The Darwin family arrived in Bournemouth, southern England, in September 1862 for a seaside holiday to help twelve-year-old Lenny and his mother Emma convalesce from recent illnesses. Darwin was not happy, fretting over their recovery while wishing he had his work to distract him. Never one to sit still for long, he took the other kids on country rambles, but he wasn’t impressed with what he saw. “This is a nice, but most barren country and I can find nothing to look at,” he complained to Joseph Hooker. “Even the brooks and ponds produce nothing—The country is like Patagonia.”57 But as is true almost everywhere, there was plenty to see once you looked closely, and he soon came upon a patch of round-leaved sundews, a curious plant with glistening dew-drop-covered leaves that he had begun to experiment with while vacationing in nearby Sussex two years earlier but then dropped as he became busy with other projects.
He immediately collected some to restart his sundew “feeding” experiments, beginning with bits of hair plucked from his own head and his toenails (both of which the picky plants decidedly rejected). It would have been interesting to have been a fly on the wall (at a safe distance from the sundews) in the Darwin’s seaside cottage that day—picture the eminent naturalist carefully feeding his toenails to a plant! But as ever, there was method to the seeming madness. Darwin’s revived interest in sundews snowballed into a series of ground-breaking investigations into plant carnivory, culminating thirteen years later in Insectivorous Plants (1875). Yet his motivation was not so much novel plant physiology, as interesting as that was, but more the way these plants emulated animals—an emulation that spoke of ancestral evolutionary union of plants and animals. Emma Darwin perhaps expressed it best when she remarked on her husband’s sundew fascination to Mary Lyell, wife of the geologist Charles Lyell, “I suppose he hopes to end in proving it to be an animal.”58 But more than that, he was sure these plants taught lessons in evolutionary gradualism to boot. What did the simple movement and sensory abilities of Drosera say about the evolutionary steps leading to their far more animal-like relatives, Venus fly trap (see p. 121)? “I began this work on Drosera in relation to gradation as throwing light on Dionaea,” he admitted to Asa Gray.59
Drosera rotundifolia, common in sunny boggy areas throughout the northern hemisphere, is one of nearly 200 species in the genus worldwide, now divided into about a dozen sub-genera. All the species have glandular hairs that glisten like dew in the sun and enable them to capture and digest insects, a vital source of nitrogen in acidic, nutrient-poor soil. At the time of Darwin’s experiments, it was well known that insects got trapped on the leaves by the sticky mucilage, but no one knew if they were digested—some thought the leaves acted as botanical flypaper, protecting the plant. Darwin was able to prove that the insects do indeed become food—Drosera is acknowledged as the first plant genus in which carnivory was confirmed—but more than that, he and his son Francis later demonstrated just how beneficial carnivory is to sundews, showing in a controlled experiment that plants “fed” insects grew more vigorously and produced more flowers and seeds than “starved” plants shielded from insects by netting. Francis Darwin published this study in 1880 in the Journal of the Linnean Society and in the second edition of Insectivorous Plants (revised by Francis in 1888).
In his first series of experiments in the 1860s, Darwin sought to determine the nature of the dewy secretion acidic enough to break down insects. He found it was similar to the digestive pepsin and acids in animals, and that it also has antiseptic properties, preventing growth of mold. For his experiments, Darwin filled his greenhouse with sundews, raiding his kitchen and medicine cabinet for all sorts of things to try out on them as food. He tested with nitrogenous and non-nitrogenous substances, liquids and solids—albumen, milk, olive oil, boiled peas, ammonia, hydrochloric acid, glycerin, turpentine, quinine, cobra venom, and more. In his usual fashion, he encouraged others to try experiments too. Writing to Daniel Oliver at Kew Gardens, Darwin marveled how his sundews could somehow detect nitrogen in fluids. Noting that “Our Drosera likes milk better than any other drink,”60 he suggested that Oliver try feeding droplets of milk and saliva to an Australian species for comparison. Ultimately, his investigations turned into a characteristically collaborative affair: another half dozen Drosera species were analyzed with the help of Oliver and others, clergyman Henry M. Wilkinson sent observations of trapped insects, John Burdon-Sanderson, professor of physiology at University College London, studied their electrical impulses, and Darwin’s sons George and Francis helped with experiments and drew illustrations of the leaves in various stages of entrapping prey with their dew-tipped glandular hairs—referred to by Darwin as “tentacles,” a reflection of his tendency to see the animal in these plants. Ultimately, he dedicated eleven of the eighteen chapters of Insectivorous Plants to Drosera, mainly his beloved D. rotundifolia: “a wonderful plant, or rather a most sagacious animal.”61
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 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 excellently adapted for the special purpose of catching insects, so that the subject seemed well worthy of investigation. …
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… The whole upper surface is covered with gland-bearing filaments, or tentacles, as I shall call them, from their manner of acting. … 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.
