A Garden of Marvels Page 26
Dorothy wrote back, “Cheers!”
Two months later, I visited again, bringing my laptop and an FTDI cable for downloading a new script. All went well, with Dorothy reporting to my mother, and my mother reporting to me. (My mother, who has been known to pour an off-brand vodka into a Stolichnaya bottle, was nonetheless a little miffed when Dorothy warned, “And none of that no-name stuff.”) In January, however, my mother reported that Dorothy was having a problem. Not a drinking problem, of that she was certain. Nor was Dorothy undernourished: My mother had added fertilizer on schedule. Nonetheless, the tree’s lower leaves were turning yellow and falling off.
Shades of Kam Kwat.
I had my mother inspect the leaves for bugs. She didn’t see any, she said, but there were some little, wispy cobwebs. And the leaves seemed oddly dusty.
I knew that syndrome. Dorothy was infested with spider mites. Spider mites are so small that to the naked eye they look like a fine, reddish dust. When an infestation gets bad, tiny cobwebs appear at the intersections of leaf and stem. Like other arachnids, spider mites spin webs, although not to trap prey but to avoid predation. Like aphids, they drill their stylets into leaves—into individual cells, in this case—in order to drain the sucrose, a practice that eventually kills the host. Outdoors, mites are controlled by predator insects and washed off by rain. On my mother’s screened veranda, they were living in a gated, all-inclusive resort.
Fortunately, the population of spider mites on a single plant is easy to reduce, although it’s hard to eliminate them all. I told my mother that she’d have to spray the leaves, particularly their undersides, with horticultural oil once a week for three weeks. The first spraying would smother the active bugs; the next two would take care of the hatchlings of eggs already laid. I expected some resistance to that chore, but to the contrary, she was eager to get started. Of greater concern was how she could get new leaves to come out on the defoliated branches. Dorothy, she said, was looking rather threadbare. I promised I would figure it out. At least I knew radical pruning was not an option. To my surprise, I discovered that Charles Darwin had something to do with the solution to Dorothy’s problem.
twenty-eight
Onward and Upward
In the early 1860s, at the same time Darwin was observing orchids and pondering how evolution shaped their extraordinary variety, he became infatuated with the sundews (Drosera) that grew along the edges of a nearby heath. Sundews are small plants that live in boggy areas in North America and northern Europe. Their leaves, round or straplike, are rimmed by sticky filaments that snag insects unfortunate enough to blunder into them. The leaves then slowly bend over the victim and dissolve it. Darwin was fascinated by the process and set out to discover what the Drosera eat. He fed his sundews raw beef, peas, olive oil, and even cobra venom, all of which they readily devoured. When sand, cinders, and glass were on the menu, however, although they closed their leaves over these tidbits, they soon reopened. Nitrogen, he concluded, is what sundews seek, and their carnivorous ways are an evolutionary adaptation that allows them to live on nitrogen-poor soils. In fact, we now know that up to 50 percent of a sundew’s nitrogen comes from digesting trapped insects.
In 1863, Darwin began to study climbing species to explain how evolution had enabled plants without trunks or stiff stalks to join in the general race toward the sun. To the congeries of exotic orchids and sundews in his heated greenhouses, he added climbers. Plants with weak cores have three strategies for using external supports to compensate for their deficiency. Some, like hops, are twiners that travel sunward by twirling their stalks around a buttress. Others have projecting tendrils or leaves (the pea and potato plant, respectively) that do the grasping. But how does the tip of a stem or tendril or leaf find something to grip? He could see the tips moved, seemingly in search of a handhold, but was there a method to their investigation?
By positioning a pane of glass above these moving extremities, Darwin was able to trace their paths throughout the course of a day. He discovered that they rotate in an elliptical, species-specific pattern, which he called circumnutation or “nodding in a circle.” When a tip encounters a vertical object such as another plant’s stalk or a fence post, by continuing to circle, it gradually bends itself around the rigid structure. Because the tips grow upward while they circumnutate, they tend to form a spiral. Darwin had noticed that when a tip encounters a crevice in a flat surface, it appeared to consciously explore and then reject the location, but he now saw that actually it is pulled out by its own constant revolving and upward motion. In The Movements and Habits of Climbing Plants (1865), he concluded that the various strategies for clinging likely evolved from some ancient capability. Twining, he wrote, is another example of modification by descent.
