Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/146

 134 INSECTIVOROUS PLANTS INSECTIVOROUS PLANTS. Insectivorous or, as they are sometimes more correctly termed, carnivorous plants are, like the parasites, the climbers, or the succulents, a physiological assemblage belonging to a number of distinct natural orders. They agree in the extraordinary habit of adding to the supplies of nitrogenous material afforded them in common with other plants by the soil and atmosphere, by the capture and consumption of insects and- other small animals. The curious and varied mechanical arrangements by which these supplies of animal food are obtained, the ways and degrees in which they are utilized, and the remarkable chemical, histologies!, and electrical phenomena which accompany these processes of prehension and utilization, can only be understood by a separate and somewhat detailed examination of the leading orders and genera. It is convenient to follow the order adopted by Mr Darwin in his work on Insectivorous Plants (Loncl., 1875), to which our knowledge of the subject is mainly due, incorporating, however, as far as possible the leading observations of other writers on the subject. We must preface this, however, by a brief summary of the facts of taxonomy and distribution. Taxonomy. The best known and most important order the Droseracex is placed among the calycifloral exogens, and has obvious affinities with the Saxifragacese. It includes six genera Byblis, Roridida, Drosera, Droso- phyllum, Aldrovanda, and Dionsea, of which the last three are monotypic, i.e., include only one species. The curious pitcher-plant, Gephalotus follicularis, is usually raised to the dignity of a separate natural order Cephalotese, though Bentham and Hooker (Gen. Plant.) place it among the Ribesiacex. The Sarraceniacese are thalamiflorals, and contain the genera Sarracenia, Darlingtonia, Heliam- 2&amp;gt;kora, while the true pitcher plants or Nepenthacese, consisting of the single large genus Nepenthes, are placed near the Aristolochiacex among the Apetalse. Finally the genera Pinguicula, Utricularia, Genlisea, and Polypom- pholix belong to the gamopetalous order Utricularix. Thus all the four leading divisions of the exogenous plants are represented by apparently unrelated orders ; certain affinities, however, are alleged between Droseracese, Sarraceniacex, and Nepenthacesc,. Distribution. While the large genus Drosera has an all but world- wide distribution, its congeners are restricted to well-defined and usually compara tively small areas. Thus Drosopliyllwn occurs only in Portugal and Mo rocco, Byblis in tropical Australia, arid, although Al drovanda is found in Queensland, in Bengal, and in Europe, a wide dis tribution explained by its aquatic habit, Dionteais restricted to a few localities in North and South Carolina, mainly around Wilming ton. Cephalotus occurs only near Albany in Western FlG, _ Lcaf of Sumlew ~. ofera ntmdifolia Australia, Heham- (After Darwin.) phora on the Koraima Mountains in Venezuela, Darling tonia on the Sierra Nevada of California, and these three genera too arc as yet monotypic; of Sarracenia, however, there are six or eight known species scattered over the eastern States of North America. The 36 species of Nepenthes are mostly natives of the hotter parts of the Indian Archipelago, but a few range into Ceylon, Bengal, Cochin China, and some even occur in tropical Australia on the one hand, and in the Seychelles and Madagascar on the other. Pinguicida is abundant in the north temperate zone, and ranges down the Andes as far as Patagonia; the 1 50 species of Utricularia are mostly aquatic, and some are found in all save polar regions ; their unimportant congeners, Genlisea and Polypompholix, occur in tropical America and south-western Australia respectively. It is remarkable that all the insectivorous plants agree in inhabiting damp heaths, bogs, marshes, and similar situations where water is abundant, a peculiarity perhaps due to their habit of copious secretion and conse quent need of water. Drosera. The Common Sundew (D. rotundifolia) has extremely small roots, and bears five or six radical leaves horizontally extended in a rosette around the flowerstalk. The upper surface of each leaf is covered with gland-bearing filaments or &quot; tentacles,&quot; of which there are on an average about two hundred. Each FIG. 2. Leaf of Sundew, enlarged, rrlanrl i cnrrrmnrlprl r &amp;lt;i lorrrp with tlic tentacles on one side in- giand is surrounded by a large flectcd ovcr a bit of mcat p]aced 011 dew-like drop Of a viscid but the disk. (After Darwin.) transparent and glittering secretion, and the popular names (Sundew, French Rossolis, German Sonnenthau) as well as the Linnsean (from SpoVos, dew) have been thus suggested. The stalk of the tentacle has the essential structure of a leaf. A small fibre-vascular bundle, consisting mainly of spiral vessels, runs up through the stalk and is surrounded by a FJG. 3. Glands of Sundew magnified. (After Podcl-Port.) A, external aspect with drop of secretion ; U, internal structure. layer of elongated parenchyma cells lined by a thin layer of colourless circulating protoplasm, and filled with a homo geneous fluid, tinted purple by a modification of chlorophyll (erythrophyll, Sorby). The epidermis bears small multi cellular prominences. The glandular head of the tentacle contains a central mass of spirally thickened cells in im mediate contact with the upper end of the fibro-vascular bundle. Around these (but separated from them by a