The Orchids surpass every plant in the vegetable world, for the variety of means employed by nature to continue the race. The blossoms are, for the most part, at once both male and female, though some plants are diœcious, but, except in rare instances, as in that of the Bee Orchis, not a blossom can be fertilized without the aid of insects. The flowers of the Orchideæ are constructed upon a fixed plan, which can be traced through the innumerable variety of beautiful, singular, and often grotesque forms which they assume. A corresponding variety is exhibited in the admirable contrivances and adaptations which enable insects to detach the pollen from one blossom, and carry it to fertilize another blossom. [Illustration: Fig. 72. Orchis mascula:—A, side view of flower, with a portion cut away; B, section through one side of rostellum, with included disc and caudicle of pollinium; _a_, anther; _r_, rostellum; _d_, disc.] The British Orchideæ belong mainly to three natural groups: the Ophreæ, which comprise the Orchis, Ophrys, and other common Orchids; the Neotteæ, comprising Epipactis, Neottia, Spiranthes, &c.; and the Malaxeæ, represented by Malaxis. Orchids have white, fibrous roots, and many of them a pair of fleshy tubercles. From thence a straight stem springs up, ending in a spike of blossoms, each of which is attached to the stem by a twisted stalk, containing the ovarium, as represented in fig. 72. Ribbed lanceolate leaves rise from the roots, and some are attached sparingly to alternate sides of the stem. The calyx is formed of three pointed sepals, one perpendicular, the others horizontal. Instead of being green they are usually coloured. Sometimes the whole plant is green. One petal (fig. 72) is much larger than the others, occasionally assuming the most extraordinary forms. It is called the labellum, or lower lip; it secretes nectar, a sweet juice, to attract insects, and is often produced into a long, hollow, spur-like nectary, as in fig. 72 _n_. On that side of the spur which is opposite to the labellum, the organs of reproduction are so placed, that an insect alighting upon the labellum cannot insert its proboscis into the nectary tube to eat the honey, without touching them. [Illustration: Fig. 73. Orchis mascula:—Front view of flower, with sepals and petals removed; _a_, anther; _r_, rostellum; _s_, stigma; _l_, labellum.] Mr. Darwin, in his admirable work on the ‘Fertilization of Orchids,’ assumes the Orchis mascula as a type for explaining the mechanism of the reproductive organs of the Orchidaceæ generally. Fig. 73 represents a front view of the flower with all the sepals and petals cut off, except the labellum, or lip; and fig. 72 is a side view of the same, with the near half of the labellum cut away, as well as the upper portion of the near side of the nectary, or spur.