|
|
Text and images from The Openlibrary.Org
On The Various Contrivances By Which British and Foreign
Orchids Are Fertilised By Insects, And On The Good Effects Of
Intercrossing
By Charles Darwin M.A., F.R.S., etc.
INTRODUCTION
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
CHAPTER II.
Ophreæ continued — Fly and Spider Ophrys — Bee Ophrys, apparently adapted for perpetual self-fertilisation, but with paradoxical contrivances for intercrossing — The Frog Orchis; fertilisation effected by nectar secreted from two parts of the labellum — Gymnadenia conopsea — Greater and Lesser Butterfly Orchis; their differences and means of fertilisation — Summary on the powers of movement in the pollinia
CHAPTER III.
Epipactis palustris; curious shape of the labellum and its apparent importance in the fructification of the flower — Cephalanthera grandiflora; rostellum aborted; early penetration of the pollen-tubes; case of imperfect self-fertilisation; fertilisation aided by insects — Goodyera repens — Spiranthes autumnalis; perfect adaptation by which the pollen of a younger flower is carried to the stigma of an older flower on another plant
CHAPTER IV.
Malaxis paludosa; simple means of fertilisation — Listera ovata; sensitiveness of the rostellum; explosion of viscid matter; action of insects; perfect adaptation of the several oegans — Listera cordata — Neottia nidus-avis; its fertilisation effected in the same manner as in Listera .. .. 130-156
CHAPTER V.
Cattleya, simple manner of fertilisation — Masdevallia, curious closed flower — Dendrobium, contrivance for self-fertilisation — Vandeæ, diversified structure of the pollinia; importance of the elasticity of the pedicel; its power of movement — Elasticity and strength of the caudicle — Calanthe with lateral stigmas, manner of fertilisation — Angræcum sesquipedale, wonderful length of nectary — Acropera, perplexing case, a male Orchid
CHAPTER VI.
Catasetidæ, the most remarkable of all Orchids — The mechanism by which the pollinia of Catasetum are ejected to a distance, and are transported by insects — Sensitiveness of the horns of the rostellum — Extraordinary difference in the male, female, and hermaphrodite forms of Catasetum tridentatum — Mormodes ignea, curious structure of flower; ejection of its pollinia — Cypripedium, importance of the slipper-like form of the labellum — Secretion of nectar — Advantage derived from insects being delayed in sucking the nectar — Singular excrescences on the labellum, apparently attractive to insects
CHAPTER VII.
Homologies of Orchid-flowers — The great modification which they have undergone — Gradation of organs, of the rostellum, of the pollen-masses — Formation of the caudicle — Genealogical affinities — Mechanism of the movement of the pollinia — Uses of the petals — Production of seed — Importance of trifling details of structure — Cause of the vast diversity of structure for the same general purpose — Cause of the perfection of the contrivances in Orchids — Summary on insect-agency — Nature abhors perpetual self-fertilisation
INTRODUCTION.
THE object of the following work is to show that the contrivances by which Orchids are fertilised, are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom; and, secondly, to show that these contrivances have for their main object the fertilisation of each flower. In my volume 'On the Origin of Species' I have given only general reasons for my belief that it is apparently a universal law of nature that organic beings require an occasional cross with another individual; or, which is almost the same thing, that no hermaphrodite fertilises itself for a perpetuity of generations. Having been blamed for propounding this doctrine without giving ample facts, for which I had not, in that work, sufficient space, I wish to show that I have not spoken without having gone into details.
I have been led to publish this little treatise separately, as it has become inconveniently large to be incorporated with the rest of the discussion on the same subject. And I have thought, that, as Orchids are universally acknowledged to rank amongst the most singular and most modified forms in the vegetable kingdon, the facts to be presently given might lead some observers to look more curiously into the habits of our several native species. An examination of their many beautiful contrivances will exalt the whole vegetable kingdom in most persons' estimation. I fear, however, that the necessary details will be too minute and complex for any one who has not a strong taste for Natural History. This treatise affords me also an opportunity of attempting to show that the study of organic beings may be as interesting to an observer who is fully convinced that the structure of each is due to secondary laws, as to one who views every trifling detail of structure as the result of the direct interposition of the Creator.
I must premise that Christian Konrad Sprengel, in his curious and valuable work, 'Das entdeckte Geheimniss der Natur,' published in 1793, gave an excellent outline of the action of the several parts in Orchids; for he well knew the position of the stigma; and he discovered that insects were necessary to remove the pollen-masses, by pushing open the pouch and coming into contact with the enclosed sticky glands. But he overlooked many curious contrivances,—a consequence, apparently, of his belief that the stigma generally receives the pollen of the same flower. Sprengel, likewise, has partially described the structure of Epipactis; but in the case of Listera he entirely misunderstood the remarkable phenomena characteristic of that genus, which has been so well described by Dr. Hooker in the 'Philosophical Transactions' for 1854. Dr. Hooker has given a full and accurate account, with drawings, of the structure of the parts, and of what takes place; but from not having attended to the agency of insects, he has not fully understood the object gained. Robert Brown,* in his celebrated paper in the 'Linnæan Transactions,' expresses his belief that insects are necessary for the fructification of most Orchids; but adds, that the fact of all the capsules on a dense spike not infrequently producing seed, seems hardly reconcileable with this belief: we shall hereafter see that this doubt is groundless. Many other authors have given facts and expressed their belief, more or less fully, on the necessity of insect-agency in the fertilisation of Orchids.
* 'Linnæan Transactions,' 1833, vol. xvi. p. 704.
In the course of the following work I shall have the pleasure of expressing my deep obligation to several gentlemen for their unremitting kindness in sending me fresh specimens, without which aid this work would have been impossible. The trouble which several of my kind assistants have taken has been extraordinary: I have never once expressed a wish for aid or for information which has not been granted, as far as possible, in the most liberal spirit.
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.
