Darwin's Backyard

by James T. Costa

  Obtaining Your Climbers

  Try to find climbers from three or four groups below. Most should be readily available at garden centers and nurseries, or from online retailers, garden clubs, and websites dedicated to climbers.

  Common ivy (Hedera helix) Root-climber

  Brewer’s hops (Humulus lupulus) Twiner

  Virgin’s bower (Clematis virginiana) Leaf-climber

  Sweet autumn clematis (C. terniflora) Leaf-climber

  Anemone clematis (C. montana) Leaf-climber

  Wild cucumber (Echinocystis lobata) Tendril-climber

  Bur cucumber (Sicyos angulatus) Tendril-climber

  Passionflower (Passiflora spp.) Tendril-climber

  Prairie Moon Nursery (www.prairiemoon.com) sells seeds of native North American climbing plants including Clematis, Sicyos, and Echinocystis.

  NOTE: Depending on the species (and vendor) your climbers may be obtained as young plants, rhizomes or tubers, or may be grown from seed. Seeds may require stratification (some period of exposure to cold and moisture) before germinating; follow instructions on the seed packet. Vines and other climbers can get, of course, inconveniently long. Work with shoots or young plants of a manageable stature, planted in pots or a suitable outdoor garden with a small trellis or pole.

  I. Climbing Styles and Behavior

  Darwin was struck by how different plant groups climb with different organs, modified in various ways—some climb with their leading shoot, others with modified roots, leaf petioles, midribs, flower peduncles, or specialized tendrils, a nice illustration of evolutionary variations on a theme.

  A. Materials

  • English ivy, brewer’s hops, clematis, passionflower, and bur or wild cucumber, potted or planted in the ground with trellis

  • Hand lens or dissecting microscope

  • Bamboo chopstick, wood skewer, or wood dowel, 4 in. (10 cm) in length, for probe

  B. Procedure

  Observation often goes hand in hand with experimentation, and as we’ve seen Darwin learned a great deal about his subjects simply by looking closely.

  1. Root-climbing. Common or English ivy (Hedera helix): Native to Europe and Eurasia, this vine sports aerial roots along its stem. Root-climbers didn’t hold much interest for Darwin, since their climbing organs show few of the active “behaviors” of other climbers. Still, for comparative purposes it is worth taking a look. Root-climbers can ascend even smooth walls by producing a kind of glue. Tugging on a vine growing up a vertical surface, note how strongly it adheres. Inspect the rootlets for adhesive disks at their tips, which secrete a glue-like substance. The “glue” includes nanoparticles just 60–85 nm in diameter, which were first described in 2008. They are being explored for applications in medicine, cosmetics, and cleaning agents.

  2. Twining. Hops (Humulus lupulus) vines are also capable of climbing smooth, unbranched supports. Lightly draw your finger up a stem to learn how: the rough sandpapery feel comes from thousands of tiny downward-pointing hooks. Inspect the stem with a magnifying glass or, better, a dissecting microscope, and you will see that the surface is covered with these hooks. Note that they are pointed at two ends, with a pivot point in the center. From the side they look like tiny anvils. These are modified trichomes, or plant hairs, that function as miniature grappling hooks. As the stem winds around its support the hooks catch on the surface, increasing stability; with the combination of the spiral twining and tiny hooks, hops is remarkably resistant to slippage, and stability actually increases the larger the vine grows. This is because the downward-pulling weight of the vine creates tension that helps the spiral stem tighten. The principle at work is similar to that of the bamboo “finger trap” toy, with helical bands of bamboo that tighten with stretching (tension) but loosen and open with inward force (compression).

  3. Leaf-climbing. Virgin’s bower (Clematis) vines provide a good example of a leaf-climber that uses its petioles to ascend. Gently stroke one side of an actively growing leaf petiole with a small twig or even your finger for a minute or two every hour, and within as little as 3–4 hours the petiole will have curled around on itself. You can uncurl it by repeating the process and stroking the opposite side of the petiole. The touch-sensitive petioles bend and clasp supporting stems or branches, or trellises in the garden. Clematis is “positively thigmotropic,” bending toward an object making physical contact.

