HOW TREES GROW

By Dr. William Chaney, Purdue University

In the early 1900s, a German plant physiologist named Klebs proposed a concept that is still very appropriate today. He was the first to note that the only way environmental factors and heredity can affect organism growth is by affecting its internal physiological processes. A tree's physiological processes constitute the mechanism by which genes and environmental factors operate to control growth. Whatever you do with a tree - whether you are selecting a superior individual, planting, fertilizing, watering or pruning - you are ultimately dealing with that tree's physiological processes.

For tree farmers to understand why any condition or treatment affects trees, they must know how this factor or treatment affects physiological processes. Tree farmers can use this kind of knowledge as a basis for developing better methods for growing and maintaining trees.

CELLS

The basic structural unit of trees is a cell, millions of which connect and coordinate into a harmonious whole. Mature cells per-form various functions, but are very similar when they are first formed, when they all have the capacity to become a whole tree. Each has a nucleus that contains all the genetic information need-ed to produce a specific free.

Cells have several organells in addition to a nucleus. The chloroplasts, the site for photosynthesis, are green because they contain the pigment chlorophyll, which can convert the sun's radiant energy into chemical energy in the form of glucose. This sugar is the basic food and building material for new cells and trees.

Another cell organelle - the mitochondria - uses sugar from the chloroplasts and releases the captured solar energy in a form useful to cells. The chloroplasts and mitochondria work in concert- one captures the energy and the other releases it to cells.

A less familiar celiorganelle is the dictyosome. Like any organism, trees must dispose of waste products resulting from cell metabolism. The dictyosomes accumulate tannins, phenols and other toxic substances in spherical membrane-bound structures called vesicles. These are pinched off from the edges of the dictyosomes, migrate to the outer cell membrane, attach themselves to that membrane and belch the material into dead cell walls. This is important to the tree because individual cells are protected from those toxic compounds, which import disease and insect resistance to the tree as a whole. Some of these waste products are dumped into a central envelope inside cells called the vacuole, which contains the cell sap. Here they are isolated from the cell's living contents. The vacuole also keeps cells fully expanded, a condition necessary for peak performance of all the organelles. Leaves on a tree wilt when water in the cell sap is lost. This allows living cytoplasm inside the cell to collapse and fold, slowing down or even stopping organelles from functioning. Hence, the vacuole is a very important part of a cell, although its contents are nonliving.

We can begin the fascinating story of tree growth with flowering. All trees have flowers, although many aren't very showy. Their purpose is to be pollinated and fertilized and to develop into seed-bearing fruits or cones.

THE SEED

Seeds contain an aggregation of cells in an embryo that reflects all the parts of a mature tree. But each cell has all the organelles described above and the genetic capacity to become a whole tree. Some parts of the tree are already recognizable under a microscope. These include cotyledons, the first leaves, where sugars and starch to fuel the growth of the new tree are stored, and the apical meristems, the growing points for the shoot and root that emerge upon germination. Incredibly, all the cells are derived from the single cell that resulted from fertilization. As the embryo develops and becomes a tree, many cells differentiate and specialize to form leaves, roots and other organs. Until the seedling can produce enough leaf area with chloroplasts to support itself through photosynthesis, it uses food stored in the seed.

The fascinating growth processes that result in the leaves, stems, wood, bark, roots and other tree tissues are controlled by hormones.

HORMONES

There are several kinds of naturally occurring hormones - aux-ins, gibberellins, cytokinins, ethylene, and some inhibitors. Each has its special functions.

In the 1930s, auxin was discovered. Being the only known hormone, plant physiologists attributed to it the control of almost all growth process-es. As other campounds were discovered, they realized that gibberellin controls cell enlargement, cytokinins influence cell division and specialization, and ethylene affects the degeneration of cells and plant response to wounding. Also, natural inhibitors interact with all of these growth promoters.

A hormone is a chemical messenger that in very low concentrations regulates the physiological processes dictated in the genes and influenced by environmental factors. Anything a tree farmer does to a tree influences the production and balance of hormones. When a tree is pruned, for example, the balance of hormones is altered enough to cause a dormant bud to grow, or perhaps a whole new bud to appear. Roots, buds and stems respond differently to a range of concentrations. Extremely low concentrations of auxins promote root growth and do not affect stems. At slightly higher concentration, auxins become an inhibitor of roots, but promote stem activity.

