this peculiar form? What regulates the length of
the tube? What is the use of the arch? What lesson do the little teeth
teach us? What advantage is the honey to the flower? Of what use is the
fringe of hairs? Why does the stigma project beyond the anthers? Why is
the corolla white, while the rest of the plant is green?

[Illustration: Fig. 7.]

[Illustration: Fig. 8.]

The honey of course serves to attract the Humble Bees by which the
flower is fertilised, and to which it is especially adapted; the white
colour makes the flower more conspicuous; the lower lip forms the stage
on which the Bees may alight; the length of the tube is adapted to that
of their proboscis; its narrowness and the fringe of fine hairs exclude
small insects which might rob the flower of its honey without performing
any service in return; the arched upper lip protects the stamens and
pistil, and prevents rain-drops from choking up the tube and washing
away the honey; the little teeth are, I believe, of no use to the
flower in its present condition, they are the last relics of lobes once
much larger, and still remaining so in some allied species, but which in
the Dead-nettle, being no longer of any use, are gradually disappearing;
the height of the arch has reference to the size of the Bee, being just
so much above the alighting stage that the Bee, while sucking the honey,
rubs its back against the hood and thus comes in contact first with the
stigma and then with the anthers, the pollen-grains from which adhere to
the hairs on the Bee's back, and are thus carried off to the next flower
which the Bee visits, when some of them are then licked off by the
viscid tip of the stigma.[20]

[Illustration: Fig. 9.]

[Illustration: Fig. 10.]

[Illustration: Fig. 11.]

In the Salvias, the common blue Salvia of our gardens, for instance,--a
plant allied to the Dead-nettle,--the flower (Fig. 9) is constructed on
the same plan, but the arch is much larger, so that the back of the Bee
does not nearly reach it. The stamens, however, have undergone a
remarkable modification. Two of them have become small and functionless.
In the other two the anthers or cells producing the pollen, which in
most flowers form together a round knob or head at the top of the
stamen, are separated by a long arm, which plays on the top of the
stamen as on a hinge. Of these two arms one hangs down into the tube,
closing the passage, while the other lies under the arched upper lip.