical expert
ments were conducted at Minnesota
are capable both of reproduction from
a single cell and by mating. From the
mating Of two several thousand organ
isms were obtained which differed
markedly in their physiological char
acters and parasitism. Other crucial
experiments were carried on with a
new type OF stem rust Of wheat which
first appeared in the Middle West
about 10 years ago and caused enor-
nous damage. Since then it has
spread all over the United States.
More than $50 strains have been OBJ
tanned. DR.. Stakman said. Some will
not attack one variety of wheat, others
another. The high selectivity OF stem
rust fungi long has amazed plant
breeders. Some will affect wheat and
barley but not rye or oats. Some at
tack wheat and rye, but not barley or
oats. Some will attack only one strain
of wheat
Immunity for Few Years.
In the past when q rust, smut or
blight attacked crop in the United
States. a common practice was to
introduce some variety from abroad
which appeared to be resistant to it.
This was interbred with the native
crop in order to get a disease resistant
strain, or supplanted it altogether. For
a few years everything would go we'll.
Then the new strain would be at
tackled, apparently by the same para
site as the one it had supplanted. And
apparently, from the data DR.. Stakman
presented to the National Academy,
this is what always will happen and
immunity can be considered good
only for few years.
Wheat farmers have observed, Dr.
Stakman said, that some years one
variety of wheat will be entirely i'm
mune, while on another they will
suffer very heavy loss because Of
disease. They may take advantage
of this observation by planting only
the immune variety the next year, only
to find that it suffers just as badly
and the loss is greater. This is be-
cause the different evolving varieties
of smut and rust are very selective as
to place. time and weather. The in
digenous varieties may be starved out
one year, but a few individuals will
be able to live under the changed
conditions and become the parents of
q countless host the next year. So
nothing can be considered entirely i'm
mune
The only remedy at present, DR..
Stakman stressed, is to keep ahead of
the natural evolution of plant fungous
diseases by breeding new species, but
there is hope for the future in dis
covering the mechanisms at work in
producing such an abundance of ma
lignant new organisms. One factor at
work in the fungi, he has found, is
that of natural gratis. The rust and
blight organisms send out tendrils.
Those on two different organisms
fuse-just as is the case with an arti-
fetal graft in the orchard. An even
more important part, he believes, is
being played by mutations such as
occasionally produce new varieties of
insects and flowers. Whatever the
mechanisms, they work very rapidly,