Insectivorous Plants

ON THE STRUCTURE AND MOVEMENTS OF SOME OTHER SPECIES OF DROSERA. Drosera anglica--Drosera intermedia--Drosera capensis--Drosera spathulata--Drosera filiformis--Drosera binata--Concluding remarks...Pages 278-285 CHAPTER XIII. DIONAEA MUSCIPULA. Structure of the leaves--Sensitiveness of the filaments--Rapid movement of the lobes caused by irritation of the filaments--Glands, their power of secretion--Slow movement caused by the absorption of animal matter--Evidence of absorption from the aggregated condition of the glands--Digestive power of the secretion--Action of chloroform, ether, and hydrocyanic acid- -The manner in which insects are captured--Use of the marginal spikes--Kinds of insects captured--The transmission of the motor impulse and mechanism of the movements-- Re-expansion of the lobes...286-320 CHAPTER XIV. ALDROVANDA VESICULOSA. Captures crustaceans--Structure of the leaves in comparison with those of Dionaea-- Absorption by the glands, by the quadrifid processes, and points on the infolded margins-- Aldrovanda vesiculosa, var. australis--Captures prey--Absorption of animal matter-- Aldrovanda vesiculosa, var. verticillata--Concluding remarks...321-331 CHAPTER XV. DROSOPHYLLUM--RORIDULA--BYBLIS--GLANDULAR HAIRS OF OTHER PLANTS-- CONCLUDING REMARKS ON THE DROSERACEAE. Drosophyllum--Structure of leaves--Nature of the secretion--Manner of catching insects-- Power of absorption--Digestion of animal substances--Summary on Drosophyllum--Roridula- -Byblis--Glandular hairs of other plants, their power of absorption--Saxifraga--Primula-- Pelargonium--Erica--Mirabilis--Nicotiana--Summary on glandular hairs--Concluding remarks on the Droseraceae...332-367 [page x.] CHAPTER XVI. PINGUICULA. Pinguicula vulgaris--Structure of leaves--Number of insects and other objects caught-- Movement of the margins of the leaves--Uses of this movement--Secretion, digestion, and absorption--Action of the secretion on various animal and vegetable substances--The effects of substances not containing soluble nitrogenous matter on the glands--Pinguicula grandiflora--Pinguicula lusitanica, catches insects--Movement of the leaves, secretion and digestion...Pages 368-394 CHAPTER XVII. UTRICULARIA. Utricularia neglecta--Structure of the bladder--The uses of the several parts--Number of imprisoned animals--Manner of capture--The bladders cannot digest animal matter, but absorb the products of its decay--Experiments on the absorption of certain fluids by the quadrifid processes--Absorption by the glands--Summary of the observation on absorption-- Development of the bladders--Utricularia vulgaris--Utricularia minor--Utricularia clandestina...395-430 CHAPTER XVIII. UTRICULARIA (continued). Utricularia montana--Description of the bladders on the subterranean rhizomes--Prey captured by the bladders of plants under culture and in a state of nature--Absorption by the quadrifid processes and glands--Tubers serving as reservoirs for water--Various other species of Utricularia--Polypompholyx--Genlisea, different nature of the trap for capturing prey-- Diversified methods by which plants are nourished...431-453 ----- INDEX...455-462 [page 1] INSECTIVOROUS PLANTS. ----- CHAPTER I. DROSERA ROTUNDIFOLIA, OR THE COMMON SUN-DEW. Number of insects captured--Description of the leaves and their appendages or tentacles-- Preliminary sketch of the action of the various parts, and of the manner in which insects are captured--Duration of the inflection of the tentacles--Nature of the secretion--Manner in which insects are carried to the centre of the leaf--Evidence that the glands have the power of absorption--Small size of the roots. During the summer of 1860, I was surprised by finding how large a number of insects were caught by the leaves of the common sun-dew (Drosera rotundifolia) on a heath in Sussex. I had heard that insects were thus caught, but knew nothing further on the subject.* I * As Dr. Nitschke has given ('Bot. Zeitung,' 1860, p. 229) the bibliography of Drosera, I need not here go into details. Most of the notices published before 1860 are brief and unimportant. The oldest paper seems to have been one of the most valuable, namely, by Dr. Roth, in 1782. There is also an interesting though short account of the habits of Drosera by Dr. Milde, in the 'Bot. Zeitung,' 1852, p. 540. In 1855, in the 'Annales des Sc. nat. bot.' tom. iii. pp. 297 and 304, MM. Groenland and Trcul each published papers, with figures, on the structure of the leaves; but M. Trcul went so far as to doubt whether they possessed any power of movement. Dr. Nitschke's papers in the 'Bot. Zeitung' for 1860 and 1861 are by far the most important ones