all these three factors to produce the hoary character, though
how this comes about we have not at present the least idea.

[Illustration: FIG. 9.

Sections of primula flowers. The anthers are shown as black. A, "pin" form
with long style and anthers set low down; B, "thrum" form with short style
and anthers set higher up; C, homostyle form with anthers set low down as
in "pin," but with short style. This form only occurs with the large eye.]

[Illustration: FIG. 10.

Two primula flowers showing the extent of the small and of the large eye.]

A somewhat different and less usual form of interaction between factors may
be illustrated by a case in primulas recently worked out by Bateson and
Gregory. Like the common primrose, the primula exhibits both pin-eyed and
thrum-eyed varieties. In the former the style is long, and the centre of
the eye is formed by the end of the stigma which more or less plugs up the
opening of the corolla (cf. Fig. 9, A); in the latter the style is short
and hidden by the four anthers which spring from higher up in the corolla
and form the centre of the eye (cf. Fig. 9, B). The greater part of the
"eye" is formed by the greenish-yellow patches on each petal just at the
opening {56} of the corolla. In most primulas the eye is small, but there
are some in which it is large and extends as a flush over a considerable
part of the petals (Fig. 10). Experiments showed that these two pairs of
characters behave in simple Mendelian fashion, short style ( = "thrum")
being dominant to long style (= "pin") and small eye dominant to large.
Besides the normal long and short styled forms, there occurs a third form,
which has been termed homostyle. In this form the anthers are placed low
down in the corolla tube as they are in the long-styled form, but the style
remains short instead of reaching up to the corolla opening (Fig. 9, C). In
the course of their experiments Bateson and Gregory crossed a large-eyed
homostyle plant with a small-eyed thrum ( = short style). The F_1 plants
were all short styled with small eyes. {57} On self-fertilisation these
gave an F_2 generation consisting of four types, viz. short styled with
small eyes, short styled with large eyes, _long styled_ with small eyes,
and _homostyled_ with large eyes. The notable feature of this generation is
the appearance of long-styled plants, which, however, occur only in
association with the small eye. The proportions in which these four types
appe