THE INFLUENCE OF ENVIRONMENT ON SEX IN HEMP, 
CANNABIS SATIVA L. 1 
By Hugh C. McPhee 
Animal Husbandman in Animal Genetics } Bureau of Animal Industry , United 
States Department of Agriculture 
INTRODUCTION 
Since environment apparently has such a potent influence on sexual expression 
in hemp, Cannabis sativa L., the writer found it necessary during the course of 
an investigation of the genetics of sex in this species, to study the effect of relative 
length of day and night upon vegetative and floral development. The problems 
connected with this phase of the work have been pursued only as far as was 
necessary to enable the major work to be carried on and the results are not to be 
considered as final from the physiological standpoint. But since the work has 
been discontinued it seems desirable to report the results that they may be 
available to others who are interested in problems of this kind. 
Physiological studies similar to those reported here have been pursued by 
several investigators, especially Garner and Allard (1, 2, 8) 2 Since they have 
reviewed the literature which pertains to this phase of physiology, it seems un¬ 
necessary to attempt further review here except to mention Schaffner's work 
on hemp. Schaffner (£, 6 , 7) found that apparently it is possible to control 
sex in hemp. When grown during the long days of summer the sexes remain 
pure and the ratio of staminate to carpellate plants is approximately 1:1. When 
grown in the greenhouse during the short days of winter, however, both car- 
pellate and staminate plants showed reversal to the opposite sex. Further¬ 
more, the proportion of plants which showed sex reversal seemed to be roughly 
proportional to the length of the darkness period. In the shortest days of winter 
some of the plats showed almost 100 per cent of intersex plants. Schaffner 
therefore maintains that sex in hemp is non-Mendelian in nature and is under 
the control of environmental factors. 
SEXUAL DIMORPHISM 
When grown under normal conditions hemp is when mature dimorphic in 
both vegetative and flower characters. When grown in the greenhouse during 
the winter the dimorphism is even more pronounced. The principal sexual 
differences are as follows: 
Staminate plant: More slender and taller than the carpellate plant because of 
the rapid elongation of the internodes just prior to anthesis; terminal inflores¬ 
cences with practically no leaves; flowers normally with five sepals and as many 
anthers; much shorter life than the carpellate type (PI. 1). 
i Received for publication March 26, 1924—issued November, 1924. The results reported in this paper 
were obtained at the Laboratory of Plant Genetics, Bussey Institution of Harvard University, and 
completed in the U. S. Department of Agriculture. 
* Reference is made by number (italic) to lt Literature cited,” p. 1080. 
Journal of Agricultural Research, 
Washington, D. C. 
(1067) 
Vol. XXVIII, No. 11 
June 14,1924 
Key No. G-431 
