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Encyclopedia Britannica - Main :: PIG-POL |
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PITCHER PLANTS , in botany, the name given to plants in which the leaves bear pitcher-like structures or are pitcher-like in form. The plant generally understood by this name is Nepenthes, a genus containing nearly sixty species, natives of tropical Asia, north Australia and (one only) of Madagascar. North Borneo is especially rich in species. They are shrubby plants climbing over surrounding vegetation by means of tendril-like prolongations of the midrib of the leaf beyond the leaf-tip.A, Honey-gland from attractive C, Transverse section of the surface of lid. same. B, Digestive gland from interior of pitcher, in pocket-like de- pression of epidermis, opening downwards. The pitcher is a development at the end of the tendril. It is generally tubular in form, but in some species two forms are produced on the same plant, lower or terrestrial goblet-shaped pitchers and upper suspended pitchers retaining the more primitive more or less tubular form; in a few species a third formfunnel- or cornucopia-shaped pitchersoccurs in the upper part. In the terrestrial type a pair of well-developed wings traverse the length of the pitcher; in the tubular or funnel-shaped form the wings are narrow or ridge
ridges, which are usually prolonged as teeth beyond the in-folded margin. Above the mouth is the lid (operculum), which varies in size from a small narrow process to a large heart-shaped expansion. A study of the development of the pitcher, especially in the young pitchers of seedling plants, shows that the inflated portion is a development of the midrib of the leaf, while the wings, which are especially well represented in the terrestrial type of pitcher, represent the upper portion of the leaf-blade which has become separated from the lower portion by the tendril; the lid is regarded as representing two leaflets which have become fused. The short straight or curved process from the back of the pitcher behind the lid represents the organic apex of the leaf (A in fig. r). The size of the pitcher varies widely in the different species, from an inch to a foot or more in depth. The colour also varies considerably, even in different pitchers of the same individual, A, Attractive surface of lid; B, conducting; C, glandular; and D, detentive surface; magnified. A and D are taken from S. flow. according to age, light exposure or soil conditions. It is uniformly green or more or less spotted, blotched or suffused with red or crimson, or sometimes, as in N. sanguinea or N. Edwardsiana, largely or wholly of a rich scarlet or crimson colour. In-sects are attracted to the mouth of the pitcher by a series of glands, yielding a sweet excretion, which occurs on the stem and also on the leaf from the base of the leaf-stalk to the lid and peristome. Embedded in the incurved margin of the rim which affords a very insecure foothold to insects, are a number of large glands excreting a sweet juice. The cavity of the pitcher is in some species lined throughout with a smooth glistening surface over which glands are uniformly distributed; these glands secrete a liquid which is found in the pitcher even in the young state while it is still hermetically closed by the lid. In other species the glands are confined to the lower portion of the cavity surface, while the upper part bear a smooth waxy secretion on which it is impossible, or at any rate extremely difficult, for insects to secure a foothold. This area is termed the " conducting " area, as distinguished from the lower or " detentive " gland-bearing area. It has been proved that the secretion contains a digestive ferment capable of rendering proteid matter soluble. Insects, especially running insects, which have followed the track of honey glands upwards from the stem along the leaf, reach the mouth of the pitcher, and in their efforts to sip the attractive marginal glands fall over intothe liquid. The smooth walls above the liquid afford no foot-hold, and they are drowned; their bodies are digested and the products of digestion are ultimately absorbed by the glands in the pitcher- wall
The side-saddle plant, Sarracenia, native of the eastern United States, is also known as a pitcher-plant. There are about seven species, herbs with clusters of radical leaves some or all of which are more or less trumpet- or pitcher-shaped. The leaf has a broadly sheathing base succeeded by a short stalk bearing the pitcher, which represents a much enlarged midrib with a wing-like lamina. Above the rim of the pitcher is a broad flattened lid, which is also a laminar development. The surface of the leaf, especially the laminar wing, bears glands which in spring exude large glistening drops of nectar
Ir\ the right hand one cut open to show internal structure. varies from yellow-green to deep crimson in different species and in individuals according to exposure to sunlight and other conditions. This forms the attractive area, and the inner surface of the lid also bears numerous glands, as well as downward-pointing hairs, each with a delicately striated surface (fig. 2, A). Below it is the conducting surface (B) of glassy epidermal cells, with short downward-directed points, which facilitate the descent, but impede the ascent of an insect. Then come the glandular surface (C), which is formed of smooth polished epidermis with numerous glands that secrete the fluid contents of the pitcher, and finally the detentive surface (D), of which the cells are produced into long and strong bristles which point A A, Ordinary leaf of Cephalotus. B, Monstrous leaf with spoon-shaped depression. C and D, Other abnormal forms more deeply pouched, showing formation of pitcher. E, Ordinary pitcher of Cephalotus. a, Apex of leaf. downwards and meet in the centre of the diminishing cavity so as to render escape impossible. The secretion wets an insect very rapidly, but, so far as is known, seems to be completely destitute of digestive powerindeed, rather to accelerate decomposition. The pitchers accumulate vast quantities of insects in the course of a season, and must thus abundantly manure the surrounding soil when they die. Moreover, the- feast is largely shared by unbidden guests. Not to speak of I chemical composition this rock resembles the trachytes rather than the rhyolites. In Eigg and Skye there arc many dikes of pitchstone, mostly of intermediate rather than of acid character, all connected with the great eruptive activity which characterized that region in early Tertiary times. The following analyses give the chemical composition of a few well-known pitchstones: insects which feed upon the pitcher itself, some drop their eggs into the putrescent mass, where their larvae find abundant nourishment, while birds often slit open the pitchers with their beaks and devour the maggots in their turn. Cephalotus follicularis, a native of south-west Australia, , a small herbaceous plant, bears ordinary leaves close to the ground as well as pitchers. The latter somewhat resemble in general form those of Nepenthes. The lid is especially attractive to insects from its bright colour and honey secretion; three wings lead up to the mouth of the pitcher, on the inside of which a row of sharp
ridge
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