Darwin's Backyard

by James T. Costa

  Therein lies an important lesson: the concepts of homology versus analogy in understanding the “animal-like” qualities of carnivorous plants, and its bearing on the broader concept of Darwin’s Tree of Life. Darwin’s overarching interest in these plants lay in what they might teach us about universal common ancestry. For Darwin, commonalities in underlying physiology, or cellular processes, was evidence for such an ancestor. Plants and animals (let alone fungi, protists, and the various prokaryotic groups) outwardly might seem to have little in common, making it difficult to see them as relatives, but Darwin sought similarities at the less obvious, physiological, level. He would have reveled in the modern understanding of the ultimate evidence for shared ancestry: our very DNA.

  Experimentising: Feed Me, Seymour!

  In pursuing his vision of universal common descent—the idea that all species are descended from common ancestors—Darwin was inevitably intrigued by plant-like animals and animal-like plants. And there may be no plants more animal-like than certain botanical carnivores. Darwin was interested in the sense perception of these plants with regard to trapping prey: what they respond to, and how they move, and what they digest.

  Obtaining Sundews and Flytraps

  For these investigations we want Drosera rotundifolia or other suitable sundew species, and Venus flytrap (Dionaea muscipula), preferably with fresh leaves. While available in garden centers, these plants are often not well cared for. Carolina Biological (www.carolina.com) is a trusted source. If you want to grow your own sundews or flytraps, an excellent source of information (and seeds) is the International Carnivorous Plant Society (ICPS, carnivorousplants.org/howto) with growing guides by species.

  I. Sensitive, Are We?

  A. Materials

  • Drosera rotundifolia or other sundew

  • Dissecting probe or stiff paint brush

  • Forceps or tweezers

  • Hand lens and/or dissecting microscope

  • Milk (small quantity—no more than 2 tsp [10 ml])

  • Pipet or eye dropper

  • Hard-boiled egg white, cubed to 1/16 in. (2 mm) on a side

  • Toothpick (wood), cut to about 1/16 in. (2 mm) pieces

  • Hard cheese and gummy bear candy, cut into about 1/16 in. (2 mm) cubes

  • Litmus paper

  B. Procedure

  1. Be careful not to touch the filaments on the inner surface of the leaf lobes before you are ready to begin. Using the hand lens, note the difference in length of the hairs (filaments) fringing the leaf versus those found in the leaf center.

  2. The sensitivity of the filaments is best demonstrated with the long outer ones, which can be individually excited to inflect. Firmly poke a single long filament at its tip (droplet) three or four times with the probe or paint brush. (Note the viscous consistency of the droplet.) Did the filament inflect toward the center of the leaf? If so, how long after stimulation? Repeat this step with several filaments on several leaves to observe a good response. How many responded? How many didn’t? Darwin found that when he poked a number of filaments four, five, or even six times with enough force to bend the filament, only a portion of the filaments became inflected. “The result was so uncertain,” he wrote, “as to seem capricious.” He suggested that it is adaptive for the plant not to be overly sensitive to touch, since the leaves are probably often contacted by blown debris, waving grass blades, etc. When a filament did respond to touch Darwin found that it did so quickly (he reported that he had “distinctly seen, through a lens, a [filament] beginning to bend in ten seconds, and . . . strongly pronounced inflection in under one minute” after stimulation. How quickly did stimulated filaments bend in your study?

  3. Selecting several fresh filaments, not touched before, apply a minute droplet of milk to each by dipping the tip of the probe or brush into milk and carefully transferring the tiny droplet to the tip of a single filament in turn (a hand lens or dissecting microscope will help). Did inflection of these filaments occur any faster?

  4. Select another leaf. Manually stimulate the short filaments at the center of the leaf disk with the probe or brush. Observe the “behavior” of the longer marginal filaments over the course of 1 to 2 hours. How many become inflected, and how long did it take them? An experiment like this was Darwin’s first indication of transmission of a “motor impulse” in sundew leaves.

