It turned out that others, too, had described the rotating and twisting of climbing plants: Ludwig Palm, another botanist at Tübingen, and the Frenchman Henri Dutrochet, perhaps most extensively. Chastened but sure that he could make a contribution, Darwin forged ahead; there was more to his interest in these climbers than their apparent sense perception. Besides, observing them in the quiet of his study and greenhouse was the perfect occupation for his increasingly jangled nerves. The years 1863 and 1864 were difficult for Darwin. The Origin, out in its third edition in April 1861, was still a lightning rod drawing strong criticism from some quarters, and he agonized over the harsh reviews. He received a constant stream of letters with all manner of questions, comments, criticisms, attacks, and the all-too-occasional congratulations. He was relieved to let friends like Huxley, Lyell, and Gray enter the fray on his behalf—a fray that he literally could not stomach. Darwin’s old illness, which seemed to ebb and flow, came on like a fast-flowing spring tide in those years, with near-daily retching and chills. By September 1863 he was in constant pain; in a letter to his friend the Rev. John Innes (formerly the local vicar, who had since moved to Scotland) Darwin commented that he had taken to botany and all he could manage was an hour or two of work each day. Emma insisted that they take a family holiday in Malvern, Worcestershire, where Darwin could undergo treatment at the hydropathic establishment of James Smith Ayerst—their youngest son Horace, aged 12, even took the “water cure” with his father, since he was displaying similar dyspepsia. They stayed several weeks but to little effect; the locale was perhaps not the best for soothing his nerves, considering it was the scene of the devastating death of their daughter Annie back in 1851. He tried different cold-water treatments when he returned home, but they failed, too, and he finally gave up the water cure for good. This period of ill health was the worst that Darwin had ever experienced, confining him to bed for some weeks by early 1864.
Under the circumstances, watching the slow drift of shoots and tendrils in the near-dark of his study was the perfect occupation. “The pace of his life,” as Darwin biographer Janet Browne nicely put it, “slowed to that of a plant.”6 His children (most now young adults) helped as they could. Darwin informed Willy, his eldest at 24 years old and working in banking in Southampton, that his current hobbyhorse was tendrils, and asked him to make some observations on dimorphic plants. George, Frank, and Lenny, all teenagers, were boarding at Clapham School, not far away in south London. When home on holidays they once again became faithful research assistants; Lenny recalled how his father, joking about his son being “infernally healthy,” sent him off to fetch plants for the sickroom-study, and George proved to be a talented botanical artist. George illustrated his father’s Linnean Society paper with drawings of plants in the gardens and greenhouse at Down. In 1864 George graduated Clapham School and entered Trinity College, Cambridge, where the mathematical instruction of Clapham headmaster Charles Pritchard, an astronomer, stood him in good stead. While his start at Cambridge was inauspicious, to the family’s surprise he ultimately graduated as “second wrangler” (second-highest scores in the university) on the grueling Mathematical Tripos examination. George went on to become an able astronomer himself, eventually Plumian Professor of Astronomy and Experimental Philosophy at Cambridge.