Balance and concentration determine if auxins promote or inhibit growth. Vegetation managers employ this dichotomy in controlling plant growth. Some of the most potent herbicides, such as 2, 4-D, are synthetic auxin compounds. Many of the chemicals used to influence tree growth are really hormones that promote or inhibit physiological processes, based on their concentration.

Cells are to tipotent, meaning that every living cell in a tree can potentially become any tissue in that tree. This is the basis for tissue culture in which new plants are produced from a few cells taken from a parent. It won't be too long before we know enough about hormonal balance to make tree tissue culture more common.

THE ROOT SYSTEM

The root system develops from the embryonic root within the seed. As cells in the apicalmeristem of the root divide and elongate, they push the root tip into the soil. Roots lengthen in this manner. The rootcap, whose cells are constantly being sloughed off, protects the tender cells in the apical meristem and lubricates the root as it is rammed into the soil. Expanded root cells differentiate to carry out functions of uptake and transport of water and nutrients to above ground parts of the tree.

Lateral roots do not grow from a bud like lateral shoots. They grow from internal root tissue. Some internal root cells, in response to the right balance and mixture of hormones, produce a new root meristem. These cells divide and elongate and are forced between and around cells in the root, rupturing root tissue and emerging us a lateral root. This creates a pathway for easy movement from the soil into roots that avoids entering living cells. If fertilizers or any other chemicals are appliced in close contact with branced roots, or even at the base of a tree, uptake will occur. Fine feeder roots are not essential to have uptake. However, water and nutrients move through soil very slowly and for only short distances. Anything near a stationary root is quickly depleted from the soil around the root. Hence, to be healthy, a tree must have existing and new roots growing in length to explore the soil for water and nutrients.

Tree roots are not very deep. Basic tree biology education should convey that most of the root system is quite shallow and can extend far from a tree, even for beyond the drip line. Much of this is genetically controlled, but the principal reason for shallow rooting is the very stressful environment, particularly poor soil conditions. Low oxygen content and soil compaction predispose trees to many other problems.

To survive, roots need 3 percent oxygen in the soil. If they are to grow, existing apical meristems require 5-10 percent oxygen. And, for new roots to form, soil oxygen content must be at least 12 percent. Our atmosphere is about 21 percent oxygen. In an undisturbed loom soil six inches below the surface, the percent oxygen is only slightly less than that in the air. A compacted loom will have about 5 percent oxygen 15 inches deep into the soil. Tree roots would survive at this depth, but new roots would be stressed. A clay loom soil at three feet has an insufficient oxygen level to support new root growth. In a sandy soil, even at five feet, the oxygen content is about 15 percent, where roots can survive and grow.

THE SHOOT SYSTEM

The first shoot of a tree emerges from the seed as the cells of the embryo expand during germination. At its tip, and at the tip of every lateral shoot that develops, is an apical, meristem where in length and height occur. Growth is similar to that in the root only in that cells divide and elongate. Nothing comparable to a root cap exists because the air provides no resistance to growth. The organization of the apical meristem, in shoots is more complex than that of the roots because buds are produced from which lateral branches, leaves and flowers form.

As the cells derived from the apical meristem of shoots differentiate to form the various tissues of the shoot system, some specialize to form two lateral meristems: the vascular cambium and the cork cambium. These account, respectively, for increase in girth and bark production. The vascular cambium is located between the wafer conducting xylem tissue to the inside and the food-conducting phloem tissue to the outside. The vascular cambium is the origin, of new cells that become xylem and phloem tissue.

.The other secondary meristem, the cork cambium, found just outside the functional phloem, also produces two kinds of cells. These constitute the bark, which, protects and insulates the succulent tissues beneath.

THE XYLEM

The water-conducting xylem tissue, or wood, consists of only four kinds of cells. But there can be great size and shape differences among those cells as well as differences in their arrangement and proportion. In fact, these characteristics are so singular that a wood anatomist can identify trees just by inspecting the wood.

The most primitive type of xylem cell is a tracheid, a norrow tapered cell, usually about 1 mm long, with pits in the sidewalls and closed on both ends. The cells are arranged vertically in the xylem and joined by pit pairs. Water, with its dissolved contents, moves upward in the cells for a short distance before it must move through the pit pairs into an adjacent tracheid. Upward conduction follows a tortuous and inefficient pathway in wood dominated by tracheids.

