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.
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 Arethuseæ (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.—Helobiæ.
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, Juncagineæ, 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 (Alismaceæ) 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 gynœcium ([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.
Fig. 91.—Types of Helobiæ. A, inflorescence of arrowhead (Sagittaria), with a single female flower, × ½ (Alismaceæ). B, section through the gynœcium, showing the numerous single carpels, × 3. C, a ripe fruit, × 3. D, a male flower, × 1. E, a single stamen, × 3. F, a leaf of Sagittaria variabilis, × ⅙. G, ditch-moss (Elodea), with a female flower (fl.), × ½. (Hydrocharideæ). H, the flower, × 2. an. the rudimentary stamens. st. the stigma. I, cross-section of the ovary, × 4. J, male inflorescence of eel-grass (Vallisneria), × 1. K, a single expanded male flower, × 12. st. the stamen. L, a female flower, × 1. gy. the stigma.