  4. Tendril-climbing. Just as there are different ways to be a climber, among tendril-climbers we see the same principle of diversification at work: there are different ways to make a tendril. Tendrils, branched or unbranched, are variously specialized stems or leaves that may be derived from shoots, leaf petioles, or branches, depending on lineage.

  a. Bur cucumber (Sicyos angulatus) and wild cucumber (Echinocystis lobata) are North American climbers of the squash family, Curcurbitaceae. It was Asa Gray’s observations of the tendrils of bur cucumber that initially got Darwin interested in climbing plants. Using either of these species, try Gray’s demonstration of tendril sensitivity, from his 1858 paper:

  A tendril which was straight, except a slight hook at the tip, on being gently touched once or twice with a piece of wood on the upper side, coiled at the end into 2½–3 turns within a minute and a half.

  Following Gray’s lead, locate a straight tendril and gently stroke the upper surface a few times with the probe, then remove the probe. Observe and time the response. How long did it take for coiling to initiate, and how long did it last? Gray found that if the coiled tendril is left alone it will straighten itself. Time this: how long does it take your tendril to become straight again?

  NOTE: Use actively growing straight tendrils, not ones that are coiled up. In many tendril-bearers the tendrils contract into a tight coil and hang limply after a few days (longer in some species) if they fail to come into contact with an object to grab. When that happens, the tendril has become more or less insensitive and won’t respond to prodding, and so is useless for our purposes.

  b.Observe older bur or wild cucumber vines with well-developed tendrils that have grasped a support to find examples of coiling and countercoiling, or coil “perversion”—the formation of coils that go in opposite directions, functioning as shock absorbers (see illustration below).

  c.Passionflowers (Passiflora spp.) are a large and mostly tropical group native to the Americas, and many species are now available as garden plants prized for their unusual and complex flowers. Many people who grow passionflowers in their garden don’t give much thought to the plant’s tendrils. Not us! Note that the tendrils are unbranched, and extend from the axil of each leaf along the stem. Plants that are shaded produce a long leading or searcher tendril that extends toward sunlight like a long finger pointing the way the plant is yearning to go. You can see how readily this “finger” will grasp by placing a wood dowel or stick in contact with the lower surface of a leader tendril held out horizontally. Observe how the tendril goes from being extended firmly to seemingly going limp at the point of contact with the dowel. After the tendril has begun its coiling, remove the dowel, and see how the angle remains in the tendril. What happens? Will it continue to coil, or straighten itself over time?

  A coiling and counter-coiling tendril. Drawing by Leslie C. Costa.

  II. Circumnutation

  Up-and-coming climbers get ahead by constantly probing in a circular or elliptical motion, a phenomenon that the Darwins dubbed circumnutation (Latin circum, circle, and nutatio, nodding or swaying). Circumnutation is slow, but plants vary quite a lot in just how slow, from the glacial to just on the edge of perceptibility by watchful humans. It’s fun to try to gauge the movement with some circumnutation speed demons (bearing in mind that qualifying as a speed demon is a relative thing in the plant world). All you need is a point of reference, and that’s where our handy “circumnutometer” comes in. Try this with brewer’s hops, one of the speediest circumnutators around.

  A. Materials

  • Hops (Humulus lupulus) vine. Roots
tock is readily available commercially, or from home-brewer friends who grow their own hops. Use a young plant with a single actively growing shoot perhaps a foot long, planted in the center of a circular planter about 10 in. (25 cm) in diameter.

  • Paper plate with a diameter a bit smaller than the planter diameter so that it fits comfortably within the planter

  • Ruler

  • Scissors

  • Markers

  • Pins

  • Watch or stopwatch

  B. Procedure

  1. Cut out the center of the paper plate, creating a ring like the example at right. Mark the cardinal directions or hours of a clock (or only the 12, 3, 6, and 9 o’clock points) around the ring using the ruler and marker.

  Potted hops (Humulus lupulus) plant with rotating shoot. “Circumnutometer” disk at right can be labeled with cardinal directions or hours like a clock, and fitted over the hops atop the planter. Observed from above, the progress of the rotating shoot can be timed. Drawing by Leslie C. Costa.

