Insectivorous Plants (Darwin)/Chapter 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.^[1] I 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 gingle 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 (Cœnonympha 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 (Æsculus hippocastanum), without thereby receiving, as far as we can perceive, any advantage ; but it was soon evident that Drosera was 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 sensitiveness of the glands to slight pressure and to minute doses of certain nitrogenous fluids, as shown by the movements of the socalled hairs or tentacles; 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 stand-

↑ 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. [In the ‘Quarterly Journal of Science and Art,’ 1829, G. T. Burnett expressed his belief that Drosera profits by the absorption of nutritive matter from the captured Insects. F. I).] There Is also an Interesting though short account of the habits of Drosera by Dr. Milde, In the ‘Bot. Zeitung,’ 1852, p. 540. In 185.5, in the ‘Annales des Sc. nat. bot.,’ tom. III. pp. 297 and 304, MM. Grœnland and Trécul each published papers, with figures, on the structure of the leaves; but M. Trécul 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 Dec. 11, 1862, Mr. J. Scott read a paper before the Botanical Society of Edinburgh, which was published in the ‘Gardener's 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 socalled hairs, entitled, "Sur la Différence 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. Trent, of New Jersey, on some American species of Drosera. Dr. Burdon Sanderson delivered a lecture on Dionæa, befor 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 Dionæa first appeared. Professor 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 addresss on Carnivorous Plants (Brit. Assoc., Belfast, 1874), has given a history of the subject. [A paper on the comparative anatomy of the Droseraceæ was published in 1879 by W. Oels as a Dissertation at Brestau.]

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[p21-1] 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. [In the ‘Quarterly Journal of Science and Art,’ 1829, G. T. Burnett expressed his belief that Drosera profits by the absorption of nutritive matter from the captured Insects. F. I).] There Is also an Interesting though short account of the habits of Drosera by Dr. Milde, In the ‘Bot. Zeitung,’ 1852, p. 540. In 185.5, in the ‘Annales des Sc. nat. bot.,’ tom. III. pp. 297 and 304, MM. Grœnland and Trécul each published papers, with figures, on the structure of the leaves; but M. Trécul 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 Dec. 11, 1862, Mr. J. Scott read a paper before the Botanical Society of Edinburgh, which was published in the ‘Gardener's 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 socalled hairs, entitled, "Sur la Différence 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. Trent, of New Jersey, on some American species of Drosera. Dr. Burdon Sanderson delivered a lecture on Dionæa, befor 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 Dionæa first appeared. Professor 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 addresss on Carnivorous Plants (Brit. Assoc., Belfast, 1874), has given a history of the subject. [A paper on the comparative anatomy of the Droseraceæ was published in 1879 by W. Oels as a Dissertation at Brestau.]

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