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twayblade

Значение термина twayblade в knolik

twayblade - Twayblade (Listera ovata)
twayblade - Only two big, egg-shaped leaves - the "tway blades" - and a slender spike of insignificant flowers whose green colouring makes the children say: "They never come out," and yet a plant of such wonderful ways that Charles Darwin spoke of it as one of the most remarkable members of a family whose every member is itself wonderful. It is an orchid and a lover of damp, shady places, whether in wood or pasture, with a strong preference for limestone or chalky soil, and though reported from most parts of Britain it is said to be specially partial to Wales. Usually it is only a foot or so high, but if it find a spot exactly suited to it it may double this height. Below ground its roots are just a brown bundle of thickish fibres, and in this it differs from all other British orchids, for it has nothing of the nature of bulbs such as most of them have, it has not even the succulence of that mass of root-fibres that is the most striking characteristic of its closest relative, the Neottia, or Bird's Nest Orchid. From the damp earth the single stem rises for, perhaps, some six inches; it is thick, yellowish, and sheathed with a colourless bract or two. And then, almost opposite to each other, stretch two broad leaves, stalkless and vivid, and marked with outstanding veins that run from base to tip. No wonder that the stalk below is but a sickly green, for they must shade it from much of the sunlight, and hence from the power that would enable it to form green colouring matter.

But if the twin leaves are the outstanding characteristic of the plant, it is in those flowers so lacking in beauty that the chief interest lies, for each is an elaborate piece of plant mechanism. There are three ordinary sepals, green, ovate in shape, and with a sharp tip; the three petals are a lighter green, the two side ones being a slighter edition of the sepals, while the lowest petal is a long, spreading lip, its tip two-cleft. Above this cleft is a furrow running up the middle, and since honey oozes out of its banks it is a miniature river of nectar.

Now, the middle of the flower in an orchid has developed quite differently from that in any other flower. There is only one stamen, and it consists simply of a two-celled anther. The ovary is there certainly, lying outside the sepals and petals; its style has coalesced with the filament of the stamen, and the two form what is known as the "column."

The "column" is sometimes spoken of as being "the prolongation of the floral axis." Normally, there should be three stigmas, but the two top ones go to form a remarkable organ known as the rostellum - no other plant has such an organ. The lower stigma alone functions as such, and is sticky. Therefore, looking straight into a flower of the Twayblade we have the column at the back, its upper part broad and protecting the anther, which is in front of it. Just below the anther the rostellum projects - in this orchid rather large and thin and somewhat scale-like, bulging in front and slightly arching over the stigma which lies below it. The anther contains in its two cells two masses of pollen - pollinia - -the pollen grains being slightly held together by elastic strands. These cells open each by a slit in front, and through this slit falls the pollinium, which is rather like a skittle in shape. Its tip just catches the edge of the rostellum, and there, at the opening of the flower, the two lie side by side awaiting developments. The tissue of the rostellum is divided into a series of locules, each of which contains a drop of thin, milky fluid and possesses the power of violently exploding if touched. So sensitive is this organ that a touch from the thinnest human hair is sufficient to bring about the explosion, and thereupon two milky drops are shot out. Like some enamels, a film forms on them instantly, and in two or three seconds they are brown and "set." Notice that that edge of the rostellum on which the pollinia tip rests is not sensitive; if it were the masses of pollen would be immediately glued permanently in the recesses of their own flower - the last thing the plant desires.

Let us now see by what means the mechanism is set working. The Twayblade is popular with beetles and small flies, who have not the aesthetic craving for colour that the bees and the butterflies possess. They are quite content with insignificance and plainness if they can secure nectar, so they come in numbers to this plant - it is sometimes suggested that they must be drawn thither by an odour impalpable to our grosser senses. The beetle, let us say, alights on the cleft tip that is specially there for platform purposes. It crawls up the lip, licking, as it proceeds, the honey from the fairy-like trough. When it reaches the end just by the "column," it naturally lifts its head and so strikes the arching, and exquisitely sensitive rostellum. Out flies the necessary cement, touching both the pollinia and the insect's head. A second suffices for the one to be firmly glued to the other, and the insect backs out, taking the pollinia with it. But as a further result of the explosion the rostellum curves downwards over the sticky stigma, and thus, in the backing out, the pollinia cannot be placed upon it. Off flies the insect with its pollinia (sometimes having received them full in its eyes), and it may thus collect pair after pair if it go to one newly opened flower after another. But if the stigma is now shielded by the rostellum, how can it receive pollen itself from another flower? Well, it does not remain shielded, for after a short time the rostellum moves again, leaving an open passage to it. At this point, too, it becomes very much stickier, and now a beetle, lapping along the nectar course with projecting pollinia horns, will strike the stigma with them. They crumble at once, and some of their pollen lies fast on the sticky surface. The beetle may again lift its head at the end of the furrow, but this time there is no explosion and no pollinia to annex, for the flower has completed the cycle of its plans. Not often, either, do they miscarry; for instance, on one fading flower-spike Darwin found every single flower had both dispatched its pollinia and received pollen in return, and hence was setting seed. He also noticed that spiders' webs were often woven over and about these plants; no doubt the spiders had discovered what an excellent fly neighbourhood they afforded. As to the Latin name, Listera ovata, the "ovata," of course, refers to the egg-shaped leaves, and Listera is after a certain Dr. Martin Lister, who was physician to Queen Anne and a naturalist of some renown.

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