  5. Observe the direction of the “motor impulse.” With the forceps carefully place a small cube of egg white on filaments near the leaf edge on one side of the medial line of a leaf (X at the lower left of the diagram). Repeat by placing a cube directly on the medial line, near the upper margin or lower margin (X at the lower right of the diagram). Inspect the leaves after 24 hours and count how many filaments on the side of the leaf opposite from where you placed the egg have responded by inflecting. In what direction do they inflect? Do any bend in a different direction? Darwin found that the “motor impulse” is transmitted more readily in a longitudinal than a transverse direction on the leaf; accordingly, we would expect more filaments longitudinally opposite the egg cube to have responded than those located transversely across the medial line.

  II. Picky Eaters

  A. Materials

  Same as for Part I.

  B. Procedure

  1. Can sundews tell the difference between edible and inedible substances? Select two fresh leaves on the same plant, preferably side by side. Place an egg-white cube in the center of one leaf, and place a similar-sized wood matchstick cube in the center of the other. Make observations on the “behavior” of each leaf at regular intervals—perhaps at 2, 4, and 6 hours—comparing the reaction of the leaf to the egg versus the wood.

  2. Select three other fresh leaves. “Feed” one an egg-white cube, the second a bit of cheese about the same size, and the third a piece of the gummy bear (cut into a like-sized piece). Observe the reaction of the leaves to each. After 7 days, gently extract any food or food remains (it may be necessary to carefully pry open the leaf or filaments). Is the sundew a picky eater? Which food was digested and which not, and why? (Hint: Drosera needs nitrogen-containing protein, and excessive sugars are not to its taste.)

  3. Stimulate two filaments using milk or egg. Carefully touch a piece of litmus paper to a droplet or two from unexcited (uninflected) filaments. Repeat with another piece of litmus paper, this time touching a droplet or two from filaments stimulated by the presence of digestible food. Take care not to touch the paper to the food item itself. What is the color difference of the two pieces of litmus paper? Which one shows more presence of acid? Can you think of why? The droplet from the excited filament should be decidedly more acidic, indicating digestive enzymes.

  III. Trigger-Happy Flytraps

  A. Materials

  • One or more fresh Dionaea muscipula plants

  • Dissecting probe or stiff paint brush

  • 3/16 in. (0.5 cm) diameter wood dowel, toothpicks, or wood kitchen matches

  • Glue or tape

  • Forceps or tweezers

  • Short (~1" [2.5 cm]) piece of thread, monofiliment, or hair

  • Hand lens and/or dissecting microscope

  NOTE: Each trap can close and reopen only about six or seven times before withering. Darwin and others found, however, that traps with insects sometimes never reopened, or if they did were sluggish or unresponsive for a time.

  B. Procedure

  1. It was the rapidity of the snap-trap action of this plant as much as its “stomach” that fascinated Darwin. Look closely at the inner surface of one of the leaf traps with a hand lens or dissecting microscope, and find the two to five small trigger hairs (trichomes) arrayed in the center of each lobe. Look at two or three other leaves; can you tell visually if the trigger hairs are always in approximately the same position? Compare the length and width of the trigger hairs with the far stouter spike-like projections fringing the lobes.

  2. Touching two or more of the trigger hairs in succession will trip the trap. Yo
u can repeat Darwin’s test of the sensitivity of the hairs by making a probe with a short length of hair. (Someone with longish straight hair might wish to donate one from his or her head; otherwise, try a substitute like a length of moderately thick thread, dental floss, or fishing monofilament.) Darwin’s “hair probe” made it possible to touch the filaments very lightly. To make your hair probe, affix a single hair with tape or glue to the end of a 1 in. (5 cm) length of 3/16 in. (0.5 cm) diameter wood dowel, toothpick, or matchstick with the flammable tip removed. This will be the probe handle. Cut the hair to extend about 1” (2.5 cm) beyond the end. (The hair can be glued to the tip of a dissecting probe, alternatively.) Now you are ready to test the sensitivity of the trigger hairs: ever-so-gently brush the hair of your probe against two of the trichomes in fairly rapid succession and observe the result; watch your fingers! (Just kidding.) Allow up to 24 hours for a closed trap to fully reopen.

  The slender trigger-hairs on the inner surface of the flytrap can be tripped with a probe. Drawing by Leslie C. Costa.

