INTRODUCTIONThe experience of geneticists and plant breeders who have worked with beans (Vicia Faba L.) is that they set seed readily enough by enforced self-pollination, but if they have opportunities for cross-pollination they do so to a troublesome extent. Sirks (1931) has observed that the earlier workers with beans disagreed as to the amount of vicinism and suggests, probably correctly, that it varies from place to place and from season to season. His own observations led to the conclusion that natural crosses within the major group (the large-seeded varieties) were not rare and within the minor group (the small-seeded varieties) were fairly frequent. He did not, however, find any natural hybrids between major and minor; emasculation followed by artificial cross-pollination could give 50 % or more success in crosses within the groups, but only about 8 % in crosses between major and minor.If information about cross-breeding is needed with a view to controlling vicinism, probably the most useful estimates are to be obtained from systematic plantings of contrasting types giving a distinctive hybrid. Shen (1947), working in Szechuan, arrived at an estimate of 24% natural crossing by this method.For the planning of selection work an estimate is needed, referring as closely as possible to the breeding relations within a population. The purpose of the present contribution is to show how to obtain the required information from the progenies of plants which have been subject to the natural pollination within a farm crop. Some attention will be given to the bearing of the results on selection procedure, but a fuller discussion of this aspect will be reserved until a later paper.

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SummaryCrosses between nine inbred lines of diploid doublecut red clover were measured for corolla tube length and nectar height, caged with honey-bees, and observations made on the number of bee visits and seed yield. There was evidence of additive genetic variation in nectar height, corolla tube length, bee visits and seed yield. The possibilities for improvement in these characters by selection are discussed. Attention is drawn to a remarkable discriminating behaviour of honey-bees, in that they could apparently recognize plants which had only one parent in common. A cage with a strong colony of honey-bees produced more than twice as much seed as a cage with a weak colony, and plants near to hives gave significantly more seed than plants distant from hives. Plants of English origin tended to have higher and more accessible nectar, more bee visits and greater seed yield than plants of continental origin.

Previous papers in this series described (Bond, Drayner, Fyfe & Toynbee-Clarke, 1964a) a type of male sterility in beans, segregating as a simple Mendelian recessive and showed (19646) that very good yields could be obtained from hybrids produced with its aid. With that type of male sterility, hybrid seed could only be grown on an experimental scale, because it had not been possible to produce progenies with more than 50 % male-sterile plants.The present paper describes another form of male sterility, apparently involving cytoplasmic inheritance and showing more promise for commercial exploitation.Discovery.-The male sterility was first discovered at King's College, Newcastle upon Tyne when, by the use of hilum colour markers it was shown that one plant underwent a much higher degree of natural crossing, under open pollination, than a group of fifty plants drawn from the same population (Bond, 1957). Subsequent examination of the progeny of this plant showed that the anthers were shrunken and released very little pollen.Description.-Pollen grains from male-sterile plants did not stain as deeply in acetocarmine as pollen grains from normal plants and none germinated in sugar solution. Seed setting following self-pollination of such plants was rare, but cross-pollinations produced normal amounts of pods and seeds. Glasshouse environments appeared to favour the development of pollen but even under these conditions selffertilization was difficult to obtain.No effect of the environment on the proportion of fertile plants in progenies of male-steriles could be detected. When progenies were divided into autumnsown and spring-sown portions, the date of sowing had no significant effect on the proportion of fertile plants. Similarly, plant density, position in the field and date of observation had no detectable effect on the degree of sterility of progenies.A small proportion of male-sterile plants had one or more completely fertile tillers. Such plants are described in this paper as tiller-sterile or partlyfertile, the term semi-sterile being reserved for plants with all flowers producing small quantities of viable pollen.

1. A method of producing hybrid beans with the aid of a cytoplasmic male-sterile is described.2. Most of the requirements of large scale production of hybrid seed can be met: use of genetic markers showed that the male sterility allowed 100% cross fertilization; there was no evidence of a deficiency of insect pollinators near Cambridge; and fertility restoring lines are available.3. The importance of being able to maintain a high level of sterility in the female parent line is emphasized. Methods of improving the proportion of sterile plants in the sterile phase of an inbred are discussed. The tolerable level of fertility in progenies of sterile parents depends on the level of sterility necessary in the female parent of the final cross. Five inbreds are being handled with less than 2% of fertile plants in their sterile phase.