The tentacles on the central part of the leaf or disc are short and stand upright, and their pedicels are green. Towards the margin, they become longer and longer and more inclined outwards, with their pedicels of a purple colour. Those on the extreme margin project in the same plane with the leaf, or more commonly are considerably reflexed. A few tentacles spring from the base of the footstalk or petiole, and these are the longest of all, being sometimes nearly ¼ of an inch in length. …
Leaf viewed from above, enlarged four times.
Leaf with all the tentacles closely inflected, from immersion in a solution of phosphate of ammonia.
Leaf with the tentacles on one side inflected over a bit of meat placed on the disc.
[Since the publication of the first edition, several experiments have been made to determine whether insectivorous plants are able to profit by an animal diet. …
… My [Francis Darwin’s] experiments were begun in June 1877, when the plants were collected and planted in six ordinary soup-plates. Each plate was divided by a low partition into two sets, and the least flourishing half of each culture was selected to be “fed,” while the rest of the plants were destined to be “starved.” The plants were prevented from catching insects for themselves by means of a covering of fine gauze, so that the only animal food which they obtained was supplied in very minute pieces of roast meat given to the “fed” plants but withheld from the “starved” ones. After only ten days the difference between the fed and starved plants was clearly visible: the fed plants were of brighter green and the tentacles of a more lively red. At the end of August the plants were compared by number, weight, and measurement. …
[The results] show clearly enough that insectivorous plants derive great advantage from animal food. It is of interest to note that the most striking difference between the two sets of plants is seen in what relates to reproduction—i.e. in the flower-stems, the capsules, and the seeds. …
… Both starved and fed plants were kept without food until April 3rd, when it was found that the average weights per plant were 100 for the starved, 213.0 for the fed. This proves that the fed plants had laid by a far greater store of reserve material in spite of having produced nearly four times as much seed. …—F. D.]
… It will be sufficient here to recapitulate, as briefly as I can, the chief points. In the first chapter, a preliminary sketch was given of the structure of the leaves and of the manner in which they capture insects. This is effected by drops of extremely viscid fluid surrounding the glands and by the inward movement of the tentacles. As the plants gain most of their nutriment by this means, their roots are very poorly developed; and they often grow in places where hardly any other plant except mosses can exist. The glands have the power of absorption, besides that of secretion. They are extremely sensitive to various stimulants, namely repeated touches, the pressure of minute particles, the absorption of animal matter and of various fluids, heat, and galvanic action. A tentacle with a bit of raw meat on the gland has been seen to begin bending in 10 s., to be strongly incurved in 5 m., and to reach the centre of the leaf in half an hour. The blade of the leaf often becomes so much inflected that it forms a cup, enclosing any object placed on it.
