Trees

Trees vary greatly in shape according to their species.

Most tree species are found among the gym-nosperms and angiosperms, with more species occurring in the latter group. Most are dicotyledons, but a few are monocotyledons, such as palms (Palmae), Joshua trees (Yucca brevifolia), which are found in desert areas, and dracaenas (Dracaena spp.l.

Botanically, trees are plants that usually have a single woody stem (the trunk, or bole) and a crown with woody branches. They may be only a few feet high, as in the undergrowth of tropical rain forests, or very tall—the Australian mountain ash (Eucalyptus regnans) and the California redwoods, for example, may attain a height of more than 300 feet (91 meters).

Only gymnosperms and dicotyledons have proper wood. This substance is a permanent, secondary tissue comprised of cells with ligni-fied walls. Plant cell walls are made mostly of cellulose arranged in microscopic fibrils. In woody plants the cellulose is impregnated with lignin—a material that imparts compressive strength to the cell wall.

Tree growth and form

The growth rings on the trunk of wych elm (Ulmus glabra) display the different annual rates of growth. The pale sapwood contains xylem, which transports water, and inner phloem (bark), which conducts food. The inner dark section is the heartwood. The small rings on the edge of the stem belong to a lateral branch.

The apical meristems of a tree—the growing regions at the tips of the trunk, twigs, and roots—are the sources of increase in height of the tree. Auxins (growth hormones) are supplied in greater quantities to the trunk meristems than to the lateral ones, which are often suppressed.

The secondary or lateral meristem, in the shape of a thin ring between the wood and the bark, is the vascular cambium. This meristem is the means by which lateral growth, or thickening, takes place in the tree because it forms new wood cells inward, and bark cells on its outer side. Monocotyledon trees do not have true wood, but numerous vascular bundles set in a matrix of parenchyma, which may become lignified. They either have no secondary thickening (such as palms) or thicken by the formation of extra vascular bundles as do dracaenas, for example.

In some tree species the trunk divides at a low level into two or more trunks. In most trees, a single trunk forms the dominant axis from which lateral branches grow to form a crown, or head, of branches. Those species in which the trunk continues to the tip of the tree as a single axis, with the lateral branches being smaller in size and of secondary importance, have a pyramidal, or excurrent shape, and are described as monopodial (single axis). Trees in which the trunk does not grow to the crown top but is replaced by variously massive limbs, are termed decurrent, or sympodial (many axes), such as oaks (Quercus spp.) and basswood (TiHa spp.). When young, all trees are monopodial. Few dicotyledons retain this form at maturity, although many gymnosperms do. In monopodial trees where the leaves occur only at the top of the trunk, as in palms, the trees may be described as columnar.

The shape of a tree varies partly according to its species and partly in response to its environment. In a specimen tree—a tree grown in the open in isolation—the branches remain, atypically, from low down on the trunk and may spread out, almost sweeping the ground. When trees grow close together, as in a forest, the lower branches receive little light and soon die. The branches forming the crown at the top of the tree are the only ones that survive and the tree grows havinq a branch-free trunk.

The influence of the environment on the final shape of a tree is well seen in exposed windy areas, such as mountaintops or near the sea. A tree that manages to grow there develops a distinctive swept-back look with the branches trailing away from the direction of the prevailing wind. As soils in these places tend to be thin or nutrient poor, the tree never grows to such a height as the same species would in a sheltered area with an abundance of rich deep soil.

These shapes can be developed artificially. The art of bonsai, perfected in Japan, involves the development of a shallow root system by growing in a small pot and regular root pruning. This has the same effect as when a tree grows in shallow soils. The branches are also pruned and trained with wires to grow in the required direction, simulating the effect of the wind. By manipulating the tree’s environment in this way, the bonsai master can produce miniature versions of any tree form found in the wild.

The growing point, or apical meristem, on a stem consists of meristem tissue in which new cells are formed by rapid cell division. Buds form in the inner Junction of each leaf with the stem and grow on stems in three ways—opposite each other, alternately opposite, or whorled, spiraling up the stem.