  See also:

  A. M. Ellison, “They Really Do Eat Insects: Learning from Charles Darwin’s Experiments with Carnivorous Plants,” in Darwin-Inspired Learning, ed. M. J. Reiss, C. J. Boulter, and D. L. Sanders (Rotterdam: Sense Publishers, 2015), 243–256.

  “Insectivorous Plants” at the Darwin Correspondence Project: www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/insectivorous-plants.

  9

  Crafty and Sagacious Climbers

  Vines and other climbers have always had the power to fascinate. They evoke associations positive and negative: in myth, metaphor, and lore such vines as grapes and hops are ancient symbols of growth, prosperity, fertility, and a good party, and vines play important supporting roles, so to speak, in the adventures of heroes like Tarzan or Indiana Jones. But more often than not we find something vaguely sinister about climbing plants, and our fascination comes with a tinge of fear or wariness. I think the reason is at least twofold: first, vines can easily bring entanglement to mind, if not strangulation. Fictional ones are almost always sinister, from J. K. Rowling’s devil’s snare to the lethal assassin vine of Dungeons and Dragons. Some are stranglers in lore, like the European swallow-wort (Cynanchum louiseae), a milkweed relative commonly known as “dog-strangling vine.” Others are stranglers in fact (albeit of other vegetation, not animals), like the notorious kudzu vine of Japan (Pueraria spp.). This viney legume was introduced to the southeastern United States in the 1940s to stabilize soil during road construction. However, it proved to be an extremely invasive plant and is now popularly known as the Vine that Ate the South. The second reason that climbing plants can seem sinister is their movements: uncannily animal-like, they inspire an uneasy sense that they are purposeful in their probing; watchful as they stealthily grow and entwine. That movement, a slither in slow motion, may underlie yet a third reason for our uneasiness: altogether too snake-like, they evoke images of perhaps the most (unfairly) feared and reviled of animals.

  Darwin had no such negative associations with climbing plants. On the contrary that same uncanny animal-like ability to move, reach, probe, and feel that underlies our unease piqued his curiosity. This was another group of plants with volition, with sense perception that underscored their fundamental relationship with animals, just like his beloved sundews and flytraps. Certainly John Horwood, the neighbor’s gardener who helped Darwin with many of his botanical experiments and oversaw the design of his heated greenhouse, thought so. Asa Gray had sent Darwin seeds of Echinocystis lobata, the North American wild cucumber plant, an excellent climber with tendrils that always seem to find something to grab in their vicinity. In a letter to Hooker, Darwin marveled at the sensitivity of those tendrils, and, referring to Horwood, commented that “A clever gardener, my neighbour, who saw the plant on my table last night, said ‘I believe, Sir, the tendrils can see, for wherever I put the plant, it finds out any stick near enough.’”1

  Darwin was in the midst of his early flush of excitement about Drosera, back in early 1861, when Gray brought tendrils to his attention. They must have been discussing the irritability of Drosera leaves. In a now-lost letter Gray evidently reminded Darwin that he had written a paper in 1858 entitled “Note on the Coiling of Tendrils.” This paper described the exquisite sensitivity of tendrils of the bur cucumber, Sicyos angulatus, which coil at the slightest touch and uncoil again in the space of an hour.

  Darwin filed this information away in his mind, for at that moment he was in thrall to sundews as well as flower polymorphisms and crossbreeding, while also hard at work on his orchid book. But a year later he turned to tendrils and wrote Gray to remind him about that paper. “I should like to try a few experiments on your Tendrils; I wonder what would be good & easy plant to raise in pot,” he asked. Ever obliging, Gray sent a reply to Darwin later that month, joking about how North American “weeds” are routed by invading species (another subject of interest to Darwin), but also providing seeds and growing instructions (with a jibe about the lack of enough warmth and sunshine in England for the plants):

  Oh—as to the weeds,—Mrs. Gray says she allows that our weeds give up to yours. Ours are modest, woodland, retiring things, and no match for the intrusive, pretentious, self-asserting foreigners. But I send you seeds of one native weed which corrupted by bad company—is as nasty and troublesome as any I know, viz. Sycios [sic] angulatus,—also of a more genteel Cucurbitacea, Echinocystis lobata (the larger seeds). Upon these, especially upon the first, I made my observations of tendrils coiling to the touch. Put the seeds directly into the ground; they will come up in spring, in moist garden soil. My observations were made on a warm sunny day. I doubt if you have warmth and sunshine enough in England to get up a sensible movement.2

