RIMULA

In the Primrose and Cowslip, again, we find quite a different plan. It
had long been known that if a number of Cowslips or Primroses are
examined, about half would be found to have the stigma at the top of the
tube and the stamens half way down, while in the other half the stamens
are at the top and the stigma half way down. These two forms are about
equally numerous, but never occur on the same stock. They have been long
known to children and gardeners, who call them thrum-eyed and pin-eyed.
Mr. Darwin was the first to explain the significance of this curious
difference. It cost him several years of patient labour, but when once
pointed out it is sufficiently obvious. An insect thrusting its
proboscis down a primrose of the long-styled form (Fig. 12) would dust
its proboscis at a part (_a_) which, when it visited a short-styled
flower (Fig. 13), would come just opposite the head of the pistil
(_st_), and could not fail to deposit some of the pollen on the stigma.
Conversely, an insect visiting a short-styled plant would dust its
proboscis at a part farther from the tip; which, when the insect
subsequently visited a long-styled flower, would again come just
opposite to the head of the pistil. Hence we see that by this beautiful
arrangement insects must carry the pollen of the long-styled form to the
short-styled, and _vice versa_.

The economy of pollen is not the only advantage which plants derive from
these visits of Insects. A second and scarcely less important is that
they tend to secure "cross fertilisation"; that is to say, that the seed
shall be fertilised by pollen from another plant. The fact that "cross
fertilisation" is of advantage to the plant doubtless also explains the
curious arrangement that in many plants the stamen and pistil do not
mature at the same time--the former having shed their pollen before the
pistil is mature; or, which happens less often, the pistil having
withered before the pollen is ripe. In most Geraniums, Pinks, etc., for
instance, and many allied species, the stamens ripen first, and are
followed after an interval by the pistil.

THE NOTTINGHAM CATCHFLY

The Nottingham Catchfly (Silene nutans) is a very interesting case. The
flower is adapted to be fertilised by Moths. Accordingly it opens
towards evening, and as is generally the case with such flowers, is pale
in colour, and sweet-scented. There are two sets of stamens, five in
each set. The first evening that the flow