by Guy Murchie
If you cut down a coniferous tree and examine its body in detail in an effort to find out what actually makes the lateral branches grow at such a different angle from the central stem, you will soon discover a reddish triangular segment of wood occupying the bottom quarter or more of each branch. Foresters call this "reaction wood" because it is wood that has had a reaction. Careful tests have shown that it is extremely sensitive to pressure. As soon as coniferous wood cells become noticeably crowded, as when a pine branch's weight compresses its lower parts, they start to redden and increase their rate of division. This, reaction makes their side of a branch grow faster than the other side, thus bending the branch as a whole against gravity or whatever exerted the pressure. If a lateral branch is tied unnaturally high, reaction wood will form on its upper side and bend its free portion back down to its natural angle.
What enables the reaction wood to steer the branch so precisely? What keeps it from ever forgetting the exact characteristics of its particular kind of tree? These are not easy questions and they obviously involve the rather formidable subject of genetics, but last century Charles Darwin took a step toward the answer with his experiments in a plant's reactions when he found that a stimulus - say, a sunbeam - received in one part of the organism could elicit response in quite another part, indeed that the behavior of a young stem is directed by the tip growing at its top end in a way comparable to how a worm's whole body is controlled by the tiny brain in its front end.
Obviously plants do not have specialized nerve cells like those the animal kingdom evolved for high-speed transmission of messages from tongue to toe, yet a detectable "influence" somehow moves down a plant stem at a measurable rate. And, early in this century, that mysterious "influence" was proved capable of passing a cut in the stem if a film of gelatin was inserted to bridge the gap between the severed ends, suggesting that the "influence" was probably chemical in nature. But many more years of experimenting had to pass before Frits W. Went and others finally in 1928 isolated 40 milligrams of what turned out to be a potent growth hormone found in plants in several forms and which soon became well known as auxin. The potency of auxin moreover was so great that, as Went calculated to the amazement of scientists, if all the growth that theoretically could be produced by a single ounce of it were strung into one continuous line, it would encircle the earth.
Plants are now known to generate microscopic amounts of auxin somehow in their meristem tips at the ends of growing twigs, whereupon the substance flows slowly rootward through the bast channels along with the leaf gruel to stimulate growth in all the cells it reaches. Since most of it is concentrated in the meristem zones, that is where growth is fastest. Lesser amounts reach the elongation zones, and still less beyond them, but the gravitation of auxin to the lower sides of limbs and to other regions of relative pressure seems to be what boosts local growth there (exemplified by reaction wood), guiding and orienting the whole structure. And the fact that light repels auxin has the effect of concentrating the hormone in the shadowed lower sides of stems where its shade-biased growth steadily pushes the growing tips up from darkness toward light.
Other vegetable hormones were discovered soon after the auxins: the gibbereilins, the cytokinins, abscisins, brassins and B vitamins such as thiamine and niacin, all either accelerating, decelerating or in other ways influencing the development of vegetation - and collectively providing more than a hint that plant growth must be a lot more complex than anyone had realized, being almost certainly controlled by an interplay of counteracting mechanisms not basically different from those involved in driving a car. At about the same time the direct effect of the duration of days and nights on blossoming was discovered, Particularly the sensitivity of many plants to red light - a phenomenon that closely parallels the light sensitivity of animals.
And then a previously unnoticed intrarelation between different branches on the same tree began to attract the attention not only of botanists but even of a few philosophers. For it had been observed that cutting off the topmost or leading stem of a pine tree induces the lateral branches just below it to bend upward in what seems to be an attempt to replace the lost leader. At least that is the ostensible purpose of the reaction, if one can believe there is any purpose in nature. The most remarkable part of the tree's response to the emergency, however, comes after the first scramble upward by the competing lateral branches, when the tree somehow singles out one (rarely two) from among these rising candidates and elevates it triumphantly in a few growing years to the vacant, vertical throne. The problem of succession is thus usually settled without a serious battle as the chosen prince of branches swings to the central supreme position with a seeming crown of authority while the other limbs drop placidly back to their accustomed places as if they recognized their new sovereign. How they recognize him, no one can say. It is known only that the cutting off of the top stem causes new reaction wood to appear at the base of the nearest lateral branches but how one of these is selected for honor and the others persuaded to forget it remains a mystery. The only thing reasonably sure is that some sort of a cooperative decision somehow gets to be agreed upon by the branches, if not by the whole organism. One might consider it the result of a kind of secret conference within the cells of the tree, perhaps involving a process analogous to voting in which proximity and responsiveness are at a premium and messages travel through the mysterious mediums of chemistry and electromagnetism.