  That spring Darwin tried to germinate Gray’s seeds in his newly completed greenhouse, designed to John Horwood’s specifications and built against the kitchen garden wall. Horwood’s boss John Turnbull had kindly lent Darwin the use of his own greenhouses for Darwin’s orchid work. Now, with a greenhouse of his own, he could maintain his plants all year long and a lot more of them to boot. Whether it was the limited English sunshine or not, just one Echinocystis seed that Gray sent germinated, and none of the Sicyos seeds. Darwin made the most of the one specimen he had: “I have been looking at its tendrils & seen with great interest their irritability; it is a very pretty little discovery of yours,” he wrote to Gray that June. He also revealed a discovery of his own: “I am observing the plant in another respect, namely the incessant rotary movement of the leading shoots, which bring the tendrils into contact with any body within a circle of a foot or 20 inches in diameter.—If I can make out anything clear about this movement, & do not find that it is known, I will perhaps write a letter to you for the chance of its being worth inserting in Silliman or elsewhere.”3 “Silliman” was a reference to the American Journal of Science and Arts, informally known as “Silliman’s journal” for geologist Benjamin Silliman Sr. of Yale, the journal’s founder.

  The regular circular motion of the shoot immediately captured Darwin’s attention, and that same month he wrote to Hooker to ask if he or Daniel Oliver at Kew were aware of the phenomenon. Observing Echinocystis lobata, he suddenly noticed that the stem between the uppermost leaves slowly rotated, tracing a full circle some 12 to 20 inches across in 1½ to 2 hours. Sometimes it stopped and rotated in the opposite direction. Curiously, the stem didn’t appear to become twisted, yet the rotation went on day and night. When a solid object was placed within the circle traced by the probing plant, the tendril immediately grabbed hold. No wonder the plant seemed to be able to see. He asked Hooker if the slow rotary searching behavior was already well known.

  Hooker hadn’t taken notice of this phenomenon and immediately jumped on the bandwagon and set some Kew staff to observing climbing plants. Darwin then wrote to ask Hooker if he had any climbing plants to spare for his greenhouse, and described in more detail some of his obser
vations. Hooker was encouraging: “Your observations on Tendrils &c are most curious & novel,” he said, “& I am delighted that you are going on with them—you are ‘facile princeps’ [easily the best] of observers.”4 Darwin’s new greenhouse was soon home to a host of climbing plants from around the world—he was ideally situated to benefit from proximity to the largest botanical collection anywhere, courtesy of his good friend who happened to be assistant director.

  It turned out that although Hooker did not know of the circular motion of the tendrils, Gray in the United States did. Darwin excitedly wrote to Gray that July with his findings, but was deflated when Gray replied that the phenomenon was well known. When he told Gray that he found nothing published on the subject, and nor did Hooker and his colleagues at Kew know anything about it, Gray came down a bit hard on the Kew botanists, telling Darwin that “As to tendrils, What are Hooker & Oliver (the latter a Professor too) about, and where have they lived not to know anything of them?” A slightly exasperated Gray told Darwin that “everybody” must have seen the way the tendrils sweep, and coil up if they fail to contact something after a certain time. Then he directed Darwin to reread the opening sentences of Gray’s 1858 paper, the one that initially piqued Darwin’s interest, where he would find reference to the work of Hugo von Mohl, the botanist at the University of Tübingen in Germany who first described the rotary movement of tendrils back in the 1820s. Gray long before had encouraged a colleague to translate von Mohl into English, and he reprimanded Darwin tongue-in-cheek for overlooking it: “and be thankful to me for having instigated (in 1851) Van Voorst to get Henfrey to translate this little book.—tho you English pay no attention to it, when you have got it.” Ouch. But he encouraged Darwin not to give up his experiments: “do not abandon this subject, for it will be fruitful in your hands.”5