Hayes and Garber - Cucurbit Breeding

38 BREEDING CROP PLANTS
mas protrude from the glumes and take a pendent position before
anther dehiscence takes place.
Cotton. Probably cotton crosses to a greater extent than any
of the other plants, except sorghums, listed as belonging to the
naturally self-fertilized group. Because of the difference in observations
by investigators it would seem that varietal differences
are one probable cause for the discrepancies.
Leake (1911) observed 5 per cent, natural crossing in India.
Figures given by Webber (1905) and Balls (1912) range from 5
to 13 per cent.
Grain Sorghums.
Ball (1910) states:
"All sorghums are adapted to open or wind pollination and most of
them are probably adapted to self-fertilization. In adjacent rows of
different varieties flowering on approximately the same date, as high as
50 per cent, of the seed produced by the leeward row was found to be
cross-pollinated. It is probable that in a fairly uniform field of any
given variety a similar percentage of natural crossing takes place."
Graham (1916), in India, made a careful study of the amount
of cross-fertilization in the Juar plant (Andropogon sorghumBrot.)
.
Crossing was more frequent in the looser types of inflorescence
than in the compact types. Single plant cultures were used for
the study, which extended over a period of seven years. The
percentage of crossing obtained by counting a given number of
plants and noting those which were untrue to type gave 97 plants
out of 1,577 (6 per cent.) in the loose headed type and only two
plants out of 292 (0.6 per cent.) in the compact type of panicle.
Preliminary studies were made by Karper and Conner (1919)
of the amount of cross-pollination in plants of white milo which
were found growing in a plot of yellow milo. The yellow and
white varieties flowered simultaneously. Forty-one heads of
white milo, which had been surrounded by yellow milo, were
planted the following year. An average of 6 per cent, of natural
crossing in plants so surrounded was noted.
Peas and Beans. Piper (1912) finds that natural crossing in
the cowpea occurs but rarely in most localities. At Arlington
Farm, in the experimental plots, instances of natural crossing
have been observed. In some instances natural crossing occurs
more frequently.
Thus an Indiana farmer, who originally grew
only eight varieties, found after several years that he had over
40 types. The new types, Piper concluded, were the result of