me plant may bear flowers of
three kinds, viz. male, female, and hermaphrodite. Or it may be that these
three forms occur in the same species but in different individuals--female
and hermaphrodites in one species; males, females, and hermaphrodites in
another. One case, however, must be mentioned as it suggests a possibility
which we have not hitherto encountered. In the common English bryony
(_Bryonia dioica_) the sexes are separate, some plants having only male and
others only female flowers. In another European species, _B. alba_, both
male and female flowers occur on the same plant. Correns crossed these two
species reciprocally, and also fertilised _B. dioica_ by its own male with
the following results:--

{121}

dioica [female] x dioica [male] gave [female] [female] and [male] [male]
" x alba [male] " [female] [female] only
alba [female] x dioica [male] " [female] [female] and [male] [male].

The point of chief interest lies in the striking difference shown by the
reciprocal crosses between _dioica_ and _alba_. Males appear when _alba_ is
used as the female parent but not when the female _dioica_ is crossed by
male _alba_. It is possible to suggest more than one scheme to cover these
facts, but we may confine ourselves here to that which seems most in accord
with the general trend of other cases. We will suppose that in _dioica_
femaleness is dominant to maleness, and that the female is heterozygous for
this additional factor. In this species, then, the female produces equal
numbers of ovules with and without the female factor, while this factor is
absent in all the pollen grains. _Alba_ [female] x _dioica_ [male] gives
the same result as _dioica_ [female] x _dioica_ [male], and we must
therefore suppose that alba produces male and female ovules in equal
numbers. _Alba_ [male] x _dioica_ [female], however, gives nothing but
females. Unless, therefore, we assume that there is selective fertilisation
we must suppose that all the pollen grains of alba carry the female
factor--in other words, that so far as the sex factors are concerned there
is a difference between the ovules and pollen grains borne by the same
plant. Unfortunately further investigation of this case is rendered
impossible owing to the complete sterility of the F_1 plants. {122}

[Illustration: FIG. 27.

Single and double stocks raised from the same single parent.]

That the possibility of a difference between the ov