Online Encyclopedia

ORCHIDS

Online Encyclopedia
Originally appearing in Volume V20, Page 172 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
ORCHIDS. The word Orchis is used in a special sense to denote a particular genus of the Orchid family (Orchidaceae) ; very frequently, also, it is employed in a more general way to indicate any member of that large and very interesting group. It will be convenient here to use the word Orchis as applying to that particular genus which gives its name to the order or family, and to employ the term " orchid " in the less precise sense. The flowers of all orchids, though extremely diverse within certain limits, and although superficially very different from those of other monocotyledons, are all formed upon one common plan, which is only a modification of that observable in such flowers as those of the narcissus or snowdrop (Galanthus). The conformation of those flowers consists essentially in the presence of a six-parted perianth, the three outer segments of which correspond to a calyx, the three inner ones to a corolla. These segments spring apparently from the top of the ovary—the real explanation, however, being that the end of the flower-stalk or "thalamus," as it grows, becomes dilated into a sort of cup or tube enclosing and indeed closely adhering to the ovary, so that the latter organ appears to be beneath the perianth instead of above it as in a lily, an appearance which has given origin to the term " inferior ovary." Within the perianth, and springingfrom its sides, or apparently from the top of the ovary, are six stamens whose anthers contain pulverulent pollen-grains. These stamens encircle a style which is the upward continuation of the ovary, and which shows at its free end traces of the three originally separate but now blended carpels of which the ovary consists. An orchid flower has an inferior ovary like that just described, but with the ovules on the walls of the cavity ( not in its axis or centre), a six-parted perianth, a stamen or stamens and stigmas. The main distinguishing features consist in the fact that one of the inner pieces of the perianth becomes in course of its growth much larger than the rest, and usually different in colour, texture and form. So different is it that it receives a distinct name, that of the " lip " or " labellum." In place of the six stamens we commonly find but one (two in Cypripedium), and that one is raised together with the stigmatic surfaces on an elongation of the floral axis known as the "column." Moreover, the pollen, instead of consisting of separate cells or grains, consists of cells aggregated into "pollen-masses," the number varying in different genera, but very generally two, four, or eight, and in many of the genera provided at the base with a strap-shaped stalk or "caudicle" ending in a flattish gland or "viscid disk" like a boy's sucker. In Cypripedium all three stigmas are functional, but in the great majority of orchids only the lateral pair form receptive surfaces (st, fig. 3), the third being sterile and forming the rostellum which plays an important part in the process of pollination, often forming a peculiar pouch-like process (fig. 4, r) in which the viscid disk of the pollen-masses is concealed till released in the manner presently to be mentioned. It would appear, then, that the orchid flower differs from the more general monocotyledonous type in the irregularity of the perianth, in the suppression of five out of six stamens, and in the union of the one stamen and the stigmas. In addition to these modifications, which are common to nearly all orchids, there are others generally but not so universally met with; among them is the displacement of the flower arising from the twisting of the inferior ovary, in consequence of which the flower is so completely turned round that the " lip," which originates in that part of the flower, conventionally called the posterior or superior part, or that A A. Floral diagram of typical orchid flower; 1, labellum; a, anther; s, rudiments of barren stamens (staminodes). B. Diagram of the symmetrical trimerous flower of Fritillary (Fritillaria). p1- $ of Orchis. s, sl, sl, The three divisions of the outer perianth. pl, p1, The two lateral divisions of the inner perianth. The superior division or the labellum, which may become inferior by the twisting of the ovary. e, The fertile stamen, with its two pollen-masses in the anther-lobes. c, The one-celled ovary cut transversely, having three parietal placentas. PS, s, s, s, The three outer divisions of the perianth. p, p,1, The three inner, 1 being the labellum, here inferior by the twisting of the ovary. e, Spur of the labellum. o, The twisted ovary. st, The stigma. a, The anther, containing pollen-masses. Sepals. Petals. Anther. Two united stigmas. Rostellum (barren stigma). s, p, a, st, r, nearest to the supporting stem, becomes in course of growth turned to the anterior or lower part of the flower nearest to the bract, from whose axil it arises. Other common modifications arise from the union of certain parts of the perianth to each other, and from the varied and often very remarkable outgrowths from the lip. These modifications are associated with the structure and habits of insects and their visits to the flowers. Cross fertilization, or the impregnation of any given flower by pollen from another flower of the same species on the same or on another plant, has been proved to be of great —g advantage to the plant by securing a more In the common orchids