Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/150

 138 INSECTIVOROUS PLANTS are excited to curve still farther inwards, and in short all the phenomena of secretion, aggregation, digestion, absorp tion, &c., may be observed which have been described in Drosem. Utricularia. The aquatic species of this plant are found floating in foul and stagnant water. Their much divided filamentous leaves bear bladders (fig. 11, A), averaging about y 1 ^- of an inch in length, each of which bears six or seven long bristles around the mouth, which is fitted with a thin transparent valve, that opens inwards and is covered with peculiar glands. The interior of the bladder is lined by quadrifid hairs (fig. 11, B), like those described in Aldrovanda, Aquatic crustaceans, worms, insect larva&amp;gt;, and other small animals easily enter by pushing inwards the posterior free edge of the valve, which is highly elastic. FIG. 11. A, bladder of Utricularia ncglecta (after Darwin), showing at c collar indistinctly seen through walls. 13, quadrilid hairs from interior of bladder of U. vulgaris (x 800). This instantly shuts against an interior thickened collar or projection around the mouth, and so renders escape impossible. The means by which the plant attracts its victims are unknown, but their success is very remarkable. Few bladders fail altogether, and many are found quite filled with crustaceans, as many as ten having been counted by Darwin within a single bladder. These bladders, however, have no secretion, and are quite unable to digest; they merely absorb the products of decomposition by means of their quadrifid hairs. The terrestrial species (e.g., U. montana), as also those of Polypompholix, bear numerous minute bladders of essentially similar structure along their creeping subter ranean rhizomes, and these usually contain the decomposed remains of small terrestrial articulate animals. Genlisea has curious long-necked pitchers, lined with long downward directed hairs, which at once aid an animal in its entrance and prevent its retreat. Sarracenia. Long supposed to be reservoirs of water for the birds, as was suggested by Linnaeus, or refuges for insects from their pursuers, as was supposed by Catesby, the true function of the leaves of this curious plant has only been elucidated of recent years, mainly by the labours of Mellichamp and Hooker. The mouths of the long radical trumpet-shaped leaves are protected by a large spreading lid, the inner surface of which is abundantly smeared with nectar, and often gaily coloured. Into one form of pitcher rain enters easily, into the other with difficulty. This with the mouth of the pitcher is furnished with numerous honey-secreting glands, and furnishes the attractive surface (fig. 12, A). A pathway too leads up wards from the ground along the broad wing of the pitcher, and is at least in some species also honey-baited ; along this creeping insects are lured to their destruction. Below it is the conducting surface (B) of glassy epidermic cells, with short downward -directed points, which like those of Genlisea 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 detentivc surface (D), of which the cells are produced into long and strong bristles which point downwards and meet in the centre of the diminishing cavity so as to render escape impossible. The secretion wets an insect very rapidly, and appears to have remarkable anaesthetic effects. It seems to be completely destitute of digestive power, indeed rather to accelerate decomposition. The pitchers accumulate vast quantities of insects in the course FIG. 12. Leaves of Sarracenia purpurea. A, attractive surface of lid; B, con ducting, C, glandular, and D, detentive surface ; magnified. A and D are taken from S. flava. of a season, and must thus abundantly manure the surrounding soil when they die. Moreover, the feast is largely shared by unbidden guests (commensals). Not to speak of 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. Darlingtonia. Of the two forms of pitcher in this genus the larger and ordinary form, that of the adult plant, is somewhat twisted, and instead of a lid has a large inflated hood overarching the small mouth. A large bilobed nectariferous and brightly coloured expansion hangs down from this, and attracts insects, particularly moths. As in Sarracenia, the plant seems merely to absorb the products of their putrefaction. Nepenthes. The pitchers of this genus are borne at the ends of long tendril-like prolongations of the leaves, and are of considerable size, varying from an inch to a foot or more in depth. Again we have two varieties of pitchers, one belonging to the young state of the plant, short, broad, and provided with broad external wings, adapted for the capture of ground game, while the adult form, intended for winged game, is long, narrow, and often destitute of lateral appendages. The mouth of the pitcher is strength ened and kept open by a thickened rim, which, like the under surface of the lid, secretes honey, and is frequently produced inwards and downwards into a short funnel-shaped tube which prevents the escape of insects, or into a row of incurved hooks sometimes strong enough to retain a small bird. The younger form of pitcher has its whole interior lined by secreting glands ; the other and more common form