SEXUAL INEQUALITY IN HEMP 



Male Plants Die While Females Grow Vigorously Under Same Conditions- 

 Possible Sex-Limited Environmental Character Suggested — Advantage to 

 Females of Death of Males — Reversal of Usual Effects of Selection. 



0. F. Cook, 

 Bureau of Plant Industry, U. S. Department of Agriculture, Washington, D. C. 



A STRIKING example of sexual 

 inequality was noticed in August, 

 1912, in a plot of hemp that 

 was being grown on the Vir- 

 ginia Truck Experiment Station, near 

 Norfolk. The nature of the inequality 

 will be apparent by reference to figures 

 9 and 10. AH of the male plants were 

 very slender and spindling, and the 

 foliage was of a pale yellowish green 

 color, in strong contrast with the deep 

 blackish green of the female plants. In 

 addition to their more robust form, the 

 female plants were often a foot taller 

 than adjacent males, the largest of the 

 female plants attaining about three 

 feet. Many of the male plants had 

 already died at the time the photo- 

 graphs were taken, August 13, and the 

 others were evidently to follow shortly. 

 But all of the female plants were still 

 fresh and vigorous, and were growing 

 and flowering under conditions that were 

 bringing the male plants to an early 

 death. 



To describe the female plants as 

 stronger and more resistant than the 

 male plants seems a rather inadequate 

 statement of so great an inequality 

 of behavior. If two species were tested 

 and were found to react so differently 

 we woiild say that one of them was 

 adapted to the conditions, and that 

 some essential character was lacking in 

 the other. With two sexes of the same 

 plant behaving so differently, the idea 

 of a sex-limited environmental character 

 is suggested. Certainly there is no 

 general reason or analogy for believing 

 that sex alone would explain so great 

 a difference of behavior under the same 

 conditions. 



In the perennial plants that have the 

 sexes represented by separate individ- 

 uals, as among the dates, figs, carobs, 

 poplars, rubber trees, etc., the male 

 individuals appear to have greater 

 vigor and longevity than the females. 

 No doubt it would be an advantage in 

 an annual plant like the hemp to have 

 the males die out after they have shed 

 their poUen. The early death of the 

 males means less competition with the 

 adjacent female plants while these are 

 engaged in maturing seed. The more 

 unfavorable the conditions the more 

 desirable that the males die early in 

 order to increase the chances that the 

 seed be matured. But the advantage of 

 having the males die earlier would 'not 

 of itself cause them to die. Some other 

 specialization or intensification of the 

 sexual differences must have arisen. On 

 account of the factor of competition 

 between the sexes the usual effects of 

 selection would be reversed. Under 

 extreme conditions short-lived male 

 plants would be likely to leave larger 

 progenies than long-hved males, for the 

 female plants that stood next to short- 

 lived males would be able to ripen more 

 seed. The natural result of such varia- 

 tion and selection would be the devel- 

 opment of a plant with short-lived, 

 ephemeral males corresponding to drone 

 bees and similar specializations of the 

 sexes among insects and other inverte- 

 brate animals. 



WHY THE PLANTS DIE. 



It would be interesting to determine 

 the extent to which the presence or the 

 competition of the female plants may 

 be responsible for the early death of the 

 male plants. This could be learned by 



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