Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/3

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EXPLANATION OF TERMS

In case any one should look at this treatise who has never attended to Botany, it may be convenient to explain the meaning of the common terms used. In most flowers the stamens, or male organs, surround in a ring the one or more female organs, called the pistils. In all common Orchids there is only one stamen, and this is confluent with the pistil forming the Column. The stamens consist of a filament, or supporting thread (rarely seen in British Orchids), which carries the anther; and within the anther the pollen, the male vivifying element, is included. The anther is divided into two cells, which are very distinct in most Orchids, so much so as to appear in some species like two separate anthers. The pollen in all common plants consists of fine granular powder: but in most Orchids the grains cohere in masses, which are often supported by a very curious appendage, called the Caudicle; as will hereafter be more fully explained. The pollen-masses, with their caudicles and other appendages, are called the Pollinia.

There are properly in most Orchids three united pistils, or female organs. The upper part of the pistil has its anterior surface soft and viscid, which forms the stigma. The two lower stigmas are often completely confluent, so as to appear as one. The stigma in the act of fertilisation is penetrated by long tubes emitted by the pollen-grains, which carry the contents of the grains down to the ovules, or young seeds in the ovarium.

Of the three pistils, which ought to be present, the stigma of the upper one has been modified into an extraordinary organ, called the Rostellum, which in many Orchids presents no resemblance to a true stigma. The rostellum either includes or is formed of viscid matter; and in very many Orchids the pollen-masses are firmly attached to a portion of its exterior membrane, which is removed, together with the pollen-masses, by insects. This removeable portion consists in most British Orchids of a small piece of membrane, with a layer or ball of viscid matter underneath, and I shall call it the "viscid disc;" but in many exotic Orchids the portion removed is so large and so important, that one part must be called, as before, the viscid disc, and the other part the pedicel of the rostellum, to the end of which pedicel the pollen-masses are attached. Authors have called that portion of the rostellum which is removed the "gland," or the "retinaculum," from its apparent function of retaining the pollen-masses in place. The pedicel, or prolongation of the rostellum, to which in many exotic Orchids the pollen-masses are attached, seems generally to have been confounded, under the name of caudicle, with the true caudicle of the pollen-masses, though their nature and origin are totally different. The part of the rostellum which is not removed, and which includes the viscid matter, is sometimes called the "bursicula," or "fovea," or "pouch." But it will be found most convenient to avoid all these terms, and to call the whole modified stigma the rostellum—sometimes adding an adjective to define its shape; and to call that portion of the rostellum which is attached to and removed with the pollen-masses the viscid disc, together in some cases with its pedicel.

Lastly, the three outer divisions of the flower are called Sepals, and form the calyx; but, instead of being green, as in most common flowers, they are generally coloured, like the three inner divisions or Petals of the flower. The one petal which commonly stands lowest is larger than the others, and often assumes most singular shapes; it is called the lower lip, or Labellum. It secretes nectar, in order to attract insects, and is often produced into a long spur-like nectary.

CHAPTER I. Structure of Orchis Power of movement of the pollinia Perfect adaptation of the parts in Orchis pyramidalis On the insects which visit Orchids, and on the frequency of their visits On the fertility and sterility of several Orchids On the secretion of nectar, and on moths being purposely delayed in obtaining it.

FOR my purpose British Orchids may be divided into three groups, and the arrangement is, for the most part, a natural one. But I leave out of consideration the British species of Cypripedium, with its two anthers, of which I know nothing. Of these three groups the first consists of the Ophrea, which have pollinia furnished at their lower ends with a caudicle, congenitally attached to a viscid disc. The anther stands above the rostellum. The Ophreæ include most of our common Orchids.