The Many Ways to Be a Climber
The sensory aspects of Darwin’s “crafty & sagacious” climbers were fascinating, but he was just as keen on another great evolutionary principle that they demonstrated: adaptive variation in climbing. Different species accomplished the same thing in different ways; that is, different structures of the plants had been modified to serve similar uses in different but related groups, illustrating principles such as homology, analogy, and convergence. In August of 1863 Darwin made a telling statement to Gray that tendril “irritability is beautiful, as beautiful in all its modifications as anything in orchids.” It was the modifications—structural variations on a theme in accomplishing climbing—that became the most important evolutionary story told by vines. Recall Darwin’s admission (also to Gray) that his “chief interest” in his orchid book was how “it bears on design, that endless question.” There he showed how orchids exemplified the great principle of different parts modified for similar ends, and similar parts modified for different ends. The underlying homologies and analogies of structure in different species pointed to the vagaries of evolution by natural selection. In response to similar selection pressure, different species may evolve the same “solution” but do so in different ways. It was as if anatomical structure were a toolbox, and different parts could be modified to serve similar functions. “Homologous” organs have a common underlying structure and origin (embryological and evolutionary), even if they outwardly look very different once fully developed. A textbook example is the limb structure of mammals: bone-for-bone the wings of bats, forelegs of moles, flukes of whales, running legs of horses, and reaching arms of humans are the same structure, though the bones have different sizes and shapes. Homologous organs may be used in similar ways, or not—most of these limbs are used for locomotion, but very different forms of locomotion, from burrowing to flying to swimming. “Analogous” organs, in contrast, are outwardly similar in structure and function, but they are anatomically different and lack a common origin. Whale flukes and fish fins are a typical example—these have the same function and superficially look alike, but are very different anatomically. That difference indicates that they evolved independently, natural selection having honed them from different ancestral starting points into the shape and maneuverability necessary for efficient swimming. They are convergent, with penguin wings representing another such convergence: these birds “fly” through the water.
In the last chapter we saw Darwin effectively asking why a Creator would bother to modify different floral structures in different orchid groups to function in essentially the same way. Vines, too, present oddities and convergences in regard to the different ways that climbing is accomplished. This variation, Darwin argued, is more consistent with descent with modification than with special creation and supernatural design. The design argument implies that, insofar as there is a best way to do something, an omnipotent designer would use that best solution—that is the essence of good design. So, whether we’re talking about orchid pollination or climbing vines, a designer would use the same excellent design solution throughout the respective groups, not mixing and matching, or jury-rigging, to accomplish the same ends in different lineages. That messiness is more consistent with an evolutionary process where each step must build upon what came before.
Climbing adaptations certainly are variations on a theme. Some ascend by means of grappling-hook thorns; others have modified roots; still others modified leaves or flower peduncles. Darwin divided them accordingly in his long 1865 paper “On the Movements and Habits of Climbing Plants” (taking care to point out that “these subdivisions . . . nearly all graduate into each other”). First there are the twiners. Familiar examples include brewer’s hops (Humulus lupulus), morning glory (Convolvulus), and honeysuckle (Lonicera). In these plants the stem itself is the climbing and “grasping” organ as it grows. The stem’s tip, or apical meristem, steadily revolves in a counterclockwise direction, thereby spirally encircling supports like the stems and branches (below a certain diameter) of other plants. This is the rotary motion that first captured Darwin’s attention. It is now thought that this circumnutation (Charles and Frank Darwin came up with the term) results from differential growth and elongation of cells of the growing meristem, which is a zone of cell division. Division and elongation of cells evenly across the meristem would result in straight (outward and upward growth), but if cell division and elongation was happening a little faster on one side, that side would in a sense “pull ahead” of the neighboring cells, forcing the apical portion of the meristem to bend or curve toward the slower-growing cells. What seems to happen is that cell division and elongation move back and forth across the meristem
face like a “fan wave” rolling back and forth across the stands at a football game, with the result that the meristem curves continuously in a circle, taking anywhere from 1 to 24 hours to complete a circuit.
The leaf-climbers include vines such as virgin’s bower (Clematis), nasturtium (Tropaeolum), and gloriosa or climbing lilies (Gloriosa). These plants climb with sensitive petioles that respond to contact by bending and hooking. Once they clasp a support these specialized petioles often undergo a metamorphosis, thickening and often becoming woody. Also included in this category are vines that climb with modified flower stalks, or peduncles. One well studied peduncle-climber is Strychnos, a genus of mostly tropical trees and vines known for their potent toxins (one gives us strychnine, the alkaloid used in rodent control). Leaf-climbers also exhibit circumnutation, and often rotate a good deal faster than the twiners.