Annual rings and wood rays

A cross section of many tree trunks reveals a number of close concentric circles that appear to be split into wedges by raylike lines. These circles are annual growth rings and are apparent in trees that grow in temperate climates only. The rings are formed because the cambium in the trunk produces large conducting cells in the spring and early summer and smaller conducting cells in late summer. The highly porous spring wood conducts water rapidly to the new growing shoots, and the less porous summer wood acts as a strengthening tissue. Because growth ceases in the fall in temperate regions there is a distinct boundary between the ring of one year’s growth and the next; but this is not so in trees that grow in equatorial climates, where there are no distinct seasons. Some tropical areas have wet and dry seasons which may form distinctive rings in the wood, but they are not always reliable annual indicators.

Not only does the historical analysis of annual rings (dendrochronology) signify the age of a tree with considerable accuracy, it also indicates good and bad growth seasons, which can give a measure of the rainfall at a certain time in the past. Radioactivity levels, the oxygen content of the air, and mineral nutrient levels in the soil, among other things, can be discovered by chemical analysis of a single tree ring.

The rays that divide annual rings into wedges radiate outward from the center of the trunk. They are known as wood rays and are made up of parenchyma cells. As the tree grows in width, the cambium produces new conducting tissues and packing cells of parenchyma. The ray parenchyma cells transport food, waste chemicals, and other materials toward the center of the trunk. The stored food (often starch) can be mobilized in spring when the buds open and is passed into the vessels to be carried to the areas of growth. In spring the sap consists of a solution of up to 8 per cent sucrose. In the sugar maple (Acer sac-charum), for example, this “sugar run” is exploited each year when people tap the vessels for the sugar-rich sap.

Maple wood is a diffuse-porous hardwood. The term applies to the vessels in the sapwood, which are more or less the same size and evenly distributed throughout the spring and summer wood.

Sapwood and heartwood

A transverse section of dicotyledonous wood reveals two types—a light, outer layer of sap-wood and an inner, darker layer (the heart-wood). The sapwood has living parenchyma cells and water-conducting xylem cells, tra-cheids, and vessels, as well as fibers. Vessels occur in angiosperm wood and not in that of gymnosperms. In the latter, the tracheids and fibers are usually not fully differentiated but exist as fiber-tracheids.

A longitudinal section through the sapwood of a beech tree displays the wood fibers and vessels, running vertically, and the rays, running horizontally.

The different cells of the xylem and their arrangement give the wood its characteristics— the vessels and fibers are often present in different proportions in spring and summer wood. In some trees—oak, for example—the broad vessels are confined to the spring wood whereas in the summer wood there are numerous fibers and fiber-tracheids. This type of wood is known as ring-porous wood. Diffuse-porous wood, as found in maples (Acer spp.) and basswood, has much smaller vessels, which are more evenly distributed throughout the annual rings. A few genera of angiosperms are exceptional in that, like gymnosperms, they have no vessels. This feature is believed to be a primitive one and is found in Drimys and Trochodendron of tropical forests.

A dicotyledonous tree trunk consists of rings of several compositions. The innermost dead section— the heartwood—is darker and denser than any other part of the tree. It is surrounded by the paler, living sapwood, which is separated from the next ring, the phloem, by a thin cambium iayer. Outside the phloem is the bark-producing cork layer. Running through the rings radiating from the heartwood outward are the wood rays which are made up of parenchyma cells.

In contrast to the sapwood, the cells of the heartwood are mostly nonconducting and are used for storage except at the boundary with the sapwood where the parenchyma cells store water. The parenchyma cells of the newly formed sapwood usually live for several years. As sapwood ages to form heartwood, its cell walls undergo a chemical change and become darker and denser. Heartwood also contains less moisture than sapwood. The cells often fill with bubblelike inclusions (tyloses) of waste material, such as tannins, resins, dyes, oils, gums, and mineral salts. In the teak tree (Tectona grandis) the inclusions are composed of silica, and in satinwood (Chloroxylon swie-teniaJ of calcium oxalate.

Sweet chestnut (Castanea sativa), left, is a rough-barked tree with deep fissures that characteristically spiral around the trunk. The fissures occur as the cork plates in the bark pull apart when the trunk thickens. Smooth-barked trees, such as the Chinese cherry tPrunus serrulal, right, have a very thin bark that flakes off as the trunk thickens. The horizontal scars are lenti-cels, which are specialized structures that contain intercellular spaces that allow oxygen to diffuse through the bark into the trunk.

