In the experiment illustrated in Figure 13, Hales clipped both ends of a leafy branch (b). He denuded another branch (z), and connected (b) and (z), both upside-down, via a water-filled glass tube (i). The other end of (z) he put in a bucket of mercury. He repeated the experiment (Figure 14) with a living tree.
But if it didn’t circulate, then where did it go and how did it get there? Hales set up another experiment to investigate those questions. He started with a sunflower growing in a clay pot, then sealed the soil’s surface and blocked the hole at the bottom of the pot. He watered the sunflower through a small tube that he corked afterward. By comparing the quantity of water he added to the pot each day with the daily changes in the weight of the pot and plant, he arrived at the amount that disappeared. The water could only have disappeared into the atmosphere. By measuring the surface area of the leaves, stalk, and stems, Hales arrived at the plant’s rate of transpiration—“perspiration,” as he called it—per square inch. By calculating the surface area of the roots, he also figured the rate at which roots take up water.
In this experiment with a sunflower, Hales measured the amount of water taken up by roots and lost to the air through the plant’s aboveground portion.
He repeated the potted sunflower experiment at different air temperatures, levels of humidity, and cloud cover, and with lemon trees and other species. Sunny skies and heat, he found, increased transpiration; humidity and clouds decreased it.
He could have concluded that the sun is somehow directly responsible for the rise of sap. Instead, he ran another experiment, testing the “imbibing” capacity of leafy versus leafless branches. While leafy branches pulled up water, leafless ones did not. Sun or no sun, water does not travel up leafless branches. Unlikely as it seemed, he realized it is the almost imperceptible force of evaporation from leaves through the pinprick stomata that moves water from the ground to treetops. In fact, through the power of transpiration, the canopy of a hundred-foot-tall tulip poplar can hoist as much as a hundred gallons of water, weighing a total of eight hundred pounds, per day.
Hales’s discoveries raised new questions. If fluid in xylem exited the plant through leaves, then what was the fluid that Malpighi and Grew discovered flowing downward just beneath the bark? Hales missed what is known as phloem sap, the sucrose-laced liquid that flows down from the leaves to the roots for storage. (It also moves sideways and sometimes even upward, taking liquid energy to cells that use it to function.) But phloem sap was impossible for him to tap and measure with his instruments. It moves far more slowly and with far less pressure than xylem sap.
Even today, the pressure of phloem sap is difficult to measure, and plant physiologists have to enlist aphids to do the job for them. Aphids and other garden pests pierce the phloem with a needlelike mouthpart (a stylet) in order to route the sugary liquid into their digestive tracts. (Their excretions become food for the unsightly “sooty mold” that gardeners find on leaves.) Modern researchers set aphids to work by placing them on a plant stem where the bugs drill in. The researchers then zap the aphid body with a laser or blow it apart with a jolt of electricity, and measure the flow exiting its disembodied stylet.
I imagine Hales would have been delighted with the technical ingenuity of this method. After the publication of his two great books, Vegetable Staticks followed by Haemostaticks, he rechanneled much of his energy into developing technology for the betterment of humanity. In the 1740s, he invented forced-air ventilators that reduced mold in granaries and brought fresh air to prison blocks and the fetid atmosphere belowdecks in merchant ships. The success of his ventilators in reducing illness convinced the Royal Navy to require them in all its ships in 1756, and many English hospitals also adopted them.
In Hales’s later years, he investigated the relationship between the mineral encrustations on the bottoms of teakettles and the salubrity of the well water boiled in them. He advised watermen on how to preserve the undersides of their boats; taught housewives to place an inverted teacup at the bottom of pies to keep the syrup from boiling over; tried to poison insects on fruit trees by drilling holes in the trees and pouring in mercury (not a good idea); designed a water conduit system for Teddington; and demonstrated that putting air holes in the outer walls of ground-floor rooms prevents floorboard and joist rot. In an era when well-connected Anglican clergymen often accepted the income from several parishes while providing little or no service to most of them, he declined several opportunities to do so, and lived modestly.