The genetic memory behind such guidance in vegetable tissue, however, does not require a large piece to tell it how to grow. For any twig of certain kinds of tree, say a willow, can be cut off in spring and planted in moist soil and will sprout both roots and shoots. It is not only an organism: it is organized. It knows its top from its bottom. It feels a definite polarity and, even if you forget which end of a piece of willow was nearer the roots of its mother tree, the piece itself cannot possibly forget. If the wrong end is planted in the ground, the twig will in effect turn itself around by sprouting roots at the upper end which grow downward into the earth while shoots rise out of the soil from the bottom end to form a clump of willows, one of which a few decades later may become a beautiful big willow tree with no trace left of the misplaced twig.
Moreover polarity does not mean that any particular cell in the planted piece of willow is predestined to sprout a root instead of a shoot or vice versa, for the cell's action depends on its position in relation to the rest of the piece. If your cut puts the cell at the root end of the piece it will sprout a root. But if you cut it so the same cell
comes at the shoot end it will grow a shoot. No matter how small the grains you cut willow into, they inexorably keep their root-shoot polarity, as surely as crumbs of magnetized iron keep their north-south polarity, and this will in willow asserts itself right down to hollow statolithic cells that contain dense, loose starch grains that keep rolling to the bottom of them in response to gravity, thus apprising the twig (which somehow "feels" them) as to which ways are up and down! Polarity is rather abstract, you see: a geometric directiveness, a builtin purpose like the homing instinct in birds. But we must defer further discussion of it until Chapter 18.
The only thing I still need to mention about plant forms is the extraordinary structure of cellulose, the basic stuff of the vegetable kingdom - generally called wood - which has long been (and will be yet a while) man's most versatile building material for anything from a bow to a barn. Most cells in a tree are tubelike and aligned parallel to the trunk and branches, yet frequently tending to curve as they grow, spiraling this way and that around the tapering, cone-shaped cambium sheath under the bast. Their walls, the toughest part of wood, are made of a fibrous matting in several layers, each one grained at a different angle (like the layers in plywood), the multitudinous, parallel, hairlike microfibrils that compose them spiraling now in a lazy many-looped helix, now in a steep corkscrew of few turns, winding around and around the tube cell, often mysteriously shifting course from clockwise to counterclockwise and
back again as they weave themselves into their tight multi-mesh of incredible intricacy. And even the individual microfibrils have an internal structure no less amazing than the tapestry they are part of, each being a submicroscopic cable precisely woven of hundreds of long-chain molecules of cellulose, which stack together into continuous crystals - yes, crystals - of carbon, oxygen and hydrogen atoms of a texture and pagodalike lattice form that still far surpasses human comprehension.
SIZES AND AGES
It is well known that some trees grow to giant sizes and almost unbelievable ages. In fact the tree is the largest, as well as the longest-living, kind of mortal organism on Earth. The tallest one ever measured, as I've mentioned, reached 368 feet which, added to the depth of its comparatively shallow root system, could well make its total height over 400 feet - and California timbermen swear they logged even taller redwoods in earlier days without bothering to measure them. Several such giants have been found to exceed 2000 years of age and their bulk can be judged by a contractor's estimate that any of them would provide "all the lumber needed for a couple of dozen. five-room homes."