Vessels, an evolutionary advancement for transport of water, have a much bigger diameter than tracheids and still have pits in the side wall. But, most importantly, they have large pores in the end walls. The vessels are arranged end to end in long stacks that in trees such as ashes and oaks, could extend from a root through the trunk and up to a leaf in the tree crown. They can reach 30-40 feet, creating a good system for moving water and nutrients up trees.

Fibers, usually shorter and narrower than tracheids, have very thick walls containing few pit pairs and closed ends. They don't conduct water, but instead function in the xylem to provide structural strength. When mature and functional, the tracheids, vessels and fibers are dead, hollow cells. They normally constitute the largest portion of the xylem.

The fourth type of xylem cell is the parenchyma, an undifferentiated cell that remains alive in the xylem for several years. These cells are scattered in the xylem and constitute the vascular rays, which provide a pathway for lateral movement across the xylem. Like the other parenchyma cells in the xylem, they the storage sites for carbohydrates essential for the vitality and growth of trees. These living parenchyma cells allow the tree to respond to wounds. The callus that forms around the edges of a wound on the trunk or a pruned branch arises from the totipotent parenchyma cells in the xylem. Parenchyma cells at the bottom of a stem cutting begin to divide and form roots whereas those at the upper end of the cut-ting develop into shoots.

The xylem accumulates in annual layers, extending from the shoots in the crown into the roots to form a tapered column of wood. When viewed in cross-section, as on the surface of a cut stump, annual rings of xylem are visible because of the different ways cells develop during a growing season. Active cell division early in the growing season in apical meristems in a tree crown produces auxins that moved downward along the trunk into the cambial zone. The high concentration of auxins promote the development of spring wood with large-diameter, thin-walled xylem cells. In mid-summer when grow-ing conditions are more stressful, shoots slow down or stop grow-ing, and the production of auxins also diminishes. Cells produced by the cambium shrink and develop thicker cell walls, forming sum-mer wood. During the winter the cambium is dormant, but will resume growing the following year with the production of spring wood. It is the transition from summer wood to spring wood that makes the annual xylem ring so apparent.

Organization of the cells in they xylem is classified in three categories. based on the kinds and arrangement of cells. Nonporous wood has only tracheids, fibers and parenchyma cells. The rays

are. generally narrow, only one or a few cells wide. The wood, which is produced by conifers and other Gymnosperm trees, is very plain. The Angiosperm or hardwood trees are separated into two xylem classes, ring-porous and diffuse-porous. These classes of xylem contain vessels as well as tracheids, fibers and parenchyma cells. Diffuse-porous wood has vessels that, are uniformly distributed in each annual layer of xylem. Ring-porous trees have large-diameter vessels restricted to the spring wood portion of each annual layer of xylem. The vascular rays are often several cells wide and quite visible.

Often, the wood in is a lighter color toward the outside and darker in the center. These two areas of xylem are the sapwood and heartwood. The sapwood is the physiologically active portion of the xylem, where tracheids and vessels are used for conduction of water and dissolved nutrients, and the parenchyma cells are alive and function in carbohydrate storage. The heart-wood is nonfunctional and even the parenchyma cells are dead. They may have died because they were buried by accumulating layers of oxygen-limiting xylem. It is more likely that they died because the tree used these cells as a dump site for its own toxic waste - tannins and phenols. The heartwood is particularly decay-resistant because of the accumulation of these compounds, which account for its darker color too.

Water that flows through the dead, hollow xylem cells is driven by transpiration - the evaporation of water from the leaves. Continuous columns of water extend from the cells of the leaves through the xylem of the branches and trunk into the roots. The water columns are essentially pulled up the tree along a gradient of decreasing pressure. Because water movement is related to transpiration, environmental factors such as air temperature and relative humidity affect the rate of movement. Understanding this relationship is important in trunk injection applications of systemic pesticides and growth regulators. The weather and its effects on water movement in the xylem influence the speed and ease of injections.

The transport of water occurs through dead cells, and the path of least resistance, which, in nonporous wood is the larger-diameter spring wood tracheids. The water conduction pathway in this type of wood is a series of concentric rings of spring wood tracheids in three to four annual layers of xylem. In porous wood, the vessels provide the principal conduit for transport because of their large diameter and open end walls.