At the age of seventy-four, he was appointed chaplain to the Princess Dowager of Wales, an honor that reflected his scientific achievements, as well as his service to his country. Still, he remained a bit of an odd duck. He had the elderly princess searching the bottom of her teacup for mineral sediments. His neighbor Gilbert White wrote of Hales that his “whole mind seemed replete with experiment, which of course gave a tincture and turn to his conversation, often somewhat peculiar.” But his experiments, neatly crafted and reproducible, created the field of plant physiology. No one would significantly advance his work on transpiration for a century.
nine
How to Kill a Hickory
About fifteen years ago, friends up the street built an addition off the back of their house. Using the soil excavated in digging out the addition’s foundation, the Carters leveled their yard. Because their small yard sloped sharply down toward their neighbors, they had to build a three-foot-high retaining wall along the joint property line.
A hickory tree nearly forty feet tall stood ten feet from the retaining wall. The Carters and their neighbors enjoyed the tree, not only for its unassuming grace and shimmering golden foliage in autumn, but because its arching boughs extended over their roofs, shading their houses from the sun and no doubt moderating their electricity bills. The Carters knew that loading soil onto the roots of the tree might endanger it, so they had a three-foot by three-foot “tree well” made of railroad ties built around the base of its trunk.
The hickory after its roots were partially buried. A root tip is magnified.
For many years, the hickory prospered. Two summers ago, however, I noticed that its leaves were smaller and sparser than usual, and I idly wondered why. Early this summer and seemingly overnight, its green leaves shriveled and turned a purplish brown. I see that the bark has peeled back from some of its boughs. The tree is now indubitably dead. Although it is still standing as I write this in January, it will be gone by spring.
The Carters assume that their tree is an ill-fated victim of a dry spell last spring. But its death was neither due to drought nor was it inevitable. Instead, its roots slowly and simultaneously starved and suffocated under the soil that was added. The tree well was a well-meant gesture, but it couldn’t save the tree. The question is, why not?
Stephen Hales made great strides in explaining how water moves once inside plants, but he didn’t explore how water gets into roots in the first place. He did notice, as Malpighi and Grew had decades earlier, a band of superfine, colorless hairs growing just behind the tips of roots. Collectively, these hairs look like microscopic bottlebrushes. All three men dismissed the hairs as unimportant. In fact, they are essential to a plant’s survival, responsible for gathering water and nutrients a plant needs from the soil.
A root hair is a single elongated cell of the root’s epidermis. At less than a millimeter in length, most root hairs are invisible, which means that their power is in their numbers. A four-month-old rye plant in a twelve-inch pot, for example, has about fourteen billion root hairs. If some modern-day Sisyphus were condemned to lay those hairs end to end, they would stretch from Los Angeles to Boston and nearly back again. Their collective surface vastly increases the area of contact between roots on the one hand and soil, water molecules, and microscopic pockets of air on the other. To enhance their critical water-funneling activity, root hairs excrete a slimy, sugary substance, called mucigel, that binds molecules of water and particles of soil to them. Water passes via osmosis (wi
thout the expenditure of energy) through a root hair’s cell wall first, then through the membrane that lines the cell, and finally through the xylem’s endodermis and into the xylem. Plants, including the hickory next door, can only replenish the water they lose via transpiration if they have sufficient root hairs.
Your garden or potted plant is in trouble if more water is evaporating from its leaves than is entering its root hairs. It begins to wilt as water is pulled out of the xylem, then from the spaces between cells in the parenchyma, and ultimately from the cells themselves, whose inner membrane deflates like a leaky balloon, sometimes tearing in the process. If rain doesn’t fall or you wait too long to water, chemical reactions inside cells that depend on water fail to occur. Repeated or extended wilting damages a plant.
The mucigel also provides a hospitable environment for nitrogen-fixing bacteria. Nitrogen is fundamental to life. Without it, neither animals nor plants can build the proteins and DNA they require to function and reproduce. Yet despite the fact that the Earth’s atmosphere is 78 percent nitrogen, we animals can’t use the nitrogen we take in with every breath. Plants take in air though their stomata but are no better at absorbing atmospheric nitrogen than we are.
