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.

In the course of our studies we have also obtained certain results that have found immediate application in crop production. An example is the work with dark-period interruptions to prevent flowering.

For many years chrysanthemum growers have extended the natural photoperiods of late summer and autumn to delay flowering of certain varieties. To do so they used artificial light for several hours each night, either immediately following sunset or just before sunrise. By beginning this treatment in the middle of the summer, and continuing it regularly the remainder of the summer and early fall, they found they could keep their chrysanthemums in greenhouses from flowering until late fall or early winter.

Just as brief dark-period interruptions prevented the flowering of Biloxi soybean, so it seemed probable they might have a similar effect on chrysanthemums. The method Was tested and was found better in certain respects than the previous method of applying several hours of artificial light at the beginning or end of the day. With this method of delaying flowering by dark-period interruption, the stems of the plants did not elongate so much as with the former method. When the plants were finally permitted to bloom, following discontinuation of the interruptions, the flowers produced were of exceptionally fine quality. The individual flower stalks were short, so that the entire inflorescence was compact, a feature that added to the beauty of the flowers. Florists have used the procedure to postpone the flowering of several varieties of chrysanthemums, for many weeks. Because some early varieties are more desirable than the late ones, the procedure served to make them available at times of the year when there would otherwise be no equally satisfactory types. The practice also allows growers to time the dates of the flower harvest to synchronize with special holiday markets.

Indeed, several growers of chrysanthemums have started work on a plan whereby they can grow chrysanthemums the year around. Such a development depends upon the photoperiodic response of the plants; it Will involve dark-period interruptions to delay blooming under some conditions, and artificial shortening of the day by shading to accelerate blooming under other conditions.

THE AUTHOR

H. A. Borthwick, a graduate of Stanford University, is a senior botanist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He has concentrated Or, studies of photoperiodism for about 10 years.