Darwin next returned to carnivorous plants, and in particular to the Venus flytrap, a North American native he called “one of the most wonderful plants in the world.” At the top ends of the plant’s leaves are a pair of large, reddish lobes joined in the middle and rimmed with needlelike tines. When the trap snaps closed—in about a tenth of a second—the tines interlock to create a barred cage.
Venus flytrap.
Flytraps, he recognized, have the same gustatory preferences as sundews, but have evolved more effective hunting methods. In addition to nectar on their surfaces and tines around their edges, each lobe has three touch-sensitive hairs, called trigger hairs, on the interior surface of each lobe. A trap closes only if two of the trigger hairs are touched within twenty seconds. In this way, unlike the sundews, flytraps don’t waste energy closing over a cinder or a raindrop that is likely to touch only a single filament. Moreover, the tines of the flytrap’s cage are spaced so that smallest insects can escape; the amount of nitrogen they contain isn’t worth the metabolic cost of digesting them. The more refined mechanisms of the flytraps, Darwin concluded, had evolved from the simpler sundews, and provided additional evidence for his theories.
In the mid-1870s, Darwin returned to studying plant movements, finding them interesting in their own right, and set out to understand the physical and chemical mechanisms that activated them. In essence, in his late sixties, he decided to branch into a new scientific discipline, plant physiology. Unlike his previous work, which was based primarily on his exquisite skills as an observer—as well, of course, as his powerful and creative intellect—his new work would involve experimentation. He started by identifying what he considered the most basic kind of plant movement, the one that climbing plants must have modified over time. That movement, he concluded, is phototropism, a plant’s ability to bend to follow the sun as it moves across the sky over the course of a day. Phototropism (or as Darwin called it, heliotropism) would be the subject of his experiments.
In 1877, however, he was sixty-eight years old, and wasn’t sure he had the energy to undertake the work. In the eighteen years since the publication of the Origin, he had revised his great opus six times, rewriting about 75 percent of the text to incorporate new evidence. In the same period, he had researched and written articles and books on subjects as diverse as human evolution, flowers and fertilization, barnacles, orchids, climbing plants, and insectivorous plants, as well as engaging in a massive and international correspondence. All of this intellectual activity took place at his home, where he also participated in the lives of his wife, Emma, and their seven surviving children. Moreover, throughout these intense years of creativity, and in fact ever since his return from the Beagle, he had battled bouts of depression and episodes of unexplained vomiting, vertigo, tremors, and other symptoms that had interrupted his work, sometimes for months at a time. Now the symptoms were worsening.
To make matters much worse, his son Francis had recently lost his young wife to puerperal fever, leaving Francis the devastated father of a newborn son, his first child. Francis and his wife Amy had lived just a quarter mile down the road from Charles and Emma, who had known Amy’s family for years. Amy’s death was not only a present loss—Charles relished he
r high-spirited, outgoing nature, so unlike his own, and treated her like a daughter—but it opened an old wound that had never fully healed, the death of his firstborn daughter, Annie.
Annie had had a similarly warm and demonstrative personality, and Darwin admitted she was his favorite. When Annie was eight, she contracted scarlet fever, and her health was never the same. Two years later, Charles had taken her to the spa at Malvern, 150 miles northwest of London, where he had often retreated to tend to his own ills. Shortly after their arrival, Annie developed a high fever, and died. In a memorial he wrote at the time, Darwin mourned her “buoyant joyousness” and the depth of her affection, which made her exquisitely sensitive to the moods of her parents. Charles also suffered with an extra burden of guilt. He worried that he had passed on his own poor health, and, moreover, that her frailty was compounded by what he had come to understand were the dangers of inbreeding. Because Emma was his cousin, he feared he had inadvertently doomed his beloved child.
