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Pubdate: 1947 Source: The Yearbook of Agriculture; 1943-1947, United States Department of Agriculture Author: H.A. Borthwick, USDA Senior Botanist in the Bureau of Plant Industry, Soils, and Agricultural Engineering Pages: 282-283

New practices that can be used in breeding hemp have resulted from recent photoperiodic studies. Hemp is a short-day plant. It flowers promptly when day lengths are less than 14 hours and very slowly or not at all when day lengths are greater than 14 hours. Under average field conditions about half of the plants are female and half are male. Under some circumstances, however, female plants, each of which may produce a very large number of female flowers, will also produce a few typical male flowers. The opposite condition occurs in the males; that is, a male plant will form an occasional female flower in addition to a great number of male flowers. This tendency for hemp plants to produce these extra flowers of the opposite sex is increased by subjecting them to photoperiods of 14 hours or less and to cool nights when flowers are being formed.

Most rapid progress in breeding plants can be made with those that can be self-pollinated. Hemp, obviously, can be self-pollinated only when it produces these occasional additional flowers of the opposite sex. The advantage of being able to increase their tendency to form these flowers is apparent.

Dr. Hugh C. McPhee of the Department made use of this behavior of hemp several years ago. He used the pollen of the male flowers that were produced in small numbers on certain of the female plants to self-pollinate those plants. When he grew the resulting seeds he made the important discovery that all of the plants were female. We have recently produced several thousand seeds in this way and have not obtained a single male plant, thus thoroughly confirming Dr. McPhee's observations.

Under greenhouse conditions, with proper control of temperature and day length, a very high percentage of female plants produced enough male flowers so that self-pollination could be effected, and in certain experimental lots of female plants produced out-of-doors in late summer when days were short and nights were cool, enough male flowers were formed so that natural pollination occurred and a quantity of pure "female" seed was produced. These results suggest that a locality can be found in which the conditions are favorable to the formation of these intersex male flowers on female plants in sufficient quantity that a good crop of seed could be obtained. If this could be done, a means would be at hand to produce commercial quantities of "female" seed, thereby enabling growers to produce a pure stand of female plants. Such pure stands would result in a more uniform fiber crop and eliminate certain harvest problems. The basis for developing such a procedure lies in finding in nature a combination of environmental factors similar to that which, under experimental conditions, has resulted in formation of abundant male flowers.