Then there are the tendril-bearers. Darwin studied a great many of these, including grapes (Vitis), peas (Pisum), creepers (Parthenocissus), crossvine (Bignonia), wild cucumber (Echinocystis), bryony (Bryonia), catbriers (Smilax), and beans (Vicia). Tendrils are specialized climbing organs derived from different plant structures depending on the plant, including leaves, stems, or peduncles. While their circumnutation is similar to twiners, they tend to move faster and in more of an elliptical pattern, and a single individual is capable of circumnutating clockwise and counterclockwise. These plants often grasp in a two-phase coiling movement. Contact coiling occurs when the tendril first touches a potential supporting object. The tendril then has a growth and coiling spurt, leading into the free coiling phase, which tightens its hold on the support. The tendril does this by coiling in the opposing direction, resulting in two helical spring-like structures coiled in opposite directions relative to one another (one a right-hand coil, twisting counterclockwise; the other a left-hand coil, twisting clockwise).
The term for the left-hand-coiling process is perversion. (Most of these climbers, like humans, are right-handed, and so left-handedness is generally seen as an oddity; consider the word sinister, from Latin for left-handedness.) The opposite-helix phenomenon found in many tendril-bearers has been studied by physicists, who have shown that as tension in a cord or stem (or tendril) is decreased by helical coiling, at a certain point an instability (termed “curvature-to-writhe” instability) spontaneously sets in, resulting in two helices of opposite handedness. This is the first step toward the formation of super-coils (coils of coils) and can be demonstrated by grasping a length of rubber band and twisting both ends simultaneously. First, at a critical juncture of tension, twists of opposite handedness will form. Then with increased twisting a super-coil will form.
Some tendril-bearers will produce adhesive disks at the end of their tendrils, which attach to surfaces with glue-like secretions. The properties of the adhesive disks, which allow the plant to climb smooth surfaces without the need to coil around supporting branches to ascend, was of great interest to Darwin. The Ivy League schools have those disks to thank for their nickname: they are the reason that Boston ivy (Parthenocissus tricuspidata—a species Darwin studied) can adorn those stately limestone and brick walls.
Tendril of bryony (Bryonia dioica), showing coil “perversion.” Note the counter-clockwise coiling in the proximate portion of the tendril, and reversal to clockwise coiling in the more distal portion. From Darwin (1865), p. 96, fig. 13.
Hook- and root-climbers, finally, consist of a grab bag of climbers, including ramblers and scramblers that ascend with the aid of grappling-hook-like thorns (multiflora rose, Rosa rugosa) or specialized rootlets that produce sticky secretions (English ivy, Hedera helix). Darwin didn’t find their behavior very interesting, so he shoehorned them into one category to acknowledge the group, but discussed them only lightly. I will do the same.
It was the leaf-climbers and tendril-bearers that he found most illuminating, groups that provide a good analogy with some of the swimmers mentioned earlier: cetaceans and penguins are related vertebrates that have separately evolved organs similar in structure and function, from a similar ancestral pentadactyl (five-toed) limb starting point. Tendrils, flower stalks, and leaves and leaf petioles, Darwin realized, have a point of common ancestral origin, too. In most plant groups, they serve different functions and so have different structural appearances. But in climbers, they have been modified into similar structures, for similar functions.
Darwin worked slowly but steadily on his climbing plants during 1863 and 1864, and by the end of 1865 he had studied scores of climbing plants. This work is breathtaking in its scope, a first in this line of research. He proceeded with observations and experiments on the motions and touch sensitivity of the plants; and he also observed their structures, with an eye toward comparative anatomy. At first, mostly he watched.
Do the Twist
As a student Darwin had been taught that climbing plants twist as they grow owing to the natural tendency for plants to grow spirally, by twisting the stem on its axis. As he now realized, that was something of a nonexplanation, and on close inspection he found that twiners and other climbers do not wholly twist on their axis as they grow. In the first section of his 1865 paper he took on the nature of the rotary movement of twiners, including the rate of rotation and the way spiral growth is achieved. He explained it with an analogy: “If we take hold of a growing sapling, we can of course bend it so as to make its tip describe a circle, like that performed by the tip of a spontaneously revolving plant. By this movement the sapling is not in the least twisted round its own axis.” That is, rotating the sapling does not twist it up like a wound rubber band. Rather, the sapling is flexible, so it just bends in each direction it’s pulled.
