ts of perigone
leaves, three in each, and these are usually much alike except the
lower (through the twisting of the ovary) of the inner set. This
petal, known as the "lip" or "labellum," is usually larger than the
others, and different in color, as well as being frequently of
peculiar shape. In many of them it is also prolonged backward in a
hollow spur (see Fig. 89, _B_). In all of the orchids except the
lady's-slippers (_Cypripedium_) (Fig. 90, _B_), only one perfect
stamen is developed, and this is united with the three styles to form
a special structure known, as the "column" or "gynostemium" (Fig. 89,
_B_, _C_). The pollen spores are usually aggregated into two or four
waxy masses ("pollinia," sing. pollinium), which usually can only be
removed by the agency of insects upon which all but a very few orchids
are absolutely dependent for the pollination of the flowers.

[Illustration: FIG. 90.--Forms of _Orchideae_. _A_, putty-root
(_Aplectrum_), x 1. _B_, yellow lady's-slipper (_Cypripedium_), x 1/2.
_C_, the column of the same, x 1. _an._ one of the two perfect
stamens. _st._ sterile, petal-like stamen. _gy._. stigma. _D_,
_Arethusa_, x 1/2. _E_, section of the column, x 1: _an._ stamen. _gy._
stigma. _F_, the same, seen from in front. _G_, _Habenaria_, x 1. _H_,
_Calopogon_, x 1. In the last the ovary is not twisted, so that the
lip (_L_) lies on the upper side of the flower.]

In the lady-slippers there are two fertile stamens, and a third
sterile one has the form of a large triangular shield terminating the
column (Fig. 90, _C_, _st._).

The ovules of the orchids are extremely small, and are only partly
developed at the time the flower opens, the pollen tube growing very
slowly and the ovules maturing as it grows down through the tissues of
the column. The ripe seeds are excessively numerous, but so fine as to
look like dust.

The orchids are mostly small or moderate-sized plants, few of them
being more than a metre or so in height. All of our native species,
with the exception of a few from the extreme south, grow from fibrous
roots or tubers, but many tropical orchids, as is well known, are
"epiphytes"; that is, they grow upon the trunks and branches of trees.
One genus, _Vanilla_, is a twining epiphyte; the fruit of this plant
furnishes the vanilla of commerce. Aside from this plant, the
economical value of the orchids is small, although a few of them are
used medicinally, but are not specially valuable.