the pollen from flower to flower.

In many species the pollen is, and no doubt it originally was in all,
carried by the air. In these cases the chance against any given grain of
pollen reaching the pistil of another flower of the same species is of
course very great, and the quantity of pollen required is therefore
immense.

In species where the pollen is wind-borne as in most of our trees--firs,
oaks, beech, ash, elm, etc., and many herbaceous plants, the flowers are
as a rule small and inconspicuous, greenish, and without either scent or
honey. Moreover, they generally flower early, so that the pollen may not
be intercepted by the leaves, but may have a better chance of reaching
another flower. And they produce an immense quantity of pollen, as
otherwise there would be little chance that any would reach the female
flower. Every one must have noticed the clouds of pollen produced by
the Scotch Fir. When, on the contrary, the pollen is carried by insects,
the quantity necessary is greatly reduced. Still it has been calculated
that a Peony flower produces between 3,000,000 and 4,000,000 pollen
grains; in the Dandelion, which is more specialised, the number is
reduced to about 250,000; while in such a flower as the Dead-nettle it
is still smaller.

The honey attracts the insects; while the scent and colour help them to
find the flowers, the scent being especially useful at night, which is
perhaps the reason why evening flowers are so sweet.

It is to insects, then, that flowers owe their beauty, scent, and
sweetness. Just as gardeners, by continual selection, have added so much
to the beauty of our gardens, so to the unconscious action of insects is
due the beauty, scent, and sweetness of the flowers of our woods and
fields.

Let us now apply these views to a few common flowers. Take, for
instance, the White Dead-nettle.

The corolla of this beautiful and familiar flower (Fig. 6) consists of
a narrow tube, somewhat expanded at the upper end (Fig. 7), where the
lower lobe forms a platform, on each side of which is a small projecting
tooth (Fig. 8, _m_). The upper portion of the corolla is an arched hood
(_co_), under which lie four anthers (_a a_), in pairs, while between
them, and projecting somewhat downwards, is the pointed pistil (_st_);
the tube at the lower part contains honey, and above the honey is a row
of hairs running round the tube.

[Illustration: Fig. 6--White Dead-nettle.]

Now, why has the flower