The problem is that in the atmosphere, every nitrogen atom is triply bonded with another nitrogen atom. A molecule of two nitrogen atoms (N2) is that couple on the dance floor, lip-locked and hands deep in each other’s back pockets, oblivious to the gyrating crowd around them. The N2 molecule cannot be chemically incorporated into protein chains and nucleotides. Fortunately for life on the planet, plants can incorporate fixed nitrogen, which is a molecule composed of one atom of nitrogen bonded with three hydrogen atoms to make ammonia (NH3). The enormous energy released in lightning can split atmospheric N2 and allow a single N to bond with three H’s. Lightning-fixed nitrogen, however, accounts for less than 10 percent of the total fixed nitrogen on the planet.
The other 90 percent of the planet’s fixed nitrogen was produced solely by nitrogen-fixing bacteria.* Some of these bacterial species live independently in the soil; others live symbiotically in nodules on the roots of leguminous plants like clover, peas, and soybeans. All the bacteria employ enzymes to slowly transform the gaseous nitrogen in pockets of air in the soil into ammonia and then into nitrate, a form of fixed nitrogen that plants can take up. To get our nitrogen, we must either eat plants, getting our nitrogen secondhand, or eat other animals that have eaten plants, snagging it third-hand.
It takes an extensive root system with trillions of bacteria supplying billions of root hairs to gather sufficient nitrogen, as well as water and other minerals, to support even a small plant. An average potato plant’s roots, for example, at the end of a growing season spread five feet in diameter and as much as three feet in depth, and explore about sixty cubic feet—twelve to fifteen wheelbarrows—of soil. Actually, the potato plant is small potatoes when it comes to root systems. The roots of a single bean bush extend into more than two hundred cubic feet of soil, or a cylinder of soil eight feet in diameter and four feet deep. When it comes to tree roots, the numbers are even more startling. The area they occupy underground can be five times greater than the area of the foliage. Contrary to popular wisdom, a tree’s roots often extend far beyond the “drip line,” the circle described by a plant’s outermost leaves. Roots can extend from the trunk as far as the tree is tall.
About 90 percent of a tree’s roots grow in the top eighteen inches of soil, and a good portion of those are in the top few inches. Here is the best grazing for roots. In a forest, many of the tiniest roots actually grow upward, a bit above the soil surface and into a zone of duff, which is made of thatch, leaves, and other decomposing plant material. Trees that grow to towering proportions in the forest rarely grow so large or live as long in suburban lawns, in large part because we assiduously remove the unattractive duff, and compact and pave over the topsoil. When it comes to topsoil, “take away the top two inches,” writes Robert Kourik in Roots Demystified, “ . . . and you have a horticultural disaster in the making.” We make matters worse by growing turfgrass under and around our trees, so tree roots have to compete with the roots of grass for water and nutrients.
In our neighborhood, the subsoil has a high clay content. Clay is one of the three inorganic components of soil, along with sand and silt. I had thought that clay was a bad thing to have in soil, but the best soil is about 20 percent clay. (Ideally, the remainder would be equally divided between sand and silt.) Of the three components, clay has by far the smallest particles. If a grain of sand were the size of a potato, a particle of clay would be the size of a pinhead. Collectively, the tiny particles of clay have a vast surface area: A baseball-sized hunk of clay rolled out to a sheet a single particle in depth would cover more than an acre. Because each particle carries a slight negative charge and minerals mixed in groundwater have a slight positive charge, the clay particles help keep water and essential inorganic molecules in contact with roots.
When the Carters’ builder pushed clayey subsoil on top of the hickory’s lateral roots, the clay became a problem. It meant too much water surrounded the roots, filling up the tiny air pockets among the soil particles. The root hairs lost some of their access to oxygen, which they need to burn the carbohydrates stored in roots to release energy. While water passes into the xylem by osmosis, it takes energy to move minerals inside because their molecules are too large to slip through roots’ cell membranes. Without oxygen to burn carbohydrates, the hickory was like a starving man too weak to lift the food to his mouth. At the same time, anaerobic bacteria and fungi that thrive where oxygen levels are low probably made themselves at home and settled in to dine on the tree’s tender root tips. Gradually, the number of root hairs declined, and the tree had contact with fewer nutrients, including nitrogen.
