Darwin's Backyard

by James T. Costa

  Darwin went about his experiments on Drosera systematically, testing the plants’ response first to nonnitrogenous and then nitrogenous fluids based on an informed guess that nitrogenous compounds were key. He hypothesized that these plants became carnivores as an adaptation to their nutrient-limited habitat. Given that bogs are especially poor in nitrogen relative to other nutrients, it stands to reason that it is nitrogen-bearing substances the plants are after. If the substances selected for his nonnitrogenous trials seem a bit haphazard—gum arabic, sugar, starch, diluted alcohol, olive oil, even an “infusion and decoction” of tea—that’s because they were: Darwin used whatever was at hand, pilfering from the kitchen and washroom medicine cabinet. His methods seem modern in some ways, but not in others: he first tried a control by applying drops of distilled water (eliciting no response), varied the concentration of the substances he was testing, and tested the water control and treatments on more than one leaf. In spirit these reflect modern ideas of experimental design, with controls, treatments, and multiple replicates. On the other hand, Darwin could be imprecise and inconsistent by other modern standards. The distilled water was applied to “30 to 40 leaves” rather than a standard number for all trials, for example, and in the gum arabic tests he reported trying four solution concentrations but only quantified one of them. Darwin next tested the drops on 14 leaves, but was casual about the duration of each test, leaving them on anywhere from a day to almost 2 days, but generally about 30 hours. The imprecision was common in that era, but it wouldn’t be long before more rigorous standards for experimental method, in the light of statistical rigor, would become the norm.

  Drosera rotundifolia leaf, viewed from above and enlarged 4x. (Left) Resting position. (Center) Filaments inflected after exposure to phosphate of ammonia. (Right) Leaf with filaments on one side inflected over a bit of meat placed on the disk. From Darwin (1875a), p. 3, fig. 1, and p. 10, figs. 4 and 5.

  In all Darwin tested over 60 leaves with nonnitrogenous substances, and in no case did the tentacles respond. He next turned to nitrogenous substances, and the response could not have been more different: drops of milk, egg white, raw meat infusion, saliva, mucus, nitrate of ammonia, even his own urine induced movement of the tentacles of nearly every one of the 64 leaves tested. The short-stalked ones in the center don’t tend to move, but the long marginal tentacles bent as much as 180° to position their glistening globe of “dew” onto the object, taking from 1½ to 6 or so hours to inflect fully. Writing to Hooker at Kew, Darwin described these “first-rate chemists” as capable of distinguishing even minute quantities of nitrogenous from nonnitrogenous substances. In another letter, he enthused that the tiniest grain of nitrate of ammonia was sufficient to elicit an effect from the sundew leaves.

  Having confirmed his hunch that nitrogenous substances whet the plants’ appetite, he came up with more questions: are solids as effective as fluids in eliciting a response? Do the leaves actually have the power of digestion? Can they can break down and absorb organic matter? Those experiments waited a decade, however, as Darwin got sidetracked with a number of other projects: his orchid book was published in 1862, a succession of new Origin editions (second through fifth) appeared throughout the decade, and the two-volume work on domestication with which he was struggling, all the while conducting experiments in hybridization, flower structure, and climbing plants. He had given a paper on Drosera at the Philosophical Club of the Royal Society on February 21, 1861, and then all was silent on the sundew front for a while. His experiments largely halted until Variation in Animals and Plants Under Domestication appeared in 1868, but sundews and their relatives were far from ignored. He continued to receive information and specimens of various sundew species from friends and other correspondents, housing a growing collection in his greenhouse.

  Sundew Stomachs

  Almost exactly a decade after the family convalescence in Bourne­mouth, Darwin had a false start getting back to these plants-cum-animals: “Began working at Drosera” his journal read for August 23, 1872. But he had the usual interruptions, so yet a year later, June 14, 1873, he wrote: “Began Drosera again.”14 In the meantime naturalists on the other side of the Atlantic had become intrigued by these plants. Mary Treat of New Jersey was a talented naturalist who published numerous papers on birds, insects, and carnivorous plants. She likely learned of Darwin’s interest in sundews in the course of her frequent correspondence with Asa Gray at Harvard, and wrote to Darwin in the early 1870s. Darwin had high praise for Treat’s work, encouraging her to publish and enlisting her help in observing the thread-leaved sundew (Drosera filiformis) and other species in the eastern United States. She was happy to assist, and many of Treat’s observations on sundews and other carnivorous plants are cited in Insectivorous Plants (and were also published separately).

