Hayes and Garber - Cucurbit Breeding

The
276 BREEDING CROP PLANTS
Stewart to
conclude that there was more evidence in favor of
purity of the clone than in favor of the value of clonal selection
as a means of producing higher-yielding strains. Similar conclusions
were reached from an experiment carried on by Tyson
brothers, in New York, with the York Imperial apple. Two
trees were selected which bore unusually similar fruits and these
were used for propagation. More than 8,000 trees were planted
in the new orchard. Examination of trees of this orchard when
they came into bearing showed them to be not superior to the
usual York Imperial apple (Dorsey, 1917).
The cited cases show the present status of the problem of selection
of bud sports as a means of improvement of fruit crops.
The studies with the citrus genus appear to justify the belief
that degenerate or inferior bud sports are of frequent occurrence.
This leads to a conclusion that only those limbs which produce
normally healthy fruit should be used for propagation purposes.
Even among the citrus fruits there is as yet no very conclusive
proof that the selection of cions from high-yielding trees will
accomplish more than to prevent possible "running out" of the
.
variety. evidence from apples would seem to justify the
belief that bud sports are very infrequent. The breeder, then,
can well afford to make careful observations with the hope of
discovering bud sports.
If apparently desirable sports are found,
these may then be used for propagation.
In such crops as citrus fruits and with such plants as
Coleus,
bud sports are of frequent occurrence. There is, then, some evidence
for the belief that sports occur more frequently in heterozygous
than in homozygous material. As Stout (1915) obtained
the same changes through asexual selection as by the use of selffertilized
seed, it seems reasonable to suppose that some sort of
segregation and recombination occurs in somatic tissue. No
cytological evidence has been given to account for such a supposition.
With heterozygous material the loss of a single dominant
factor would be immediately apparent in the soma. This is one
reason why bud sports occur more frequently in heterozygous
forms (East and Jones, 1919). Nabours (1919) has shown that
similar cross-overs occur in parthenogenetic reproduction in the
grouse locust as in those forms which are produced by the recombination
of gametes containing the haploid number of chromosomes.
If the usual sort of cross-overs occurred in homozygous
material, there would be no change in the homologous parts of