stigmas; and this
stigma, perhaps in consequence of the copious secretion, is rendered
sterile. When an insect visits a flower of this kind, it rubs off some
of the viscid matter, and thus at the same time drags away some of the
pollen-grains. From this simple condition, which differs but
little from that of a multitude of common flowers, there are endless
gradations--to species in which the pollen-mass terminates in a very
short, free caudicle--to others in which the caudicle becomes firmly
attached to the viscid matter, with the sterile stigma itself much
modified. In this latter case we have a pollinium in its most highly
developed and perfect condition. He who will carefully examine the
flowers of orchids for himself will not deny the existence of the above
series of gradations--from a mass of pollen-grains merely tied together
by threads, with the stigma differing but little from that of the
ordinary flowers, to a highly complex pollinium, admirably adapted for
transportal by insects; nor will he deny that all the gradations in
the several species are admirably adapted in relation to the general
structure of each flower for its fertilisation by different insects. In
this, and in almost every other case, the enquiry may be pushed further
backwards; and it may be asked how did the stigma of an ordinary flower
become viscid, but as we do not know the full history of any one
group of beings, it is as useless to ask, as it is hopeless to attempt
answering, such questions.

We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those which I
have called leaf-climbers, and to those provided with tendrils. In these
two latter classes the stems have generally, but not always, lost the
power of twining, though they retain the power of revolving, which the
tendrils likewise possess. The gradations from leaf-climbers to tendril
bearers are wonderfully close, and certain plants may be differently
placed in either class. But in ascending the series from simple twiners
to leaf-climbers, an important quality is added, namely sensitiveness
to a touch, by which means the foot-stalks of the leaves or flowers, or
these modified and converted into tendrils, are excited to bend round
and clasp the touching object. He who will read my memoir on these
plants will, I think, admit that all the many gradations in function and
structure between simple twiners and tendril-