Diffuse-porous trees conduct water in the vessels scattered throughout two to three annual layers of xylem. Ring-porous wood, in contrast, has a conduction pathway that utilizes the large-diameter vessels of the spring wood in only the current year's xylem layer.

To get good uptake and distribution of material injected into the xylem, it is important to inject into the activity conducting portion of the xylem. For ring-porous trees, this is very shallow, since only the new xylem tissue conducts. In diffuse-porous and nonporous trees, materials can be injected deeper.

THE PHLOEM

Although the phloem constitutes only a small portion of a tree's tissues, its function in transporting food and hormones is exceedingly important. Phloem is derived from the vascular cambium, but the phloem does not accumulate in annual layers as does the xylem.

Five kinds of cells are found in the phloem. The specialized phloem cells are the sieve cells and sieve tube members. Sieve cells are the most primitive and the counterpart to tracheids in the xylem. Sieve cells have pits in the side and end walls that allow movement between cells. The evolutionary advanced sieve tube member characterizes the phloem of hardwood Angiosperm trees. The sieve tube member also has pits in the side walls but, more importantly, has performation plates with large openings at the ends of the cells. Stacked end to end, they provide efficient con-duits for transport. Two types of phloem cells, the fibers and parenchyma, are exactly like those in the xylem. Scierids or stone cells are small and fiber-like.

Every year a new ring of phloem is produced. The fleshy phloem cells are located between the woody xylem and the dead outer bark. The cells of the phloem must be alive with their protoplasm intact. The phloem's fibers and sclerids prevent the active phloem cells from being crushed. The living cells are finally destroyed and the contents reabsorbed or incorporated into the bark and shed from the tree. Thus, phloem does not accumulate like the xylem.

Movement in the phloem occurs both upward and downward to allow for distribution of food and hormones to and from sites of production, storage and utilization. Conduction in the phloem results in a positive pressure in the cells. The best evidence of this is the feeding of aphids. The aphid is a clever little insect that can delicately stick its feeding tube into a phloem cell just under the bark. The pressure in the cell forces more sugary solution through its body than it can digest. The resulting overflow, called honeydew, drips on sidewalks and cars beneath infested trees.

THE BARK

Bark is formed from the other secondary meristem, the cork cambium. The anatomy and development of bark, basically, a protective tissue, is probably the least understood of all the tissues in trees. The variations in bark appearance are enormous and change as individual trees age. However, the characteristics of the bark are frequently so closely associated with each particular kind of tree that they can serve to identify the tree species.

At least four types of bark development have been recognized. Smooth bark trees have a single cork cambium that remains with a tree for its entire life. The cork cambium produces a layer of cork cells toward the outside each year.

The other kinds of bark are variations of the same theme. Ring bark trees, such as eastern red cedar, produce a new and complete cork cambium each year, resulting in concentric rings of cork cambia and the cells derived from them. These are eventually forced outward by diameter growth, rupture, and cling to trees as long, stringy strips. For scale bark trees such as pines, the first cork cambium does not increase in circumference rapidly enough to avoid being torn apart by increases in diameter of the xylem. Beneath each point of rupture, a new cork cambium is formed. The process is repeated thousands of times. The shape and size of each new cork cambium is reflected in the shape of the scales that cling to the bark for a few years before they are shed. The furrowed bark of trees such as ashes develops similarly to scale bark trees. The difference, however, is that the new cork cambia form in the old phloem and its fibers become incorporated into the bark. Event these tough fibers are finally forced apart by diameter growth. The deep furrows, ridges and diamond-shaped patterns of bark reflect the original orientation of the phloem fibers.

Cork cells are so impervious to both water and gasses that they could limit oxygen from reaching the living cells beneath. However, a specialized structure, the lenticel, consists of loosely arranged cells extending across the bark to provide for gas exchange. The short, horizontal lines that often are so apparent on smooth bark species such as Black Cherry, for example, are lenticels. They are an essential part of all bark, just not as obvious when the bark is furrowed and rough. The next time you examine a wine bottle cork, notice the dark lines running perpendicular to the annual rings of cork. These are the lenticels that provided aeration for the cambia and other living cells.

The growth of a tree from a single cell in a fertilized flower to a coordinated accumulation of millions of cells with diverse sizes, shapes and functions is a wondrous phenomenon. Tree farmers should be proud to work with trees, realizing that through their care and maintenance practices they are dealing with the physiological processes of immensely complex and massive organisms.

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