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. Fru- 

 wirth (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 



