Hayes and Garber - Cucurbit Breeding

MODE OF REPRODUCTION IN RELATION TO BREEDING 39
natural crosses. Similar crosses have been observed at the
Michigan station. Harland (1919) has recorded a supposed case
of a natural cross which occurred in one of his hybrid cowpea
families.
Natural hybrids of soybeans have been observed at the United
States experimental farm in Virginia and also at the Kansas
experiment station (Piper 1916). They were detected by the
peculiar color of their seed. Varieties of soybeans were interplanted
at the Wisconsin station and the amount of natural
crossing was determined by testing the progeny. More than
10,000 plants were tested and only a fraction of 1 per cent, of
natural crossing was found (Russell and Morrison, 1919).
Although horticultural peas and beans are largely self-pollinated,
cross-pollination does occasionally occur. Howard and
others (1910a) give observations in India which indicate natural
crosses both in garden and field peas.
Tomatoes. Jones (1916) planted alternate plants of dwarf and
standard varieties of tomatoes 3 ft. apart in a field. Seed from
the dwarfs was tested the following year. As standard habit is
a dominant character, pollen from a standard plant fertilizing a
dwarf would give a standard in FI.
A total of 2,170 plants were grown from seed of dwarfs and
43 proved to be standards. This is practically 2 per cent. As
there was nearly as great opportunity for dwarfs to be crossed
with dwarf pollen it would seem that between 3 and 4 per cent, of
crossing occurred in this experiment.
THE OFTEN CROSS-POLLINATED PLANTS
Maize. Maize has been placed at the head of the often
cross-pollinated group, as crossing is its normal form of reproduction.
Fruwirth (1909) found a setting of 24 per cent, in unenclosed
corn plants when far enough from other plants to prevent
crossing. Knuth (1909), in similar experiments, found 16 per
cent, selfing on the upper ear and 4 per cent, on the lower.
Preliminary experiments have been made by planting corn with a
recessive endosperm color in a field of a variety with a dominant
endosperm character. Self-fertilization in these experiments
was probably less than 5 per cent. (Waller, 1917, Hayes, 19186).
Rye. The flowers of rye are very similar to those of wheat
and barley. According to Hildebrand the anthers project