by Guy Murchie
To understand the specific vegetable function, then, we need to realize that trees are not just standing there twiddling their twigs. We need to look closer at what actually goes on inside a tree, at the workings and chemistry of plants, including the dynamic virility in the tree's driving, thirsty roots that spiral shrewdly downward with force enough (when needed) to split boulders weighing tons; in the breathing of its leaves that almost sniff their way upward in search of carbon and "glance" around for sunshine to cook their airy food; in the restless traffic of tenuous arteries that not only heft their daily tonnage of water to the sky but tote food and building materials from highest twig to deepest root hair, in subtle electrical discharges that transmit messages little less sophisticated than those sent through human nerves, and in the far-ranging migrations of spores that entrust a forest's future to the winds, rains, rivers and the wandering animals of the world.
One of the first things to be remembered about the tree is that, like all terrestrial life, it evolved from sea organisms. In fact it may be regarded as basically a seaweed that learned, after millions of years of desperate struggle, how to live on dry land. And, like the more mobile creatures that followed it, its body is still composed largely of water and depends utterly on water for life and nourishment. For these reasons the tree today has little choice but to make its home in soil, the most stable source of moisture on land. But it also needs the energy and building materials it can only get from sunlight and volatile air. Its architecture therefore runs to large surfaces both below and above ground, these systems perforce united by a trunk bulky enough to hold all the channels and pumps required for a continuous topbottom interchange of materials, yet resilient enough to stand up to the fiercest winds.
What on Earth could fill such a bill better than a tree? And the beauty of its being develops apparently by random chance, while every root and branch makes its seemingly independent search for water and light, the whole tending to a highly efficient radial symmetry about the central bole, from which the foliage, according to actual mathematical analysis, literally disposes itself so the largest number of leaves receives the greatest amount of sunshine for the longest possible period. The fast upward flow of sap (more than 99 percent water) is through tubelike cells in the solid interior wood of the tree, while the much slower downward flow of syrupy gruel (only about 95 percent water) is through smaller cells in the outer layer, known as bast, just under the bark. The two systems normally move in opposite directions but are partially interconnected and, in the growing season, may go locally in the same direction for a while, particularly where new shoots are being supplied by both systems at once. It is helpful to remember that these two systems are separated only by the most vital part of the tree, the cambium layer (about an inch under the surface) where new "green" wood is continuously being born. Obviously most of this new wood adds a fresh ring each year on the outside of the layers of old rings that form the sapwood and heartwood, the main mass of the tree. A small portion of the growth, however, also produces a new yearly layer of bast inside the bark, the outside of which is inevitably aging, cracking, slowly splitting off and being discarded like animal scales, feathers or fingernails by the pressure within.
The reason I emphasize the cambium as the most alive part of the tree is that, although it is only one cell thick, it is the tree's only layer where cells thus divide and subdivide in birth, rapidly increasing their numbers, simultaneously augmenting their substance in two directions: at once building new rings outside the old wood and new layers inside the old bark all the way down to the deepest root hair and up to the highest fluttering leaf. And the fact that the level of the tree where the original seed sprouted is its oldest part naturally makes the trunk thickest there (just below ground) and causes it to taper gracefully and efficiently both upward and downward. The full three-dimensional form of each tree ring thus turns out to be a double cone, the main ring inside the tree trunk with its point upward, balanced by another inside the roots pointed downward, each complicated by branching and subbranching and created all over again each year as it is stacked outside last year's ring, like tomorrow's ice cream cones in a continuous nest - a symmetrical configuration that, engineers point out, enables the tree to reach twice the height of a cylinder of equal mass.
