I festation of heterosis k n o w n as h y b r i d vigor, has received increased attention from cotton breeders. Heterosis was first utilized in obtaining increased production in asexually propagated plants. Through the use of malesterility, gametocides, and imperfect-flowered plants, heterosis is now used to advantage in many species. Considerable evidence exists (4) t h a t heterosis is expressed in crosses between Gossypz'um hiisutum (Upland cotton) and G. barb~rdeizse (e x t r a-l o n g staple cotton). There are indications that heterosis may exist also in crosses among different varieties of Upland cotton and that varieties may vary considerably in combining ability. Data from crosses among three varieties of Upland cotton, in a study begun in 1754 at the Georgia Experiment Station, indicated that appreciable heterosis existed in i n t e r va r i e t a1 crosses. Becauses of these favorable results, a larger study designed to obtain further information on heterosis and combining ability of other varieties was begun in 1756. The purpose of this paper is to report the magnitude of the heterotic effect in crosses a m o n g four varieties of Upland cotton. REVIEW OF LITERATURE Loden and Richmond (4) presented a thorough review of work on heterosis in cotton. One of their conclusions was that the maximum heterotic effect was obtained in the F1 generation and few significant increases in yield occurred in subsequent generations. Ware (8) , working with crosses between G. hirszLtum and G. b~zrbade~re, found that heterosis occurred in the F, generation of species crosses in cotton but was not expressed for all plant charackers. In discussing heterosis in intraspecific crosses, Kime and Tilley (3) reported that the general assumption had been made that heterosis did not occur in crosses within cotton species. However, Brown (1) reported that F, hybrids were frequently larger, more vigorous, and more productive than their parents. Loden and Richmond (4) were of the opin
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