SUNDEW, in botany, the popular name for a genus of plants, Drosera, so called from the drops of viscid, transparent secretion borne by the tentacles which cover the leaf-surface. It is a cos mopolitan genus of slender glandular herbs, with leaves arranged in a basal rosette or alternately on an elongated stem, and is repre sented in Great Britain by three species, and in North America by seven species, found in spongy bogs and heaths. They are in cluded in the family Droseraceae, and comprise about ion species, most numerous in Australia.
The common sundew (D. rotundifolia) has extremely small roots, and bears five or six radical leaves horizontally extended in a rosette around the flower-stalk. The upper surface of each leaf is covered with gland-bearing filaments or "tentacles," of which there are on an average about 200. Each gland is surrounded by a large dew-like drop of the viscid secretion. A small fibro-vascular bundle consisting mainly of spiral vessels, runs up through the stalk of the tentacle and is surrounded by a layer of elongated parenchyma cells outside of which is the epidermis filled with a purple fluid. The glandular head of the tentacle contains a cen tral mass of spirally thickened cells (tracheids) in immediate con tact with the upper end of the fibro-vascular bundle. Around these is a layer of large colourless thin-walled cells which reach the surface at the base of the head and act as absorbing cells. Outside these are two
layers filled with purple fluid.
Insects are attracted by the leaves; a fly alighting on the disk, or even only touching one or two of the exterior ten tacles, is immediately held by the viscid secretion; the tentacles to which it is ad hering begin to bend, and thus pass on their prey to the tentacles next succeeding them inwards, and the insect is thus carried by a curious rolling movement to the centre of the leaf. The tentacles on all sides become similarly inflected, and the insect, bathed in the abundant secretion, is drowned in about a quarter of an hour. Closely allied to Drosera is Drosophyl lum lusitanicum, which catches such vast numbers of flies in a state of nature that the Portuguese cottagers call it the fly catcher, and hang up branches of it in their houses for this purpose. Its long narrow leaves are thickly cov ered with stalked glands, which resemble in the main the tentacles of Drosera, save in that they are incapable of movement, and that the secretion is more fluid. There are many minute colourless sessile glands, which, when stimulated by the absorption of nitrog enous matter, excrete an acid digestive secretion similar to that of the sundew.