ROOTS
If you are among those who think of roots as nothing but dull appendages sleeping peacefully in the stuffy dirt under a plant, you may be interested to know of their real adventures while aggressively hunting for water, air and mineral foods, which means fighting many a pitched battle against competing roots or animals, intermingled with making friendly, constructive deals with rocks, sociable molds, worms, insects and, more and more frequently, man. At the tip of each advancing thread of root is a root cap, a sort of pointed shoe or shovel made of tough, barklike, self-lubricating stuff that the root pushes ahead of it and replaces constantly by cambium cell division inside as the outside is worn away and turned into slippery jelly by passing stones, teeth, running water or other antagonists. But the tiny root cap is only the first of several specialized parts which, working together, enable the root to steer its zigzag or spiral course, skirting serious obstacles, compromising with offensive substances, judi~ ciously groping for grips on the more congenial rocks, secreting powerful acids to dissolve the uncongenial ones, heading generally downward in search of moisture and minerals while ever careful not to run completely out of air.
Close behind the root cap as it advances comes the meristern, the botanists' name for the concentrated terminus of the cambium layer (shown in the illustration) which literally never grows old. This is where the root cells divide at their fastest, and it is followed immediately by the zone of elongation where they gorge themselves with water and expand like hydraulic jacks, relentlessly forcing the root cap ahead of them. Then in turn comes the zone of root hairs, the unseen beard of microscopic filaments sprouting straight out of cellular protoplasm and providing the tremendous surface needed to absorb water out of dry soil fast enough to sustain not only root growth but the health and metabolism of the whole tree.
There is nothing fixed about these specialized root parts, mind you, except their relative order, for they are advancing continuously with the moving threads of root. And you may be sure the same cell that was born in the meristem cradle today will have grown enough to octuple in length by tomorrow, will sprout its first root hairs the next day, be covered with them the day after and probably lose them again before the end of the week as the advancing root leaves the cell farther and farther behind. The tree thus drinks in the manner of a very water-dependent beast, its total thirst actually far exceeding that of any animal on Earth. In fact, an ordinary cornstalk must drink and evaporate 50 gallons of water in order to reach maturity while a full ton of the same liquid asset is invested in the field for each loaf of bread.
Is not this the reason why many plants, like icebergs, remain about nine-tenths hidden beneath the surface where their roots distribute themselves in all directions as they systematically probe and search the soil, why a small alfalfa plant was found with its longest root groping thirstily and doggedly downward through dry sand for 31 feet, why mesquite on the desert is known to have reached a depth of more than 100 feet, why a random two-year-old tuft of grass carefully dug up by a scientist in 1937 had such a luxuriance of fine roots that their individual lengths, after being meticulously washed and separated out of 4 cubic meters of loam, added up to an amazing 315 miles? And more recently, and more astonishingly, why a botanist measured and calculated an innocent-looking little rye plant to have 14 billion root hairs with an end-to-end length of 6ooo miles and growing at a cumulative rate of 100 miles a day while drinking from a total surface which, if it could have been spread out flat, would have exceeded the area of ten football fields! I know it is high-handed of anyone to rearrange roots into one long statistical strand, but doing it helps my wallowing mind grasp the incredible multiplicity with which even the gentlest plants stead
ily stitch themselves into the earth. Sometimes when I look at a blossoming shrub or tree, I imagine its myriad root caps needling their seemingly independent ways outward like a million tiny sewing machines, dutifully binding and weaving their central organism to the ground. And if one should project such a picture of a root system into accurate perspective with its upper body, it would be revealed as a sort of foreshortened mirror image of the visibly moving upper tree, the central taproot corresponding to the trunk though usually shorter, surrounded by great branchlike primaries often more outspread than their counterparts above, along with lesser secondaries, slim tertiaries, twiggy "obliques" and finally the threadlike capillaries which hold the root hairs that correspond to the smallest veins in the upper leaves.