A very different sort is the Dragon Tree of the Canary Islands, genetically a member of the lily family, one giant specimen on Tenerife being reputedly around 4000 years old. And most extraordinary of all is the famous Montezuma cypress of Tule near Oaxaca, Mexico. It is only 110 feet high but an amazing 112 feet around the fluted trunk, which would make it perhaps the only large tree on Earth whose trunk circumference actually surpasses its height. Its age is adjudged to be over 5000 years, though this is hard to prove until the rings are revealed by coring or cutting.
Still another type is the banyan tree, a tropical species of strangler fig that continually drops roots from its branches, growing these into new trunks called pillars, until the whole looks like a dense grove. The world's most famous banyan, I think, is one in India said to have been described by Nearchus, the admiral of Alexander the Great, that today has a mother trunk 44 feet around and, at a recent count, 246 offspring trunks or pillar roots, some of them more than 10 feet in girth. They say it now spreads over more than an acre of ground and that a full brigade of 7000 soldiers can sit in its shade. If pillar roots were real trunks with sprouting branches there might be almost no end to the lateral growth of a single banyan tree, but somehow the pillar genes refuse to sprout, thereby putting a practical limit on a vegetable's span. Height, on the other hand, seems restricted only by the fact that vegetal weight, being three-dimensional, increases disproportionately to its 2-D supporting surfaces, as explained under Galileo's Principle of Similitude (page 15). In view of this you may wonder how, if land animals stop at elephants, a tree is enabled to reach a height twenty-five times greater. The answer appears to be that animal life, likewise 3-D and occupying perhaps 99 percent of its supporting bulk, cannot expand without soon outstripping its vital 2-D lung surfaces, while the life of a tree, essentially 2-D and residing mainly in the cambium layer (one cell thick), may occupy less than one percent of its body bulk, the rest being inert wood and bark that require neither air, water nor nourishment. This surface nature of living tissue in plants, moreover, not only gives a tree a tremendous reach, but the reach often spells the difference between life and death since the tree may not gain direct sunlight until it is hundreds of feet tall nor sufficient water until its roots have drilled scores of feet below ground.
When a plant does not have to hold itself upright against gravity, naturally it does not need to keep its girth in any particular proportion to its length, so thin vines that use trees for support can go to great lengths without overextending themselves - which explains how a jungle liana has been measured at an incredible 650 feet, and a single strand of sargasso seaweed, floating vertically in the buoyant Atlantic, at an unconfirmed, therefore even more incredible, 900 feet. But, curiously enough, many plants succeed at least as well by specializing in smallness, perhaps making up with their large numbers what they lack as individuals. The smallest any so-called normal plant (with stem, leaves, blossoms and roots) can be is about 1/5 inch tall, because it needs a minimum number of cells for this degree of organization, and cells, unlike computers, evidently cannot be further miniaturized. The tiniest plants of all (not counting viruses) are one-celled plankton, algae and bacteria, which move and feed so much like animals there is doubt in some cases as to which kingdom they belong.
SOCIAL LIFE OF TREES
The animal-like capacity of banyans to strangle other trees here logically brings up the subject of plant aggression and the variety of ways vegetables barter, bargain or battle with animals, minerals and themselves. The most dramatic examples are to be found in the tropics, where there are perhaps twenty times as many species of plants (not to mention animals) as in temperate zones and where their vegetable parasites are not just gentle mosses, lichens or an occasional fungus, as in Vermont or Germany, but much bigger and fiercer peppers, bromeliads with built-in water reservoirs, voracious orchids, armed cactuses, sword ferns and assaulting snakelike lianas.
In the rain forests there are more than thirty families of perching plants (called epiphytes) that begin life innocently enough, sprouting out of a tuft of debris in some high tree crotch, silently dropping root threads till they penetrate the ground, slowly thickening as they drink until, years later, the dainty dangler may have become the vegetable equivalent of a giant python that stealthily coils itself tighter and tighter about its mother tree until it literally smothers and chokes it to death - then, after a few years of decay, stands smugly in its place until it too is eventually strangled into oblivion by younger, stronger rivals. Such is the slow, silent and terrible warfare of the jungle which, being observed so-to-say in slow motion, is much more apparent to the human eye than the swift, infrequent clashes between visible-sized animals, few of which take place in anything but utter darkness.
