June 14,1924 
Influence of Environment on Sex in Hemp 
1079 
of “sex reversal” in one plat which was grown in the greenhouse during the short 
days of winter, but when we consider that the plat contained only 16 plants, we 
must be careful in drawing the conclusion that the same percentage of plants 
in a much larger plat would show the same changes. Until it is possible to 
obtain the same degree of sex modification in several plats grown under the same 
conditions, only tentative conclusions can be drawn concerning the exact control 
of sexual expression in hemp. It may well be that the genetic balance in different 
hemp plants is such that a unit change in the environmental factors will pro¬ 
duce more visible change in the sex of some than of others. That this balance 
can be upset and various sexual changes produced by varying the length of 
daily exposure to light there can be no doubt; but to say that any desired degree 
of sexual change can be obtained by this means is quite another thing. 
The general term “sexual reversal” which has been used by investigators to 
designate a modification of sexual expression is not entirely a satisfactory one. 
Pritchard { 4 ) and Schaffner ( 6 , 7 ) have used the term to include all those plants 
which were first either pure carpellate or pure staminate, but which later pro¬ 
duced some flowers of the opposite sex. In the majority of cases these secondary 
flowers are intersexual and in many cases entirely sterile. Such cases, in the 
strict sense of the word, are only modifications of the sex of a few flowers. A 
plant can not be said to have completely reversed its sex until it produces exclu¬ 
sively flowers of the opposite sex. It will be recognized that such a condition 
is rather rare in any of the so-called dioecious species. Schaffner (5, 6 , 7 ) has 
undoubtedly obtained many instances of modification of the sexual expression 
but not many cases of “sex reversal” in the sense that the plants which were 
one sex at first produced nothing but flowers of the opposite sex later in their 
life. A few cases which approach this degree of sex change have been obtained 
by the writer, however. At least two of the plants which were at first apparently 
pure staminate, later in their lives produced nothing but carpellate flowers and 
matured seed. Such cases are as near a complete reversal of sex as probably 
can be obtained in hemp. 
At the present time so little is known concerning the genetic complex of 
dioecious species of plants that it is not easy to draw conclusions concerning the 
behavior of the various intersex types. It is known that sex in hemp is some¬ 
how influenced by the environment, especially by the relative length of day and 
night, and also known that the time of flowering of this species is almost 
entirely controlled, within certain limits, by this same factor. But since the 
dimorphic vegetative types occur in about equal numbers regardless of the 
environment there must be something genetic concerned and the fact that one 
or both of these types later in life produce flowers of the opposite sex, is not 
necessarily a valid argument that genetic factors are not concerned. Many 
characters which are genetic in nature are affected by various environmental 
factors and there is no reason why sex can not be affected likewise. But since 
the genetics of this particular case is to be published in a later paper, further 
discussion of the point will not be entered into here. 
SUMMARY 
1. Hemp possesses a distinct vegetative and sexual dimorphism, but either 
sex may, under certain conditions, produce flowers of the opposite sex. The 
vegetative differences are the same for the plants which remain sexually pure 
and those which do not. 
2. The ultimate height attained by hemp appears to be due largely to the 
length of the period of rapid rate of growth. This period of rapid rate of growth 
seems to be roughly proportional to the length of daily exposure to light. In a 
long daily light exposure the high growth rate extends over a long period and 
the plants are tall at maturity, but in a short daily exposure to light the period 