  Two of the most important lines of investigation Darwin took up once he restarted his sundew studies in earnest included a return to the question of digestive ability, and the motor stimulus that leads to the inflection of the tentacles and leaves. In these he had the assistance of one John Burdon-Sanderson, a professor of physiology at University College, London. Darwin invited him to visit Down House in June of 1873 to discuss Drosera, and soon had him hooked. A key question for the digestion experiments was whether the plants could render animal matter soluble: can they digest matter, or simply absorb what is already broken down? In Insectivorous Plants he summarized his experiments systematically.15 First, he did what he called the “acid test”—testing the droplets from 30 leaves in an unstimulated state with litmus paper, he found little to no discoloration. He then tested another set of leaves fed, so to speak, with a range of materials from glass particles to hard-boiled egg white to bits of raw meat. After 24 hours the droplets of the tentacles inflecting toward the material were shown by the litmus paper to be decidedly acidic. Darwin’s conclusion was that the viscid secretion from the leaves becomes acidic once the tentacles contact edible objects. Darwin also made an incidental discovery: the dewdrop secretions of the tentacles has antiseptic properties, preventing the growth of mold and other microorganisms and in this way also acts like the gastric juice of animals.

  Following the suggestions of Burdon-Sanderson, Darwin next carefully tested for digestion using small cubes of hard-boiled egg. Some cubes were set up on wet moss as a control, while the others were placed on sundew leaves and observed at intervals ranging from 21 to 50 hours. The egg cubes on the leaves turned into liquid globules and disappeared (presumably absorbed). Those left on the moss simply rotted. This was all very suggestive, but the real acid test then followed: if he could neutralize the acid and shut down the digestion process, Darwin felt this would be “the best and almost sole test of the presence of some ferment analogous to pepsin in the secretion.” Pepsin is an animal digestive enzyme released by cells lining the stomach. It was in fact the first enzyme discovered, isolated by German physiologist Theodor Schwann in 1836. Schwann derived the name “pepsin” from the Greek for digestion: πε′ψς, pepsis. It was later joined by trypsin and chymotrypsin as a trio of proteolytic (protein-degrading) enzymes—proteases—playing the leading role in animal digestion. The enzymes depend upon the acidic environment of the digestive system for both their own activity and to assist in the breakdown of digestible matter, which is why Darwin’s litmus paper tests showing acidity of the sundew droplets were so encouraging. He now wanted to test whether that acidity played a role in sundew digestion.

  Darwin used a control and two treatments. With the control plants, cubes of hard-boiled egg were permitted to break down as usual, with Darwin adding only minute water droplets (on the order of 5 micro­liters) a few times daily. The plant absorbed the egg in 48 hours. He then divided treatments using similarly sized cubes of egg into two groups: he “watered” one with equally minute droplets of a weak hydrochloric acid solution (1 HCl:437 H2O), and one with a weak sodium carbonate solution (1 Na2CO3:437 H2O). Voila: the hydrochloric acid accelerated the absorptio
n of the egg-white cubes to little more than 24 hours, while the sodium carbonate neutralized the droplets altogether. Darwin was then able to neutralize the neutralizer, countering its effects with weak hydrochloric acid until breakdown and absorption proceeded normally. “From these experiments,” he concluded, “we clearly see that the secretion has the power of dissolving albumen, and we further see that if an alkali is added, the process of digestion is stopped, but immediately recommences as soon as the alkali is neutralised by weak hydrochloric acid.” His experiments “almost sufficed to prove that the glands of Drosera secrete some ferment analogous to pepsin, which in the presence of an acid gives to the secretion its power of dissolving albuminous compounds.”16 Darwin again proceeded to test a range of substances both digestible and indigestible to the plants. Protein or protein-rich substances like roast beef, fibrin, cartilage, gelatin, and even pollen were readily digested, while substances like fats, starches, hair balls, mucus, and bits of nail were not, as he had found in Bournemouth years before. (Keratin, the chief constituent of hair and nails, is a protein but its highly ordered structure renders it immune to digestion under the conditions of animal—or plant—stomachs. This is why cats cough up their hairballs instead of simply recycling the protein by digesting them.)