of British meadows, Orchis Morro, mascula (Shakespeare's long purples), &c., the general structure of the flower is as we have described it (figs. 2, 3). In addition there is in this particular genus, as indeed in many others, a long tubular spur or horn projecting downwards from the back of the lip, whose office it is to secrete and store a honeyed juice; the forepart of the lip forms an expanded plate, usually larger and more brightly coloured than the other parts of the flower, and with hairs or ridges and spots of various kinds according to the species. The remaining parts of the perianth are very much smaller, and commonly are so arranged as to form a hood over-arching the " column. " This column stands up from the base of the flower, almost at right angles to the lip, and it bears at the top an anther, in the two hollow lobes of which are concealed the two pollen-masses, each with its caudicle terminating below in a roundish gland, concealed at first in the pouch-like rostellum at the front of the column. Below the anther the surface of the column in front is hollowed out into a greenish depression covered with viscid fluid—this is the two united stigmas. The other parts of the flower need not detain us. Such being in general terms the mechanism of the flower of a common orchis, let us now see how it acts. A bee, we will assume, attracted by the colour and perfume of the flower, alights on that part of it which is the first to attract its attention—the lip. There, guided by the hairs or ridges before-mentioned, it is led to the orifice of the spur with its store of honeyed juice. The position of this orifice, as we have seen, is at the base of the lip and of the column, so that the insect, if of sufficient size, while bending its head to insert the proboscis into the spur, almost of necessity displaces the pollen-masses. Liberated from the anthers, these adhere to the head or back of the insect by means of the sticky gland at the bottom of the caudicle (fig. 4). Having attained its object the insect withdraws, taking the pollen-masses, and visits another flower. And now occurs another device or adaptation noless marvellous than those of which mention has been made. The two anther-cases in an orchis are erect and nearly parallel the one to the other; the pollen-masses within them are of course in like case, as may be thus represented II, but immediately the pollen-masses are removed movements take place at the base of the caudicle so as to effect the bending of this stalk and the placing the pollen-mass in a more or less horizontal. position, thus -, or, as in the case of 0. pyramidalis, the two pollen-masses originally placed parallel I I diverge from the base like the letter V. The movements of the pollen-masses may readily be seen with the naked eye by thrusting the point of a needle into the base of the anther, when the disks adhere to the needle as they would do to the antenna of an insect, and may be withdrawn. Sometimes the lip is mobile and even sensitive to impressions, as are also certain processes of the column. In such cases the contact of an insect or other body with those processes is sufficient to liberate the pollen often with elastic force, even when the anther itself is not touched. In other orchids movements take place in different ways and in other directions. The object of these movements will be appreciated when it is re-membered that, if the pollen-masses retained the original direction they had in the anther in which they were formed, they would, when transported by the insect to another flower, merely come in contact with the anther of that flower, where of course they would be of no use; but, owing to the divergences and flexions above alluded to, the pollen-masses come to be so placed that, when transplanted to another flower of the same species, they come in contact with the stigma and so effect the fertilization of that flower. These illustrations are comparatively simple; it would have been easy to select others of a more complicated nature, but all evidently connected with the visits of insects and the cross fertilization of the flower. In some cases, as in Catasetum, male flowers are produced so different from the female that before the different flowers had been found on the same pike, and before the facts of the case were fully known, they were taken to be representatives of distinct genera. The fruit is a capsule splitting generally by three longitudinal slits forming valves which remain united above and below. The seeds are minute and innumerable; they contain a small rudimentary embryo surrounded by a thin loose membraneous coat, and are scattered by means of hygroscopic hairs on the inside of the valves which by their movements jerk out the seeds. The floral structure is so curious that perhaps less attention has been paid to the vegetative organs than the peculiarities of their organisation demand. We can only allude to some of these points. The orchids of British fields are all of terrestrial habit, and their roots are mostly tuberous (fig. 6), the tubers being partly radical partly budlike in their character. There is often a marked alter-nation in the production of vegetative and flowering shoots respectively; and, some-times, from various circumstances, the flowering shoots are not produced for several years in succession. This fact will account for the profusion with which some orchids, like the common bee orchis for instance, are found in some seasons and their scarcity in others. Tropical orchids are mostly epiphytal—that is, they grow upon trees without deriving nourishment from them. They are frequently provided with " pseudo-bulbs, " large solid swellings of the stem, in the tissues of which water and nutritive materials are stored. They derive this moisture from the air by means of aerial roots, developed from the stem and bearing an outer spongy structure, or velamen, consisting of empty cells kept open by spiral thickenings in the wall; this sponge-like tissue absorbs dew and rain and condenses the moisture of the air and passes it on to the internal tissues. 