LEAVES
Raising our attention by this comparison to the more familiar trunk and branches of the tree above ground, we find that things up here almost match those below also in the sequence of growth, the tough, slippery root cap being replaced by a protective bud or growing tip, followed in order by a meristem zone of cell division, a zone of elongation, then one of specialization, which, however, here needing to breathe rather than drink, sprouts leaves with nostrils instead of root hairs with throats. Certainly it is the leaf that is the most distinctive as well as the most vital part of most plants, which properly value it more than a root. For the leaf is to a vegetable what a lung is to a vertebrate and maybe more. If you stick a tiny piece of leaf from an African violet into moist soil, it will normally grow roots directly out of the leaf. And seaweed (in essence nothing but a leaf) can live and breathe indefinitely while drifting on the ocean's surface without any roots at all. By contrast a bare twig on a tree in winter can breathe only slightly through microscopic holes in its tender bark, doing so much less efficiently than an insect through its spiracles, for the twig without leaves is in effect asleep and, until it gets them, can hardly grow at all. Which may be why it is so eager - in some cases desperate - to unfurl its sprouts each spring. Failing in this indeed would be like a porpoise neglecting to surface for air, without which he must surely die.
The leaf thus turns out to be much more than a symmetrical flap of verdure. Indeed, surpassing the dreams of the greatest philosophers of old, it is revealed by the microscopes of modern science as not only a breathing organ but a bustling, automatic food factory full of tubes, retorts, chambers, valves and shutters operated by more timers, thermostats, hygrostats, feedback and catalytic controls (some of conflicting or unknown motivation) than anyone has yet been able to assess. Among its tubes are the upper ends of the same ones that raise sap through the wood all the way from the root hairs, as well as those that carry the gruel made by the leaf back down into the bast to nourish the whole tree. All such channels, by the way, not only convey vital liquids but also serve as structural bones in the leaf as in the tree. In a house it would be like having water pipes strong enough to double as joists and studs, to hold up the roof and bind everything together.
The retorts and chambers on the other hand are spaces between the microscopic cells and chlorophylled subcells, called chloroplasts, which are so arranged as to give the leaf access to the great quantities of light and air it needs. Most of a leaf's volume, you see, is normally air, which permeates the spongy green interior so completely that there is up to a hundred times more atmospheric exposure there among the chloroplasts than out upon the waxy, nonchlorophyll exterior skin. Botanists have calculated that there are about 600 square inches of surface inside a leaf for every cubic inch of its bulk and that a large elm tree has in all some 15 million leaves with an area, if spread out whole, of nearly 10 acres or, if unfolded into the sum total of airbreathing light-absorbing surfaces of all the internal chloroplasts, something like 25 square miles.
A tree, however, no more breathes at a constant rate than does an animal or a combustion engine, for it too has something equivalent to nostrils or intake manifolds that open or close as they want more or less air. These apertures may be described as innumerable invisible pores in the skin of the leaf, mostly on its underside, that look (through the microscope) rather like little vaginas and are controlled each by a pair of "guard cells" not unlike vulvae which automatically change shape under the stimulus of moisture, temperature and light. When the weather is very dry and hot these pores remain mere slits but begin to cleave apart as humidity makes the guard cells turgid, flaring wide open when it is cool and rainy and the chloroplasts have an excess of moisture to evaporate back into the sky, plus a related need for air and the nourishment they can derive by absorbing its carbon dioxide under the impact of sunlight. Pores in a leaf may number anywhere from a hundred on small arid plants to a hundred thousand on the large lush foliages growing in swamps or rain jungles. And the magnitude of their gas exchange is suggested by the calculation that every day on the average a leaf requires as much carbon dioxide as would be contained in a column of air standing 150 feet high upon its surface, while in the same period the average tree in leaf evaporates more than a hundred gallons of water.
The pores of course are only one of many means by which the leaf adjusts to its changing environment. Another is the chloroplasts' ability to change their shapes and move around like monks within their cells, dispersing to intercept the maximum of light on cloudy days and lining up in each other's shadows to escape the sun when it has been bright too long. Sunlight penetrates right through leaves, you see, and is somehow sensed by the chloroplasts, evidently through submicroscopic lenses that amount to vegetable eyes - and the leaves' translucency or (conversely) absorptivity increases and decreases by as much as a third as the chloroplasts arrange and rearrange themselves. And often, it seems, leaves have to make decisions or compromises among conflicting motivations, such as choosing between closing their pores to conserve water (which slows their breathing) and opening their pores to snuff up carbon dioxide (which dehydrates them through evaporation). Usually the leaf settles such an issue by leaving its pores partly open like a man reconciling thirst and hunger by swigging drinks between mouthfuls of dinner. But experiments show that leaves on the whole are much more apprehensive of running out of water than out of air (actually just as vital), perhaps because, like many animals, they so often feel thirsty but almost never know suffocation.