  Darwin concluded the “digestion” chapter of Insectivorous Plants arguing that the so-called ferment of Drosera is very similar if not identical to the digestive juices of animals, specifically a combination of pepsin and some sort of acid. “That a plant and an animal should pour forth the same, or nearly the same, complex secretion, adapted for the same purpose of digestion, is a new and wonderful fact in physiology,” he declared. There was something missing, however; something that bothered him as well as his son Frank, by now (in his late 20s) a regular research collaborator with his father. It was very suggestive, sundews producing a pepsin-like “ferment” and dissolving nitrogenous treats. But were they digesting per se—gaining nutritional benefit? After all, these plants have chlorophyll and can survive even if deprived of treats. The crucial experiment was attempted—feeding trials to see if “fed” plants grew better, flowered more, and set more seeds than unfed plants. But many of the plants fed and unfed alike unaccountably died, so the experiment was left undone by the time Insectivorous Plants came out in 1875. It was soon taken up again by Frank, who devised a new approach.

  Planting round-leaved sundews (D. rotundifolia) in a set of six Wedgwood soup dishes in the summer of 1877, Frank arrayed these on a tray fitted with a removable wood frame covered with gauze to exclude insects, and so that all the plants received about the same amount of light. He also kept them moist. He fed 86 sundews pieces of roasted meat weighing about 1.3 mg each every 5 days or so. He did not feed the control group, similar in number. Within a few weeks Frank saw results: “The first difference noticed between fed and starved halves of the plates was on July 17th, when the fed side, viewed as a whole, was clearly greener than the starved half. The difference was quite distinct in all six plates, as both my father and I observed.”17 Toward the end of August, he noted the number of flowering stems (a total of 116 in the unfed plants versus 173 in fed), number of stems with at least one flower (19 unfed, 34 fed), and the number of healthy leaves, reckoned by the presence of glandular secretions (187 unfed, 256 fed).

  Frank also measured the diameter of a sample of leaves from fed and unfed plants, and obtained their respective dry weights, finding longer leaves and higher weights among the fed sundews. Finally, he turned to the key measures of vigor and reproductive success in a plant: numbers of seed capsules and the weight of seeds produced. In all cases the fed sundews outperformed the unfed controls, and the net weight of seeds produced by fed plants exceeded that of the unfed plants by a factor of nearly 4:1. The stumbling block to realizing this benefit before, he pointed out, was the fact that these plants seem to do okay without such nutritional supplements. Darwin was proud of his son, but didn’t live long enough to see the results of Frank’s experiment included in Insectivorous Plants. The second edition appeared in 1888, 6 years after Darwin’s death, and Frank duly appended the results to the first chapter.

  Other aspects of the nutritional biology of sundews were beyond the ability of the Darwins (or anyone of the time) to explore. Stable nuclear isotope technology, for example, developed in the following century, found a multitude of applications from medicine and agriculture to geology and ecology. This technology was put to good use in further exploring sundew nutrition by the British and Scottish team of Jonathan Millett, Roger Jones, and Susan Waldron, who approached Darwin’s question from another angle. These researchers sought to distinguish between animal- and non-animal-derived sources of nitrogen in two sundew species, D. rotundifolia (Darwin’s favorite, the laboratory rat of the sundew world) and the related species D. intermedia, capitalizing on the fact that the “heavier” of the two stable forms of nitrogen, 15N, is found in a higher concentration in animals than in (most) plants, owing to differences in the way animals and plants accumulate body mass. Animals accumulate this isotope by eating other organisms, while most plants derive theirs more slowly from the soil. This study found that on average some 50 percent of the sundew’s nitrogen was derived from insect prey.

  Characterizing the digestive “ferment” of sundews similarly had to await the advent of new technology. A series of studies in the twentieth century showed it to be a species of protease very similar to one called nepenthesin, described from pitcherplants of the genus Nepenthes in the 1960s. The proteases of Nepenthes pitcherplants and those of sundews and Venus flytraps have since been found to be very closely related—a fact that should not surprise us given that these carnivorous plants are cousins. Darwin would have been delighted to know that his hunch had proved correct: the “ferment” of Drosera is indeed chemically similar to animal pepsin, and functions in exactly the same way.

