t a series of extraordinarily beautiful and remarkable adaptations
which ensure the transference of pollen by insects from one flower to
another. It is impossible to describe adequately in a few words the
wealth of facts contained in the Orchid book. A few examples may,
however, be quoted in illustration of the delicacy of the observations
and of the perspicuity employed in interpreting the facts.

The majority of orchids differ from other seed plants (with the
exception of the Asclepiads) in having no dust-like pollen. The pollen,
or more correctly, the pollen-tetrads, remain fastened together as
club-shaped pollinia usually borne on a slender pedicel. At the base of
the pedicel is a small viscid disc by which the pollinium is attached
to the head or proboscis of one of the insects which visit the flower.
Darwin demonstrated that in Orchis and other flowers the pedicel of
the pollinium, after its removal from the anther, undergoes a curving
movement. If the pollinium was originally vertical, after a time it
assumed a horizontal position. In the latter position, if the insect
visited another flower, the pollinium would exactly hit the sticky
stigmatic surface and thus effect fertilisation. The relation between
the behaviour of the viscid disc and the secretion of nectar by the
flower is especially remarkable. The flowers possess a spur which in
some species (e.g. Gymnadenia conopsea, Platanthera bifolia, etc.)
contains honey (nectar), which serves as an attractive bait for insects,
but in others (e.g. our native species of Orchis) the spur is empty.
Darwin held the opinion, confirmed by later investigations, that in the
case of flowers without honey the insects must penetrate the wall of the
nectarless spurs in order to obtain a nectar-like substance. The glands
behave differently in the nectar-bearing and in the nectarless flowers.
In the former they are so sticky that they at once adhere to the body of
the insect; in the nectarless flowers firm adherence only occurs after
the viscid disc has hardened. It is, therefore, adaptively of value
that the insects should be detained longer in the nectarless flowers (by
having to bore into the spur),--than in flowers in which the nectar is
freely exposed. "If this relation, on the one hand, between the viscid
matter requiring some little time to set hard, and the nectar being so
lodged that moths are delayed in getting it; and, on the other hand,
between the viscid matter being