172 The number of species of orchids is greater than that of any other monocotyledonous order—not even excepting grasses—amounting to 6000, contained in 400 genera. This large number is partly accounted for by the diligent search in all countries that has been made for these plants for purposes of cultivation—they being held at present in the greatest esteem by plant-lovers, and prices being paid for new or rare varieties which recall the days of the tulipomania. The economic uses of orchids are not remarkable. When we have mentioned vanilla (q.v.), which consists of the fleshy pods . of an orchid, we have mentioned about the only economic product that now comes into market. Salep (q.v.), still used in the Levant, consists of the dried tubers of a terrestrial orchid, and contains a relatively large amount of nutritious matter. The cultivation of orchids is treated under HORTICULTURE. The order is divided into two main groups based on the number of the stamens and stigmas. The first Diandreae, has two or rarely three fertile stamens and three functional stigmas. It contains two small genera of tropical Asia and Africa with almost regular flowers, and the large genus Cypripedium containing about 8o species in the north-temperate zone and tropical Asia and America. In Cypripedium two stamens are present, one on each side of the column instead of one only at the top, as in the group Monandreae, to which belong the remaining genera in which also only two stigmas are fertile. What may be considered the normal number of stamens is, as has been said, six, arranged in two rows. In most orchids the only stamen developed to maturity is the posterior one of the three opposite to the lip (anterior before the twisting of the ovary), the other two, as well as all three inner ones, being entirely absent, or present only in the form of rudiments. In Cypripedium two of the outer stamens are wanting; the third—the one, that is, which corresponds to the single fertile stamen in the Monandreae—forms a large sterile structure or staminode; the two lateral ones of the inner series are present, the third being undeveloped. This arrangement may be understood by reference to the following diagram, representing the relative position of the stamens in orchids generally and in Cypripedium. The letter L indicates the position of the labellum; the large figures indicate the developed stamens; the italic figures show the position of the suppressed stamens. 4 6S 465 2 L 3 2 L 3 Arrangement of stamens Arrangement of stamens in Orchis. in Cypripedium. The Monandreae have been subdivided into twenty-eight tribes, the characters of which are based on the structure of the anther and pollinia, the nature of the inflorescence, whether terminal or lateral, the vernation of the leaf and the presence or absence of a joint between blade and sheath, and the nature of the stem. The most important are the following: Ophrydineae, with about 45 genera, of terrestrial orchids, mainly north temperate, including the British genera Orchis, Aceras, Ophrys, Herminium, Gymnadenia and Habenaria. Also some genera mainly represented in South and tropical Africa, such as Satyrium, Disa andtemperate Asia and South Africa; Eulophia and Lissochilus are important African genera. Cataselinae, with three tropical American genera, two of which, Cataselunc and Cycnoches, have di- or tri-morphic flowers. They are cultivated for their strange-looking flowers. Dendrobiinae, with six genera in the warmer. parts of the Old World; the chief is Dendrobium, with 300 species, often with showy flowers. Cymbidiinae, with 8 genera in the tropics of the Old World. The leaves are generally long and narrow. Cymbidium is well known in cultivation. Oncidiinae, with 44 genera in the warmer parts of America. Odontoglossum and Oncidium include some of the best-known cultivated orchids. Sarcanthinae, with 42 genera in the tropics. Vanda (Asia) and Angraecum (Africa and Madagascar) are known in cultivation. The flower of Angraecum sesquipedale has a spur 18 in. in length. The order is well represented in Britain by i8 genera, which include several species of Orchis:—Gymnadenia (fragrant orchis), Habenaria (butterfly and frog orchis), Aceras (man orchis), Herminium (musk orchis), Ophrys (bee, spider and fly orchis), Epipactis (Helleborine), Cephalanthera, Neottia (bird's-nest orchis), one of the few saprophytic genera, which have no green leaves, but derive their nourishment from decaying organic matter in the soil, Listera (Tway blade), Spiranthes (lady's tresses), Malaxis (bog-orchis), Liparis (fen-orchis), Corallorhiza (coral root), also a saprophyte, and Cypripedium (lady's slipper), represented by a single species now very rare in limestone districts in the north of England.
End of Article: ORCHIDS
[back]
ORCHHA, or URCHHA (also called Tehri or Tikamgarh)
[next]
ORCHOMENUS (local form on coins and inscriptions, E...

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.