Experiments were carried out to determine what degree of intervarietal cross-pollination occurs when two radish varieties are grown at varying distances apart. The varieties chosen were Icicle and Scarlet Globe. Hybrids are distinctive; thus crossing is readily determined.In closely planted experiments, where one half of the plot was Icicle and the other half Scarlet Globe, the amount of intercrossing ranged from 30 to 40% at a distance of 9 in. to an average of about I yo at I j ft. Intercrossing decreased fairly regularly and rapidly with distance.Single plants of Icicle were run out from plots of Scarlet Globe; in 1941 at distances from 9 in. to 9j ft. and in 1942 from 9 in. to 408 ft. In the plants near the red plots crossing was approumately IQO 7;. In 1941 there was a regular and rapid fall wifh distance and intervarietal crossing stopped at 95 ft. In 1942 the plots were closer to beehives; intercrossing was less regular and extended farther than in 1941, but beyond 240 ft. it was only 0 1 yo, and in an all-white plot 360 ft. from a red plot with no intervening stringer plants it was nil.Given a mass of plants and a profusion of flowers, bees confind their flower visiting to a small area. When radishes are grown in quantity 300 ft. guards against contamination of stock due to intervarietal crossing. If only small numbers are grown intercrossing is likely to occur over a greater distance.T w o kinds of practical problems arise in the pollination of crop plants : (I) in crops which are propagated vegetatively, such as fruit trees, where owing to the prevalence of self-and cross-incompatibility it is necessary to plant different varieties of the right kinds together to ensure cross-pollination; (2) in crops raised and grown for seeds, such as Brassicae, carrots, etc., where to maintain purity of stock different kinds and varieties must be grown apart to prevent cross-pollination.With vegetable seed crops common questions are : What kinds readily cross together? How far must different kinds or varieties be grown apart to prevent the occurrence of cross-pollination? Precise answers are difficult owing to lack of information. Experiments were carried out to determine what degree of natural cross-pollination takes place when different varieties of radish are planted at varying distances apart. The results are given in this report.
MATERIALSThe radish proved to be a convenient plant as it has a number of distinct varieties, seedlugs grow rapidly and large numbers can be grown on a small area of land, and with glasshouse space available during the winter months, seedlings can be grown and their root characters recorded at almost any time of the year.The varieties chosen were Icicle with white, long, tapering roots, and Scarlet Globe, with deep red, short, round roots. These varieties are distinct and htercrossing is clearly indicated by the root characten : crossed seedlings have purple-coloured, intermediate, conical-shaped roots, no matter which App. Biol. 30 variety was the female parent. Scarlet Globe and Icicle...
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