  Irritable Plants

  Darwin was also keenly interested in the “irritability” of his sundews. Not that his fascination with their animal-like characteristics led him to believe they could become irate; rather, his fascination was irritability in the sense used by physiologists of the eighteenth and nineteenth centuries. The Oxford English Dictionary defines it as “the capacity of being excited to vital action (e.g., motion, contraction, nervous impulse, etc.) by the application of an external stimulus: a property of living matter or protoplasm in general, and characteristic in a special degree of certain organs or tissues of animals and plants, esp. muscles and nerves.” Plants with muscles and nerves? Precisely why he was interested. He tried to figure out the mechanism behind the sense perception and movement of sundew tentacles, tracking their mobilization across leaves in response to point stimuli like carefully placed bits of meat. He experimentally determined that the “motor impulse,” as he put it, is transmitted more readily in a longitudinal than transverse direction across the leaves, a pattern that maps onto the main vascular architecture of the leaves.

  Placing a morsel on a single tentacle, he watched as the impulse spread to the surrounding tentacles, which promptly bent toward the point of excitement. “Nothing could be more striking than the appearance of [the leaves] each with their tentacles pointing truly [toward the morsel],” he declared in Insectivorous Plants. “We might imagine that we were looking at a lowly organised animal seizing prey with its arms.” Images of sea anemones come to mind. But he had to conclude that the comparison with animals was only superficial in the case of these movements. He described the motor impulse of tentacular movement as a “reflex action” (and “the only known case of reflex action in the vegetable kingdom” to boot), where the food morsel stimulated the tentacles it was in immediate contact with, the excitation of which in turn stimulated other nearby tentacles to slowly arc toward the food. He had to concede, however, that the mechanism of this “reflex action” was very different from that of the animal nervous system. Rather than electrical conducting tissue (nerves) triggering contractile fibers (muscle), he
concluded that the movement was related to the mobilization of protoplasm causing the walls of particular cells to contract and relax.

  The prominent plant physiologist Julius Sachs of the University of Würzburg had proposed in 1874 that movement of plant structures result from the osmotic flow of fluid in and out of specialized cells, creating rapid changes in tension that translate into contraction and relaxation (more on this in Chapter 9). This was later confirmed to be an important mode of movement in many plant groups, but Darwin ruled it out in the case of his sundew tentacles, which tend to move rather slowly. It likely plays a role, however, in the movement of the “snap tentacles” of the Australian pimpernel sundew, Drosera glanduligera. These specialized tentacles fringe the leaves of this ground-hugging sundew, arrayed horizontally awaiting the footfall of inquisitive insects. They lack the dewy droplets of normal tentacles, instead tapering to a fine upcurved point which functions not so much to pierce as to trap. In milliseconds they snap toward the leaf, flinging their insect prey onto the nearby glandular tentacles where they become entrapped—a phenomenon that would have captivated Darwin too, no doubt.

  The Most Wonderful Plant in the World

  Darwin’s darling of a snap-action plant was the Venus flytrap (Dionaea muscipula), the small plant endemic to a narrow band along the coastal Carolinas in the United States. Its bilobed leaves with their fringe of stout spike-like bristles resemble the iron-toothed snap traps that do their job with such devastating effectiveness. Early observers had correctly interpreted them as traps—the bristles function more to imprison than pierce hapless insects. But traps for what? Before the virtues of pollination were appreciated the traps were imagined to protect the flowers from insects. Darwin’s grandfather Erasmus, perhaps even more famous for his scientifically inspired poetry than his celebrated medical skills, expressed this interpretation in his botanical poem Loves of the Plants (1788). He described the “wonderful contrivance to prevent the depredations of insects” as follows: “The leaves are armed with long teeth, like the antennae of insects, and lie spread upon the ground round the stem; and are so irritable, that when an insect creeps upon them, they fold up, and crush or pierce it to death.” Two hundred years later, it likely inspired the monster plant of Little Shop of Horrors.