In addition, as root cells burn carbohydrates, they release carbon dioxide, the same way wood burning in the fireplace or gas combusting in an engine releases carbon dioxide. When the hickory’s roots lived in airy topsoil and duff, the carbon dioxide quickly dissipated into the atmosphere. But under three feet of dense soil, the waste carbon dioxide would have accumulated. When a person drowns, he dies not only from the lack of oxygen but from an excess of poisonous carbon dioxide. Likewise, while the tree was drowning, losing its root hairs and weakening from a lack of nutrients, it was also suffocating.
Why didn’t the tree die soon after the yard was leveled? There’s no way to know for sure, but most likely the soil was more friable at first and gradually compacted over time. As the soil grew denser, the number of root tips gradually decreased until finally the tree lacked enough nutrients to produce healthy foliage: hence, the two seasons of dwarfed leaves. Last spring, with an inadequate crop of leaves to make new sugars, the tree was finished. The hickory’s demise seemed sudden to us, but it had been dying from the day the bulldozer first covered its roots.*
ten
Our Fine Fungal Friends
In the early days of my fascination with citrus tree varieties, I came across the strangest one I had ever seen, in the online catalog of a California nursery. The foliage of Citrus medica var. sarcodactylis looks perfectly ordinary, but its fruit is another matter. It looks like the offspring of a large lemon and a small octopus. The result is a yellow “hand” with long, pointed fingers, giving rise to the species’ popular name, Buddha’s Hand. California growers are permitted to send citrus trees to non-citrus-growing states, so I ordered a three-year-old specimen.
The fruit of a Buddha’s Hand is bright yellow when ripe.
Citrus trees of that age are grown in five-gallon pots that hold about twenty pounds of moist soil. To reduce the shipping cost, the nursery washes the soil off the trees’ roots and encloses them in a plastic bag packed with damp sawdust. The directions that accompanied my small sapling advised me not to use ordinary yard soil or bagged topsoil to repot the tree. These would be too dense and retain too much water, possibly waterlogging the roots. A lightweight potting mi
x without wetting agents was called for, and I bought a bag from the garden center. I had never dealt with a bare-rooted plant before, but managed to arrange the roots as instructed. After the tree had settled in for a few weeks, I applied the recommended six-month, slow-release fertilizer.
All went well for several months, but gradually the tree began to look a bit tired. The leaves lost their luster, and although it was by then early spring, no flower buds appeared, much less any weird yellow hands. I added more fertilizer, but if anything, the little tree looked worse. I called Edie for help. The problem, she suggested, was in the soil, or rather what wasn’t.
The soil I had used, like many products labeled potting mix, was actually a soil-less mix of composted bark, coir (the fiber of the coconut husk), and the mineral perlite. Such mixes are designed to drain well while retaining an ideal level of moisture. Because the ingredients had been sterilized with heat, my soil-less mix didn’t contain soil-borne pathogens, insects or their eggs or larvae, or viable seeds that might sprout and compete with my tree for nutrients. It also didn’t have fungi. And that, Edie said, was unfortunate.
You would think all those billions of root hairs would be sufficient to get the job of plant nutrition done. But despite their mind-boggling numbers, they typically contact only about 1 percent of particles in the volume of soil they occupy. That means that a plant has access to only 1 percent of the potential nutrients nearby. Fortunately, roots have developed another strategy for retrieving those nutrients: contracting out.
The soil surrounding roots (the rhizosphere) is the most life-packed, biodiverse ecological niche on the planet. Under a microscope, it seethes with microbes, protozoa, and fungi, as well as nematodes, mites, insects, worms, and other creepers and crawlers. It’s a zoo down there. Among the estimated 1.5 million different species of fungus trying to grind out a living amid the hungry masses is a group of fungal species called mycorrhizae (“my-koh-RYE-zee”). Mycorrhizae are single-celled creatures a thousand times narrower than human hair. Under a microscope, a single fungus looks like a white thread. In the soil, it insinuates one end of itself into a root hair and extends its other end into the soil, where it branches like a microscopic shrub. Lacy, intersecting networks of mycorrhizae, called mycelia (“my-SEEL-ee-ah”), are possible to see with the naked eye. Until the late nineteenth century, however, few people remarked on the whitish fuzz they occasionally noticed on tree roots. Those who did assumed the stuff was a harmful parasite or a sign of decay.
A Garden of Marvels Page 9