He devised a characteristically simple experiment to test for twisting in twiners. He put a dab of paint on the outer curved or bowed part of a twining shoot, and watched. As the twiner grew and slowly rotated around, the dot of paint seemed to change position. First it inched over to one side of the bowed tendril, then in the concavity beneath the tendril, opposite the outer bow. Later still it appeared on the other side of the bowed tendril, and finally ended up back where it began, on the outermost bowed surface. “This clearly proves,” Darwin wrote, that tendrils “during the revolving movement, become bowed in every direction. The movement is, in fact, a continuous self-bowing of the whole shoot, successively directed to all points of the compass.”7 He explained it again as a thought experiment:
As this movement is rather difficult to understand, it will be well to give an illustration. Let us take the tip of a sapling and bend it to the south, and paint a black line on the convex surface; then let the sapling spring up and bend it to the east, the black line will then be seen on the lateral face (fronting the north) of the shoot; bend it to the north, the black line will be on the concave surface; bend it to the west, the line will be on the southern lateral face; and when again bent to the south, the line will again be on the original convex surface.8
This process is counterintuitive; most people likely assume that the tendril gets twisted up when it rotates. Not so. But what is happening at the cellular level? Darwin explained this by imagining a wholesale contraction of cell growth on each side of the stem in turn; this would have the effect of bowing the sapling in the direction of the contracting cells and forcing it to point in different directions. If the “zone” of cell contraction rolls around the sapling stem, the whole stem rotates without twisting. “In fact,” he concluded, “we should then have the exact kind of movement seen in the revolving shoots of twining plants.”
Darwin was largely correct. Changes in turgor pressure (the pressure within plant cells created by an inflow of water, like filling a balloon) leading to rapid expansion and contraction are indeed thought to lie behind circumnutation and many other plant movements. It took many years, however for plant physiologists to appreciate the significance of osmosis (movement of water across a semipermeable membrane like a cell’s membranous wall) for circumnutation and other plant movements. A con
siderable step in that direction was made by Hugo de Vries in Holland and Wilhelm Pfeffer in Germany, working on cell turgor and what we call today osmotic pressure. The work of these experimentalists was foundational for the modern understanding of how the elasticity of young plant cells and the active movement of substances like sugars into and out of them permit rapid and repeated changes in turgor pressure, and how this in turn can lead to rapid movement when cells act in concert. Until about the mid-nineteenth century botanists thought that bending movements in plants were due to an increase in cell growth on the convex side, opposite the bending. That can happen, but the physiologists showed that increased turgor pressure on the concave side is more important. As a set of cells in one area puff up like balloons or deflate in others they collectively exert pressure on neighboring cells, enough to bend a tendril.
Inspiring Revolutions
The slow rotation of stems and tendrils in wild cucumber, Echinocystis lobata, that first caught Darwin’s attention proved to be nearly universal in climbing plants, though wild cucumber seemed to move especially fast. He recorded 35 revolutions of the upper internodes (sections of stem between leaves) and tendrils, and found that they could complete a circuit in an average of an hour and 40 minutes. He cut off the tendrils, yet the shoot continued to rotate. He then turned to tendril-free brewer’s hops, grown in abundance in beer-loving Britain: “When the shoot of a Hop (Humulus lupulus) rises from the ground, the two or three first-formed internodes are straight and remain stationary; but the next-formed, whilst very young, may be seen to bend to one side and to travel slowly round towards all points of the compass, moving, like the hands of a watch, with the sun.”9 The average rate of rotation of hops shoots was just over 2 hours.
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