which have been published, both on the habits and structure of this plant; and I shall frequently have occasion to quote from them. His discussions on several points, for instance on the transmission of an excitement from one part of the leaf to another, are excellent. On December 11, 1862, Mr. J. Scott read a paper before the Botanical Society of Edinburgh, [[page 2]] which was published in the 'Gardeners' Chronicle,' 1863, p. 30. Mr. Scott shows that gentle irritation of the hairs, as well as insects placed on the disc of the leaf, cause the hairs to bend inwards. Mr. A.W. Bennett also gave another interesting account of the movements of the leaves before the British Association for 1873. In this same year Dr. Warming published an essay, in which he describes the structure of the so-called hairs, entitled, "Sur la Diffrence entre les Trichomes," &c., extracted from the proceedings of the Soc. d'Hist. Nat. de Copenhague. I shall also have occasion hereafter to refer to a paper by Mrs. Treat, of New Jersey, on some American species of Drosera. Dr. Burdon Sanderson delivered a lecture on Dionaea, before the Royal Institution published in 'Nature,' June 14, 1874, in which a short account of my observations on the power of true digestion possessed by Drosera and Dionaea first appeared. Prof. Asa Gray has done good service by calling attention to Drosera, and to other plants having similar habits, in 'The Nation' (1874, pp. 261 and 232), and in other publications. Dr. Hooker, also, in his important address on Carnivorous Plants (Brit. Assoc., Belfast, 1874), has given a history of the subject. [page 2] gathered by chance a dozen plants, bearing fifty-six fully expanded leaves, and on thirty-one of these dead insects or remnants of them adhered; and, no doubt, many more would have been caught afterwards by these same leaves, and still more by those as yet not expanded. On one plant all six leaves had caught their prey; and on several plants very many leaves had caught more than a single insect. On one large leaf I found the remains of thirteen distinct insects. Flies (Diptera) are captured much oftener than other insects. The largest kind which I have seen caught was a small butterfly (Caenonympha pamphilus); but the Rev. H.M. Wilkinson informs me that he found a large living dragon-fly with its body firmly held by two leaves. As this plant is extremely common in some districts, the number of insects thus annually slaughtered must be prodigious. Many plants cause the death of insects, for instance the sticky buds of the horse-chestnut (Aesculus hippocastanum), without thereby receiving, as far as we can perceive, any advantage; but it was soon evident that Drosera was [page 3] excellently adapted for the special purpose of catching insects, so that the subject seemed well worthy of investigation. The results have proved highly remarkable; the more important ones being--firstly, the extraordinary FIG. 1.* (Drosera rotundifolia.) Leaf viewed from above; enlarged four times. sensitiveness of the glands to slight pressure and to minute doses of certain nitrogenous fluids, as shown by the movements of the so-called hairs or tentacles; * The drawings of Drosera and Dionaea, given in this work, were made for me by my son George Darwin; those of Aldrovanda, and of the several species of Utricularia, by my son Francis. They have been excellently reproduced on wood by Mr. Cooper, 188 Strand. [page 4] secondly, the power possessed by the leaves of rendering soluble or digesting nitrogenous substances, and of afterwards absorbing them; thirdly, the changes which take place within the cells of the tentacles, when the glands are excited in various ways. It is necessary, in the first place, to describe briefly the plant. It bears from two or three to five or six leaves, generally extended more or less horizontally, but sometimes standing vertically upwards. The shape and general appearance of a leaf is shown, as seen from above, in fig. 1, and as seen laterally, in fig. 2. The leaves are commonly a little broader than long, FIG. 2. (Drosera rotundifolia.) Old leaf viewed laterally; enlarged about five times. but this was not the case in the one here figured. The whole upper surface is covered with gland-bearing filaments, or tentacles, as I shall call them, from their manner of acting. The glands were counted on thirty-one leaves, but many of these were of unusually large size, and the average number was 192; the greatest number being 260, and the least 130. The glands are each surrounded by large drops of extremely viscid secretion, which, glittering in the sun, have given rise to the plant's poetical name of the sun-dew.