Hayes and Garber - Cucurbit Breeding

COTTON AND SORGHUM 177
homozygous. Correlation between length of lint and size of
seed may explain some complications.
Mutations in Cotton.- The cotton plant, like (Enothera, has
often been spoken of as having germinal instability and likely
to produce mutations. While mutations undoubtedly do occur,
it is likewise highly probable that many of the so-called mutations
are simply segregates of a former natural cross. The ease with
which natural crossing occurs and the large number of chromosomes
(20 according to' Balls) contained in the cotton gamete
facilitate the appearance of forms differing from the general type.
The larger the number of haploid chromosomes the more difficult
it is to secure homozygous individuals after a cross. Egyptian
cotton is described by Kearney (1914) as being a mutating type.
From it the varieties Yuma, Pima, and Gila are supposed to
have arisen. In this connection it is of interest to point out that
the common belief as to the origin of Egyptian cotton is that it
arose by hybridization between a brown-linted tree cotton and
American sea island. The subsequent development is unknown.
In view of the foregoing and the fact that no convincing evidence
has been presented to the contrary, the present writers
believe that many of these supposed mutations are in reality
factorial
recombinations resulting from natural crossing.
Cotton Breeding.
Cotton improvement by breeding may be
sought along lines similar to those followed with all naturally
selfed crops. In producing pure-line material for scientific
study and subsequent hybridization it is essential to obtain absolute
self-pollination.
From a commercial standpoint, a productive cotton with long
lint and smooth seed is desirable. Webber (1905) crossed
Klondike, a productive upland variety, and sea island, which has
long lint and smooth seed. Out of an F z generation consisting
of several thousand plants, only 12 combined the large blunt bolls
of the upland with the long lint and black seed of sea island.
The progeny of each of these 12 plants was grown in isolated
plots and subjected to vigorous selection. In the fifth generation
a number of plants gave progeny " nearly fixed in type."
Resistance to wilt disease is a character of considerable commercial
importance. This disease is caused by Fusarium vasinfectum
Atk. which according to Orton attacks only the cotton and
its near relatives. By the plant-to-row method and under wilt
infection conditions, it was found possible to build up varieties
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