  2. Carefully guide the ring over the hops plant and central support, centering the ring inside the planter laying it on the soil surface; secure if necessary with pins. This is your circumnutometer.

  3. Looking down on the planter with 12 o’clock at the top, record at about what “time” or direction the hops shoot is pointing.

  4. Check the shoot periodically, perhaps at hourly intervals, each time recording the “time” or direction indicated on the circumnutometer. Which way is the shoot revolving? Darwin’s hops took about 2½ hours to complete one revolution. Is your hops revolving faster or slower than what Darwin observed? This may be expressed in CPH (circles per hour).

  III. Geotropism: Downwardly Mobile Beans

  If shoots are up-and-coming, roots are downwardly mobile. Here we show, like Darwin, that the expanding young roots (radicles) of sprouting beans sense gravity at their tips.

  A. Materials

  • 15–20 Dried pinto, black, or kidney beans

  • 20 Paper towels

  • 4 Quart-sized plastic sealable bags

  • 4 Widemouthed jars (canning, peanut butter, mayonnaise, etc. jars; glass or plastic)

  • Thick cardboard (e.g., from a corrugated cardboard box), cut into squares a bit wider than the mouth of the jars

  • Waxed paper, cut into squares matching the size of the cardboard squares

  • Cotton balls

  • Straight pins (long)

  • Craft knife or razor blade

  • Small bowl and water

  • Protractor and ruler

  B. Procedure

  1. First germinate your beans:

  a. Divide beans into 3–4 groups of up to five beans each.

  b. Place five paper towels on top of each other and wet them so they are saturated, but not dripping wet.

  c. Place up to five beans in the center of the paper towels about one in. (2.5 cm) apart, and fold the paper towels.

  d. Place the paper towels with beans inside the quart-sized sealable bag and seal. Follow this same procedure for the remaining groups of seeds.

  e. Allow the beans to germinate and the radicles develop, about 2–3 days.

  2. The germinating radicles will often point in various directions, even doing loop-the-loops. Select four beans with well-developed radicles extending more or less horizontally, straight out from the bean.

  3. With the knife or razor blade carefully cut the last 1–2 mm of the radicles off two of the beans (treatments), and leave the other two intact (controls).

  4. Fold a saturated paper towel and place it in the bottom of a jar, or pour in a small amount of water.

  5. Place a pin through a bean, and then carefully through a wet cotton ball.

  6. Place a waxed-paper square on a cardboard square. Next pass the pin with the bean and cotton ball through the waxed paper and cardboard square. Secure the pin with a dab of glue if necessary. Place the cardboard on the opening of a jar, with the bean inside the jar to maintain humidity and prevent the bean from drying out.

  Pinned bean gravitropism experiment, with radicle initially held horizontally in jar. Different jars can be set up with treatment (excised) and control (intact) radicle tips to test for response to gravity. Drawing by Leslie C. Costa.

  7. Repeat with the other three beans and jars, and label the jars according to whether they contain treatment or control beans.

  8. Place jars in a dark space like a cabinet or closet, or cover with black construction paper, and allow radicles to further develop over a few days.

  9. Make daily observations: the pinned radicles can be removed from the jars for measurement. Use a protractor to measure the angle (deviation) of each radicle from the horizontal, and a ruler to measure radicle length.

  10. Graph your data to obtain growth rate of the radicles and rate of change in angle of deviation over successive days. How rapidly did the intact radicles respond to gravity? Did the growth rate of the intact and excised radicles differ? By how much did the radicles deviate from the horizontal? Graph the deviation in degrees on successive days. The radicles with the tips removed should no longer sense gravity, and so should continue to grow more or less horizontally. The intact radicles, on the other hand, should strongly dip as they develop.

  See also:

  “Climbing Plants” and “Power of Movement in Plants” at the Darwin Correspondence Project: www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/climbing-plants and www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/power-movement-plants.

