Hayes and Garber - Cucurbit Breeding

36 BREEDING CROP PLANTS
crosses.
Smith (1912) reported eight natural hybrids in 96 rows
of Turkey winter wheat and Saunders (1905) told of a natural
hybrid which occurred at Ottawa. During the last three years
at University Farm, St. Paul, at least 2 to 3 per cent, of natural
crossing in wheat has occurred in the plant-breeding plots.
Cutler (1919) mentions frequent natural crosses at Saskatoon,
Canada.
Barley. Barley frequently is self-fertilized while the spike is
in the sheath. In four-rowed barley the lateral rows overlap in
such a way as to form a single row instead of two rows at each
edge of the rachis, as in the normal six-rowed varieties. Fruwirth
(1909) observed natural crosses in four-rowed barleys and
concluded there was practically no crossing in six-rowed forms.
He records the observations of Rimpau, who noted only eight
suspected natural crosses in barley after growing 40 varieties side
by side for a period of eight years. Harlan, after several years'
observation at University Farm, Minn, noted only two or three
natural crosses. Barley probably, therefore, crosses much less
frequently than does wheat.
Oats. The form of the individual flower of oats is very similar
to that of wheat and barley. Tschermak (1901) reports four
natural crosses observed by Rimpau, and Fruwirth (1909) records
five or six crosses observed by Rimpau after cultivating 19
varieties side by side for eight years. A natural cross between
a variety of Avena sterilis and A . nuda was noted by Pridham in
1916. These facts and numerous statements by breeders as to
self-fertilization show that natural crossing occurs much less
frequently in oats than in wheat.
Tobacco. In the tobacco plant the flowers are frequently
visited by insects and some natural crossing doubtless takes
place. As a rule only one variety of tobacco is grown in a locality.
Howard and others (1910 b, c), in India, concluded that there is
between 2 and 3 per cent, crossing in tobacco. They emphasize
the necessity of producing artificially self-fertilized seed. In
breeding experiments, artificially selfed seed is generally used
and therefore few records regarding the degree of cross-pollination
are available. As it is so easy artificially to self-fertilize tobacco
and as each flower produces many seeds (98,910 seeds per plant,
Jenkins, 1914) the amount of natural cross-pollination is of little
"*
breeding importance.
Flax. The flax flower, like the tobacco flower, is frequently