A gland, when excited, not only sends some influence down its own tentacle, causing it to bend, but likewise to the surrounding tentacles, which become incurved; so that the bending place can be acted on by an impulse received from opposite directions, namely from the gland on the summit of the same tentacle and from one or more glands of the neighbouring tentacles. Tentacles, when inflected, re-expand after a time, and during this process the glands secrete less copiously, or become dry. As soon as they begin to secrete again, the tentacles are ready to re-act; and this may be repeated at least three, probably many more times. …
Movement ensues if a gland is momentarily touched three or four times; but if touched only once or twice, though with considerable force and with a hard object, the tentacle does not bend. The plant is thus saved from much useless movement, as during a high wind the glands can hardly escape being occasionally brushed by the leaves of surrounding plants. Though insensible to a single touch, they are exquisitely sensitive, as just stated, to the slightest pressure if prolonged for a few seconds; and this capacity is manifestly of service to the plant in capturing small insects. Even gnats, if they rest on the glands with their delicate feet, are quickly and securely embraced. The glands are insensible to the weight and repeated blows of drops of heavy rain, and the plants are thus likewise saved from much useless movement. …
In the fifth chapter, the results of placing drops of various nitrogenous and non-nitrogenous organic fluids on the discs of leaves were given, and it was shown that they detect with almost unerring certainty the presence of nitrogen. These results led me to inquire whether Drosera possessed the power of dissolving solid animal matter. The experiments proving that the leaves are capable of true digestion and that the glands absorb the digested matter, are given in detail in the sixth chapter. These are, perhaps, the most interesting of all my observations on Drosera, as no such power was before distinctly known to exist in the vegetable kingdom. It is likewise an interesting fact that the glands of the disc, when irritated, should transmit some influence to the glands of the exterior tentacles, causing them to secrete more copiously and the secretion to become acid, as if they had been directly excited by an object placed on them. The gastric juice of animals contains, as is well known, an acid and a ferment, both of which are indispensable for digestion, and so it is with the secretion of Drosera. When the stomach of an animal is mechanically irritated, it secretes an acid, and when particles of glass or other such objects were placed on the glands of Drosera, the secretion, and that of the surrounding and untouched glands, was increased in quantity and became acid. But the stomach of an animal does not secrete its proper ferment, pepsin, until certain substances, called peptogenes, are absorbed; and it appears from my experiments that some matter must be absorbed by the glands of Drosera before they secrete their proper ferment. That the secretion does contain a ferment which acts only in the presence of an acid on solid animal matter, was clearly proved by adding minute doses of an alkali, which entirely arrested the process of digestion, this immediately recommencing as soon as the alkali was neutralised by a little weak hydrochloric acid. From trials made with a large number of substances, it was found that those which the secretion of Drosera dissolves completely, or partially, or not at all, are acted on in exactly the same manner by gastric juice. We may, therefore, conclude that the ferment of Drosera is closely analogous to, or identical with, the pepsin of animals. …
Most of the acids which were tried, though much diluted (one part to 437 of water) and given in small doses, acted powerfully on Drosera; nineteen, out of the twenty-four, causing the tentacles to be more or less inflected. Most of them, even the organic acids, are poisonous, often highly so; and this is remarkable, as the juices of so many plants contain acids. … Many acids excite the glands to secrete an extraordinary quantity of mucus; and the protoplasm within their cells seems to be often killed, as may be inferred from the surrounding fluid soon becoming pink.
In the ninth chapter, the effects of the absorption of various alkaloids and certain other substances were described. Although some of these are poisonous, yet as several, which act powerfully on the nervous system of animals, produce no effect on Drosera; we may infer that the extreme sensibility of the glands, and their power of transmitting an influence to other parts of the leaf, causing movement, or modified secretion, or aggregation, does not depend on the presence of a diffused element, allied to nerve-tissue. One of the most remarkable facts is that long immersion in the poison of the cobra-snake does not in the least check, but rather stimulates, the spontaneous movements of the protoplasm in the cells of the tentacles. Solutions of various salts and acids behave very differently in delaying or in quite arresting the subsequent action of a solution of phosphate of ammonia. Camphor dissolved in water acts as a stimulant, as do small doses of certain essential oils, for they cause rapid and strong inflection. Alcohol is not a stimulant. The vapours of camphor, alcohol, chloroform, sulphuric and nitric ether, are poisonous in moderately large doses, but in small doses serve as narcotics or anaesthetics, greatly delaying the subsequent action of meat. But some of these vapours also act as stimulants, exciting rapid, almost spasmodic movements in the tentacles. …
In the tenth chapter, it was shown that the sensitiveness of the leaves appears to be wholly confined to the glands and to the immediately underlying cells. It was further shown that the motor impulse and other forces or influences proceeding from the glands when excited pass through the cellular tissue and not along the fibro-vascular bundles. A gland sends its motor impulse with great rapidity down the pedicel of the same tentacle to the basal part which alone bends. The impulse, then passing onwards, spreads on all sides to the surrounding tentacles, first affecting those which stand nearest and then those farther off. But by being thus spread out, and from the cells of the disc not being so much elongated as those of the tentacles, it loses force, and here travels much more slowly than down the pedicels. Owing also to the direction and form of the cells, it passes with greater ease and celerity in a longitudinal than in a transverse line across the disc. …
I have now given a brief recapitulation of the chief points observed by me, with respect to the structure, movements, constitution, and habits of Drosera rotundifolia; and we see how little has been made out in comparison with what remains unexplained and unknown.
Echinocystis lobata. Lithograph by George Endicott, printed in Natural History of New York.