Poisons of course are used by many more plants than the familiar poison ivy, oak, sumac or toadstools (producing deadly muscarine), and there are so-called vicious trees, like the deadly upas of Java and the manchineel of tropical American shores with its luscious little "apples" that may kill anyone who eats them, and whose milky sap can blister flesh, paralyze muscles or blind the eyes of woodcutters. Less known are the plants that poison other plants, such as the aloof black walnut tree that stands by itself excreting a toxic chemical into the soil, the brittle bush of American deserts that sheds poisonous leaves, the jealous rubber-producing guayule of Mexico whose roots exude potent cinnamic acid to kill even most of its own seedlings, and not a few irascible grasses capable of retarding trees that threaten to drink away their water.
Vegetables can be benign as well as brutal, however, we must not forget, a good example being the Madre de Cacao tree (Gliricidia sepium) of tropical America, which, although its leaves, seeds and roots are apparently poisonous to rodents, so obviously helps neighboring cacao and other crops (perhaps partly through its nitrogenous root nodules) that it is widely known as the "cacao mama," "coffee mama," "clove mama," etc. Many northern trees likewise give shade and shelter to smaller neighbors. Indeed to such sun-shy ones as hemlock saplings in a forest or to wind-torn oaks on a stormy coast, this may make all the difference. Young white pines in an abandoned New England field naturally mother their rival black cherry seedlings (planted by birds perching in their branches), followed a decade or two later by maples and oaks that enjoy cherry tree shade in turn.
Many such sequences of tree species, each one shielding the next in a developing forest, are known to ecologists as important. shelter chains in evolution. A somewhat more specific mothering relationship has evolved in certain northern forests where almost all spruce, hemlock and redwood seedlings get trampled or elbowed to death in the relentless bustle of ground plants, but where the rare lucky seed that lodges in a cranny of a rotting log several feet above the mob is thereby enabled to survive - perhaps literally on its grandfather's back. Thus fallen wood both enriches the soil and cradles infant trees whose ro
ots eventually grope their way into the ground while their trunks slowly. rise skyward in characteristic rows often remarked by woodsmen - rows that clearly commemorate the disintegrated ancient trunks that once lay down there, to die and, dying, handed on the baton of life. Even logs floating in a lake or river sometimes sprout seedlings that, on being washed ashore, take firm root in solid ground or, as in a case I heard about, put out roots into the water which collect debris and gradually build up a floating island complete with soil, underbrush, earthworms and birds' nests!
SPECIAL ENVIRONS
When plants learned to live on land, put down roots and breathe air, their main problem was how to obtain or conserve enough moisture to survive. Some found it easiest to remain partly underwater and so evolved air tubes down their stems, like the water violet, or gills like the pondweed, which can exist (if need be) totally submerged. And a few even evolved back to the briny deep, apparently without noticing that it's a lot brinier now than when life first evolved there. One of these is a real tree, the mangrove, which, like the seal and the walrus, is apt to be uncomfortable if it is not within easy reach of the ocean. Although the red mangrove cannot mature without taking root in earth or sea bottom shallow enough to let it reach the air and bear its fruit above the waves, it does not require either land or rain and has been known to exist unsheltered upon submerged shoals scores of miles from the nearest coast. It drops seedlings that normally get waterlogged at the root end and float vertically sometimes for thousands of miles on ocean currents as living driftwood, putting out secondary roots and budding leaves at sea, floating on and on until at last the roots touch bottom, take hold, and the plumule branches and bursts into flower at the top. Even if the floating seedling sinks before touching bottom, it has a chance to survive if it lodges in shallow water, for it can be submerged a year and still shoot a long trunk to the surface, where it will joyfully catch its breath and bloom against the sky, its trunk eventually attaining a girth of nearly ten feet. These trees often build new islands too, for the advancing sand naturally piles up around their prop roots, making more room for seedlings, and the complex undergrowth harbors everything from driftwood, crabs and nesting cranes to oysters, which, at low tide, offer the makings of a stew ready for plucking right off the tree!