ENERGY
Having discussed how the vegetable functions from its roots to its leaves, it is time to ask such basic questions in botany as: What are plants made of? Do they breathe? What do they eat? Where do they really get their substance? And energy?
That the answers are far from obvious is shown by the long history of man's fumbling struggle to find them. Aristotle thought plants somehow ate their substance from the dirt, and it was not until two thousand years later that a Flemish alchemist-turned-chemist named Jan Baptista van Helmont tested the not-unreasonable assumption by accurately weighing the soil around a willow tree he kept growing in a pot for five years.
This classic first-ever scientific experiment in botany convinced van Helmont that plants do not consume earth, but it took many more tests before Joseph Priestley, an English amateur chemist, finally proved that plants breathe as they grow. In 1772 he enclosed a growing plant in an airtight chamber and found that it suffocated and died just as surely, if not as quickly, as an animal sealed in a similar container. But much more surprising and wonderful was his famous subsequent experiment of enclosing a plant and an animal together in the same airtight chamber and discovering that both could live! - a seeming miracle that was soon proved due to an exchange of chemically different kinds of breaths or gases shortly to be named oxygen and carbon dioxide, the animal inhaling oxygen (O 2) and exhaling carbon dioxide (CO 2) while the plant accommodatingly did just the reverse.
This great discovery led to an unprecedented use of flowers in sickrooms (in those days usually tightly shuttered against "harmful" outside air), a custom that has continued ever since. But a Dutch physician and chemist, one Jan Ingenhousz, was skeptical enough to want to experiment further and
by 1779 he had found that a plant's exhaling of oxygen is done only by its green parts, particularly the leaves, and only while they are illumined - never in the darkness of night when the plant inhales more oxygen than it exhales. By 1796 he had learned enough to postulate specifically that leaves by day decompose the carbon dioxide they inhale from air, using the carbon (plus some water) to build their bodies while they exhale the leftover oxygen. It was the first definite human awareness of the process we now know as photosynthesis, which, because it involves some 300 billion tons of sugar in vegetation each year, is today considered the most massive chemical process on Earth!
To summarize the essential chemistry: when green plant tissue is under strong light its water molecules (H 2 O) are split into separate hydrogen and oxygen atoms, of which the hydrogen (H) combines chemically with part of the inhaled carbon dioxide (CO 2) to make carbohydrate molecules (C 6 H 12 O 6), particularly sugars, starches and cellulose to build the plant, while excess oxygen is exhaled.
That is a nutshell account of photosynthesis, this fundamental life process that, when it was discovered, seemed so incredible and complex that many researchers spoke of it as miraculous. And I doubt not they were right. In the first place, its prime source of energy is literally unearthly, being photons of light streaming at 186,282 miles a second out of the sun. Then, although light seems to have little effect on water away from plants, light striking water inside green plants receives a special tool for operating on it: the chlorophyll molecule. This curious, green, magnesium-centered, daisy-shaped molecule (C 55 H 72 O 5 N 4 Mg) with its long carbon "stem" and 12 "petals" surrounding the 12th element is the key to the whole operation. As it absorbs the light's energy, its hundreds of subatomic particles jump momentarily from their normal orbits in each atom to larger, more potent orbits. Although this leap of excitement only lasts about a ten billionth of a second, the jolt when they revert back to normal somehow (not yet clearly understood) splits hydrogen from oxygen in adjoining water molecules, whereupon the separated hydrogen is grabbed