  10

  Earthworm Serenade

  In the summer of 1880 Emma Darwin commented dryly on her husband’s latest hobbyhorse. In a letter to their son Leonard, she wrote that his father “has taken to training earthworms but does not make much progress, as they can neither see nor hear.” They were a source of amusement, however, spending hours “seizing hold of the edge of a cabbage leaf and trying in vain to pull it into their holes.”1 Most people simply assumed that earthworms could neither see nor hear, but Emma had firsthand experience: her husband had experimentally demonstrated this, and she even helped with some of the experiments. Emma was joking about Darwin trying to “train” the earthworms, but Leo, then 30 years old and teaching chemistry and photography at the School of Military Engineering in Chatham, surely chuckled on reading her letter—he knew all about his father’s worm-wrangling, with his flowerpot “wormeries” in the study, midnight worm-watching excursions on the sandwalk, and experiments designed to test earthworm IQ. He had helped with his share of experiments too over the years, most recently procuring colored glass at the request of his father to use as light filters in other earthworm investigations.

  For the past few years, even as Darwin struggled to complete his sixth and final botanical book (Power of Movement in Plants, published in 1880), earthworms had taken center stage at Down House. He was fascinated by them, even smitten. But this latest hobbyhorse was not new: it was in fact a return to one of the first studies he had undertaken after the Beagle voyage, namely, earthworms as a geological force. He had long thought that, despite being dismissed as mere fish bait, earthworms taught object lessons in Lyellian uniformity. Now he was sure there was even more to them than that—they may seem senseless, but they were animals, after all, cryptic but exquisitely attuned to their environment. They had a certain wormy intelligence, he came to appreciate: problem solvers, with personality. Darwin’s fascination with the industriousness and psychology of worms might seem like the obsession of an eccentric on the surface of it, but by now you surely appreciate what a mistake it is to judge Darwin on surface appearances. No, beneath the surface his mind was always churning, turning out remarkable insights from the grist of simple observation and experiment, like those worms continually turning the soil, unseen yet remaking the world.

  The Maer Hypothesis

  Darwin was making quite a mark in London geological circles in 1837. He was just 2 months back f
rom his voyage around the world, and the new year barely opened when he read his first paper, on the elevation of the Chilean coast. Geology was a grand subject, awe-inspiring in its scope, from the vast sweep of time to the power of its forces to the inexorability of its changes. He followed that paper with another on the extinct mammals of South America, and then another on his theory of coral reef formation. The cycle of time and slow geological building up and wearing down were on his mind: the coastal elevation and coral reef papers were all about uplift, growth, and construction, while the one treating long extinct and fossilized mammals was all about erosion, subsidence, and burial. Visiting his Wedgwood relatives at Maer, as he often did, Darwin’s ever-observant Uncle Jos (Josiah II) showed him a curious thing: some years earlier he had spread over different fields an early form of fertilizer containing granular lime, cinders, and burnt marl (reddish nodules of clay mixed with calcareous material like shell bits). Now, he showed his nephew, they were all buried in a recognizable layer a few inches beneath the surface. Farmers had long noticed that objects spread on their fields were buried over time, but they simply assumed these things somehow just “worked their way down,” through the action of rain and gravity. Josiah Wedgwood thought differently: he opined to his nephew that the explanation lay in the constant churning of the soil by earthworms.

  Earthworms are segmented (annelid) worms of the class Oligochaeta, a group of terrestrial and aquatic species with a simple and mostly smooth, tubular body form. They are hermaphroditic (bearing both male and female sexual organs), and deposit their eggs in a sac produced by the glandular clitellum, the thickened body section resembling a wormy Band-Aid. There are some 4,100 or so described earthworm species worldwide, and their distribution often bears the signature of historical geologic or climatic conditions; in North America, for example, all earthworms in northerly ice-covered areas of the last glacial period were extirpated, and the northern extent of native earthworms on the continent today still largely mirrors the glacial front. Despite their name some earthworms have taken to an aquatic or semiaquatic lifestyle, but the majority are terrestrial. These make their living in rather different ways in the soil, reflecting the kind of divergence and niche partitioning that Darwin explored with plants (Chapter 3). There are smallish pigmented epigeic worms that feed on decaying organic matter, living in the area where the soil and leaf litter interface, under logs, or in compost; topsoil-burrowing endogeic worms that make and live in horizontal burrows and rarely surface; and the often large-bodied and deeper-dwelling anecic worms that make and live in permanent vertical burrows from which they surface to forage for leaves.