THE GENETICS OF SEX IN HEMP 1 
By Hugh C. McPhee 
Associate Animal Husbandman in Animal Genetics , Animal Husbandry Division, 
Bureau of Animal Industry , United States Department of Agriculture 
INTRODUCTION 
In a recent paper (7) 2 the writer showed that the relative length 
of day and night exerts a profound influence on the development of 
both vegetative and floral parts of hemp. The results show that 
the length of time between planting and flowering in this species may 
vary from about 30 days to 90 days or more, depending on the rela¬ 
tive length of day and night, other conditions being optimum for 
growth. The effect on the flowers was both quantitative and quali¬ 
tative. When the period of daylight was long the plants were either 
pure carpellate or pure staminate, but when the period of daylight 
was short a number of intersex types appeared. Since the carpellate 
and the staminate flowers are normally produced on separate plants, 
such behavior is important because of its bearing on the mechanism 
of sex determination in this as well as in other dioecious species of 
plants. 
There is no valid reason at present for doubting that environ¬ 
mental factors are largely responsible for the production of intersex 
types when hemp is grown in hothouses during the winter months. 
The results obtained by Sehaffner ( 10 , 11, 12) and by the writer (7) 
show quite clearly that individuals of either sex may produce flowers 
of the opposite sex when grown under such conditions. Sometimes 
such flowers are normal, but more often they are abnormal and 
sterile. The peculiarity of the case lies in the fact that some in¬ 
dividual plants are affected much more than others grown under the 
same conditions and at the same time. In fact, some individuals do 
not produce flowers of the opposite sex under any conditions. The 
other extreme is reached in a case reported by the writer (7) in which 
a staminate-type plant Matured several seeds. This is truly a 
reversal of the sexual function of the individual. This behavior, 
however, has led some botanists to suggest that sex in hemp is there¬ 
fore nonmendelian in nature and can not be explained by a chromo¬ 
somal mechanism. With such a suggestion the writer can not fully 
agree. The mere fact that the normal sexual expression of the indi¬ 
vidual is modified by certain factors does not prove that there is not 
a definite mechanism which is responsible for its development. It is 
merely the manifestation of the effect on the mechanism. 
The point at issue, then, is whether or not the phenomenon^ sex 
in hemp can be explained on a genetic basis. • It would seem that 
the solution of this problem can be brought about only by a series of 
1 Received for publication March 20, 1925; issued January, 1926. The results reported in this paper 
were obtained at the Laboratory of Plant Genetics, Bussey Institution of Harvard University, and com¬ 
pleted in the United States Department of Agriculture. 
2 Reference is made by number (italic) to “Literature cited,” p. 942. 
J ournal of Agricultural Research, 
Washington, D. C. 
( 935 ) 
Vol. XXXI, No 10 
Nov. 15, 1925 
Key No. A-102 
