mens ripen first, discharging the pollen and withering before the
stigma is ready to receive it, _e.g._ willow-herb (Fig. 113, _D_),
campanula (Fig. 123, _A_, _D_), and pea; in the two latter, the pollen
is often shed before the flower opens. Not so frequently the stigmas
mature first, as in the plantain (Fig. 121, _G_).

In many flowers, the stamens, as they ripen, move so as to place
themselves directly before the entrance to the nectary, where they are
necessarily struck by any insect searching for honey; after the pollen
is shed, they move aside or bend downward, and their place is taken by
the pistil, so that an insect which has come from a younger flower
will strike the part of the body previously dusted with pollen against
the stigma, and deposit the pollen upon it. This arrangement is very
beautifully seen in the nasturtium and larkspur (Fig. 99, _J_).

The tubular flowers of the _Sympetalae_ are especially adapted for
pollination by insects with long tongues, like the bees and
butterflies, and in most of these flowers the relative position of the
stamens and pistil is such as to ensure cross-fertilization, which in
the majority of them appears to be absolutely dependent upon insect
aid.

The great orchid family is well known on account of the singular form
and brilliant colors of the flowers which have no equals in these
respects in the whole vegetable kingdom. As might be expected, there
are numerous contrivances for cross-fertilization among them, some of
which are so extraordinary as to be scarcely credible. With few
exceptions the pollen is so placed as to render its removal by insects
necessary. One of the simpler contrivances is readily studied in the
little spring-orchis (Fig. 89) or one of the _Habenarias_ (Fig. 90,
_G_). In the first, the two pollen masses taper below where each is
attached to a viscid disc which is covered by a delicate membrane.
These discs are so placed that when an insect enters the flower and
thrusts its tongue into the spur of the flower, its head is brought
against the membrane covering the discs, rupturing it so as to expose
the disc which adheres firmly to the head or tongue of the insect,
the substance composing the disc hardening like cement on exposure to
the air. As the insect withdraws its tongue, one or both of the pollen
masses are dragged out and carried away. The action of the insect may
be imitated by thrusting a small grass-stalk or some similar body into
the spu