The altered cell walls and inclusions give the timber its high degree of polish. In some species heartwood is so dense and hard that it is almost impossible to cut. Hematoxylin, which is used as a stain in the study of cells (cytology), comes from the heartwood of the logwood tree of Central and South America (Haematoxylon campechianum) and is a typical example of the dyes stored in trees. Tannins act as a protective antibiotic—the more tannin there is in the heartwood (making it a darker color), the more durable it is likely to be, as in, for example, mahogany (Swietenia mahogoni) and ebony (Diospyros sp.). In some species heartwood does not develop, which is why some trees, such as poplar (Populus sp.) and willow (Salix sp.), have a tendency to become hollow when they are old.

Phloem and bark

The formation of rough bark starts in the phloem when the cells differentiate to form cork cambium, which produces the cork cells (A). A second layer of cork cambium cells differentiate from parenchyma cells in the secondary phloem. Successive layers of cork cambium (B) form inside the previous layers, separated from one another by secondary phloem fibers. Each new layer cuts off the connection of the previous layer with the secondary phloem and so the outer bark dies. The cork layers tear as the trunk ages and widens, creating the deep fissures in the bark.

The outermost regions of the trunk, outside the cambium, are comprised of food-conducting phloem tissues. The phloem, like the xylem, consists of a number of different cell types—sieve tubes, each with a companion cell, parenchyma, and fibers. Whereas the flow of water through the xylem is upward, the movement through the phloem just beneath the bark is mainly downward, carrying foods and nutrients from the leaves down to the roots (and up again in the spring in temperate regions).

As the trunk increases in thickness, through cell division in the cambium layers, the phloem often forms wedges that taper toward the periphery (this is very obvious in oak trees). The spaces between the blocks of conducting phloem are filled with wedges of parenchyma cells: greatly widened phloem rays, continuous with the xylem rays in the wood, which function in the storage of starch and tannins. They remain active until cut off from the living part of the plant by a layer of cork cambium.

As a woody stem increases in width, a cork cambium forms in the outer layer of the phloem. On its outer surface it produces small, regular, rectangular cells whose walls become impregnated with a water-repellent material called suberin. These cells are cork cells and form a protective layer around the trunk that prevents evaporation, penetration of pests and disease organisms, and, in some species, is fire-resistant. The cork oak (Quer-cus suber) in Europe is unusual because after being damaged it develops a thick cork layer. This can later be stripped off for human use.

The bark of a tree consists of the living inner bark (secondary phloem) and dead outer bark. In a smooth-barked tree, the outer bark is superficial. In rough-barked trees, however, new outer bark forms deeper and eventually cuts off the outside cells from the inner tissue. The outer layer—a mixture of many tissues— then forms a thick, dead, outer bark. As the trunk expands, the bark cracks in a pattern characteristic of the species, while new bark is continually formed.

The color of bark is due largely to the presence of tannins, which also prevent decay of the bark cells. Spaced out over the bark are tiny pores (lenticels) filled with loosely-packed cells without suberin. They open at the surface and allow the cells of the trunk to breathe.

The movement of food and water in a tree occurs in the outer layers of its branches, trunk, and roots. If a tree is stripped of these layers, or they are damaged, it will die. Water movement (markedin blue) is upward from the roots, responding to suction pressure from the transpiring leaves. Food, which is manufactured by the leaves, is usually transported downward in the phloem (indicatedin red).

Transpiration

Deciduousness is characteristic of many trees growing in temperate regions. The brilliant reds of birch trees (Betula spp.) are the result of the disappearance from the leaves of the green pigment chlorophyll, to reveal the secondary pigments carotene, xantho-phyll, and anthocyanin.

One of the problems a tree has to overcome is the transportation of water to the top of its crown. This is achieved by “suction pressure” from the leaves. As water is lost from the leaves by evaporation through transpiration, more water is drawn up the stem through the xylem from the roots, and the water in the conducting tissues of the xylem is under considerable tension. The leaf area in a large tree may be sufficient to evaporate many gallons of water in an hour. The amount of water lost during transpiration varies with weather conditions; dry, windy days increase the transpiration rate, and humid, calm days reduce it. The amount of water lost from a transpiring forest may be greater than from an open water surface of the same area, such as a lake, because of the very large leaf surface.