In an effort to distract Francis from his grief and in great need of distraction himself, Charles proposed to his son that they engage in a joint research project. Together, they would look for the answer to how it is that stems, without any muscle tissue, manage to bend toward the sun.
Francis agreed. He had earned his B.A. from Cambridge in 1870 and a medical degree from the University of London in 1875, and had seemed headed toward a career in medical research, with three significant research papers on animal physiology to his name. Nonetheless, after Amy’s death in 1876, father and son formed a professional partnership. Charles was accustomed to creating controlled conditions in his greenhouse and collecting data, but he was not a laboratory scientist. Francis was newly trained in the latest lab techniques and design of experiments.
Francis’s experience is clear in the approach the pair took in investigating phototropism. The Darwins’ experimental subjects were the emerging seedlings of Canary grass and oats. After germinating seedlings in darkness, they illuminated them from one side only. The seedlings curved toward the light. To determine what part of the seedling could “see,” they wrapped the tiny stems, except for their tips, in an opaque cloth. The seedlings still bent toward light. Next, they excised the seedlings’ tips, and found that the stems were blinded and continued to grow upright. To eliminate the possibility that the stems failed to curve, not because the tips were missing but because the seedlings had been damaged, they fashioned little caps from feather quills, blackened them with ink, and set them on the seedlings. Again, the seedlings failed to respond to light. When they removed the quill caps, the little plants curved once again. The Darwins concluded that some chemical agent in the tip transmits an “influence” that causes the stem to curve.
What was this influence? Scientists of the period didn’t have the tools or the chemical knowledge to find out. But in 1926, the Dutch botanist Frits Went ran a most elegant iteration of the Darwins’ experiment. Went cut off the tips of grass seedlings and rested them on a sheet of gelatin, so that any liquid in the tips would drain into the gelatin. Sometime later, he cut the gelatin into tiny cubes, and placed one cube on the top of each decapitated stem, positioned so that it covered only one-half of a stem. He found that the half of the stem beneath a cube grew more than the uncovered half, inducing a curve. Moreover, the greater the number of seedling tips he placed on the gelatin before carving it into cubes, the more sharply any stem would curve after being partially capped. In other words, the greater the concentration of this mystery substance, the more curvature it induced. Went was able to isolate the chemical and named it auxin, from the Greek auxein, meaning to grow or increase.
Meristematic tissue in stem tips produce auxin. When light hits the meristem from one direction, auxin migrates away from the light and toward the opposite side of the stem tip. The extra auxin causes those cells on the far side to elongate. As the extra auxin flows down through the cells along the far side of the stem, they also elongate. The result is a stem that gracefully arcs toward light. As the sun moves across the sky, different cells are flooded with auxin, causing the stem to follow the source of light.
Auxin was later identified as one of five major types of chemical messengers, phytohormones, manufactured by plants. Phytohormones control gene expression, germination, plant development, the time of flowering and fruiting, leaf drop, the opening and closing of stomata, senescence, and other activities. Unlike animal hormones, which are produced in glands, phytohormones are produced in a plant’s cells. Without them, a plant would look like a blob of algae.
In modern tissue culture labs, technicians use phytohormones to turn a few cells from a parent plant into thousands of identical seedlings in a few weeks. By applying various synthetic hormones in exact amounts at precise intervals, the technicians completely control cell multiplication and root and shoot development. The seedlings, often barely an inch long, are then sold to wholesale nurseries, where growers pot them, add more hormones to make them grow bushy, and then later add more hormones to make them flower synchronously. One of the reasons houseplants often fail to thrive after I bring them home from the garden center is that I don’t have access to synthetic phytohormones. On the other hand, if I were to use a weed killer like Verdone on my lawn, I would be deploying phytohormones to ensure that certain weeds don’t thrive. These herbicides contain a synthetic auxin that spurs dandelions, chickweed, clover, and other broad-leaf plants to grow so fast that their stems twist and contort, and the plant dies.
So, what do Darwin’s discoveries have to do with Dorothy and her dropped leaves?
