Of the five thousand species known, the great majority are inhabitants
of the tropics, but nevertheless there are within the United States a
number of very beautiful forms. The largest and showiest are the
lady's-slippers, of which we have six species at the north. The most
beautiful is the showy lady's-slipper (_Cypripedium spectabile_),
whose large, pink and white flowers rival in beauty many of the
choicest tropical orchids. Many of the _Habenarias_, including the
yellow and purple fringed orchids, are strikingly beautiful as are the
_Arethuseae_ (_Arethusa_, _Pogonia_, _Calopogon_). The last of these
(Fig. 90, _H_) differs from all our other native orchids in having the
ovary untwisted so that the labellum lies on the upper side of the
flower.

A number of the orchids are saprophytic, growing in soil rich in
decaying vegetable matter, and these forms are often nearly or quite
destitute of chlorophyll, being brownish or yellowish in color, and
with rudimentary leaves. The coral roots (_Corallorhiza_), of which
there are several species, are examples of these, and another closely
related form, the putty-root (_Aplectrum_) (Fig. 90, _A_), has the
flowering stems like those of _Corallorhiza_, but there is a single,
large, plaited leaf sent up later.

ORDER VII.--_Helobiae_.

The last order of the monocotyledons is composed of marsh or water
plants, some of which recall certain of the dicotyledons. Of the three
families, the first, _Juncagineae_, includes a few inconspicuous plants
with grass-like or rush-like leaves, and small, greenish or yellowish
flowers (_e.g._ arrow-grass, _Triglochin_).

The second family (_Alismaceae_) contains several large and showy
species, inhabitants of marshes. Of these the water-plantain
(_Alisma_), a plant with long-stalked, oval, ribbed leaves, and a
much-branched panicle of small, white flowers, is very common in
marshes and ditches, and the various species of arrowhead
(_Sagittaria_) are among the most characteristic of our marsh plants.
The flowers are unisexual; the female flowers are usually borne at the
base of the inflorescence, and the male flowers above. The gynoecium
(Fig. 91, _B_) consists of numerous, separate carpels attached to a
globular receptacle. The sepals are green and much smaller than the
white petals. The leaves (_F_) are broad, and, besides the thickened,
parallel veins, have numerous smaller ones connecting these.

[Illustration: FIG. 91.--Types of _H