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 inter- 

 planted 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-polli- 

 nated, 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 F\. 



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 repro- 

 duction. Fruwirth (1909) found a setting of 24 per cent, in un- 

 enclosed 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 