Trees have evolved to hoist their leaves above their competitors. In order to concentrate their energy on growing taller, they have developed a phenomenon called apical dominance, which means the stem or stalk tips or apices (the plural of apex) grow more strongly than shoots emerging at the sides. Apical dominance is primarily a function of auxin. While auxin causes cells in the apical meristem to divide and expand, it also influences other plant tissues. Specifically, as it travels down the stem, it has the opposite effect on meristematic tissues farther down the trunk, stalk, or stem. Auxin prevents axillary buds, which sit atop a dome of meristematic tissue, from opening and slows the growth of lateral branches.
In some conifers, the effect of apical dominance is dramatic. Auxin produced in the apical meristem of a fir tree trunk diffuses downward and arrives in nearly full strength at the topmost branches, where it greatly inhibits their growth. A little less auxin makes it to the next lower branches, which grow a little longer than the ones above them. So it goes, down the trunk, resulting in a tree with a classic Christmas tree shape. Strawberry plants, on the other hand, have little auxin in their stem tips and therefore exhibit little apical dominance and hardly grow in height at all. Instead, they compete for sunlight by spreading out along the ground. I had been right to prune my kumquat, thereby removing the apical meristems on its branch tips and their inhibiting auxin. My mistake was in my drastic application of the principle.
Deciduous trees, like the crape myrtle Ted successfully prunes so radically, have evolved to deal with surviving cold months. In winter, the energy that a tree’s leaves are able to generate during short daylight hours is less than the energy required to maintain cell function in the leaves. In addition, the loss of water through transpiration exceeds the amount that the roots are able to absorb when groundwater is locked up in ice. So, in autumn, deciduous trees cut their losses. First, thanks to hormonal signals, they drain the sucrose from their leaves and send it to their roots and branches for storage. Then they seal off the leaves at their bases with a corky substance. Without water and nutrients, the leaves’ cells die. In the spring, the trees send stored sugar dissolved in water up the xylem to fuel the growth of new leaves and branches. It is that rising sap in sugar maples that New Englanders tap to make syrup.
Tropical and subtropical evergreens like the kumquat and Dorothy, on the other hand, have evolved in regions where the number of daylight hours har
dly varies throughout the year and temperatures rarely fall below freezing. These trees have less need to store carbohydrates in their roots. After I gave my kumquat a buzz cut, it didn’t have enough stored energy to rebuild its leaves, and died.
Dr. Timothy Spann at the University of Florida’s Citrus Research and Education Center told me how I should take care of Dorothy. There are two techniques, he said. I could prune one-third of her branches. Then, after the axillary buds on the pruned branches have opened and their new leaves fully matured, I could prune another third of the branches. After waiting again, I would prune the final branches. Or, for faster results, I could try tricking my tree. Faster is always better in my book, so I took notes.
My first step would be to grab any branch flexible enough that I could bend it into a -shape, so its tip pointed to the ground and an axillary bud was at the apex of the arc. Then I would tie the branch in that position. (If a branch was too stiff to bend, I could cut partway through its diameter—a technique known as “lopping”—before bending and tying the branch.) Because auxin at the branch tip cannot flow against gravity, it would not reach the axillary buds, which meant the lower buds would be uninhibited and free to grow. Meanwhile, the upside down leaves would continue to photosynthesize and transpire, providing energy and pulling water and nutrients up from the soil.
So, on a pleasantly cool March morning when I was visiting in Fort Myers, I set to work on Dorothy with my mother in anxious attendance. First, as if I were prepping a patient for surgery, I trimmed off Dorothy’s wicked thorns. (As Parker once said, “The first thing I do in the morning is brush my teeth and sharpen my tongue.”) Then I doubled over a slender branch and held it in place while my mother tied the limb in place with a piece of green twine. We worked our way around the tree from bottom to top. Most of her branches were sufficiently supple to double over, and only a few needed lopping. It wasn’t long before Dorothy looked like a thoroughly trussed turkey. I left for home, leaving my mother on duty.