Deciduousness

The leaves, being the main source from which water is lost by the tree, may be shed during periods of water shortage. Some trees regularly shed all their leaves in temperate regions at the beginning of winter, and in tropical regions, at the beginning of the dry season. These trees are said to be deciduous.

There is often a wintertime water shortage in temperate and boreal regions, but that is not the only reason for leaf loss. Low temperatures would destroy the living cells as ice would form in the leaves. Leaf fall is the response to day length. So deciduous trees of temperate and boreal regions shed their leaves and remain dormant throughout winter.

The mechanism that causes the leaves to fall involves processes whereby the leaf is separated from the twig on which it is growing without the twig being damaged and at the same time protecting the exposed surface from drying out and enabling infection. Two layers form—the separation and the protective layers. At the leaf base there is a structurally weak zone known as the abscission layer. After food reserves and mineral nutrients have been withdrawn into the woody branches, a periderm grows over this zone at the leaf base, and eventually the leaf falls off, leaving a scar.

Before the leaf abscises, the chlorophyll in it breaks down, and it loses its green color. Secondary pigments, such as carotene and xanthophyll, become visible and give the leaves their autumnal yellow, orange, and purple colors.

Evergreen trees retain some of their leaves all year round. Each leaf has a life of about three to four years, and there is a continual exchange of leaves. To conserve water, evergreen leaves are often covered with a thick, waxy cuticle, which gives them a glossy appearance. Deciduous leaves have a much thinner cuticle and tend to be softer and less glossy.

In summer (A) when days are long and warm, the leaves of trees can photo-synthesize at a fast rate, drawing vast amounts of water from the roots to replenish the water loss through transpiration. As the days shorten and temperatures drop, food reserves are withdrawn from the leaves, and the chlorophyll breaks down. Secondary pigments become apparent and give the leaves their autumnal colors (B). An abscission layer grows across the base of the leaf stalk (C) and finally the leaf drops off the branch (D).

Roots

The crown and trunk of a tree depend on the roots for anchorage, water, and mineral salts. The vascular structure of the roots is much simpler than that of the trunk, and only the bigger roots are surrounded by bark. The roots branch laterally, crossing over each other, and may fuse to form a network. There are two types of root—thick, cablelike roots which anchor the tree, and finer, feeding roots. The anchor roots tend to grow 1 to 3 feet below the soil surface and may do so in search of water, especially in arid areas. The feeding roots mostly form a mat just below the soil surface. The finest of these have root hairs near their tips through which water and mineral salts are absorbed. In tropical rain forests the soil is typically very poor in mineral nutrients, and most of the tree roots are very near the surface to take advantage of the nutrients deposited by fallen leaves. This is also true for northern and mountain forest trees.

Large root systems are needed to anchor trees, which are top-heavy structures. Frequently, more than half a tree’s bulk will be in the form of roots underground. These roots also serve to extract water and mineral nutrients from the soil, which are transported by the xylem to other parts of the tree.

Flowers

Trees may have spectacular, showy flowers, such as those of the buckeye (Aesculus octan-dra) and flowering magnolia (Magnolia grandi-floraJ or they may be insignificant, like those of the ash (Fraxinus Americana) or oaks (Quercus spp.). The difference in the appearance of the flowers is due to their different methods of pollination. Large, bright flowers attract animals to them. In temperate and boreal forests, these animals are insects, such as beetles, moths, and flies. In the tropics, the pollinators also include birds, such as sunbirds and hummingbirds, bats, and other mammals, such as the brush-tongued opossum of Australia. The pollinators seek out the nectar that is usually found at the base of the petals inside the flowers, pollen, or the insects that feed in the depth of the flowers. These animals collect pollen grains on them as they brush against the male anthers and transfer them to the female stigmas as they move around.

The vivid scarlet of the flowers of the flame, or flamboyant tree (Delonyx regia), attracts birds and insects. These animals aid the pollination of the flowers by collecting and transferring pollen as they move from flower to flower.

Wind-pollinated flowers are not as conspicuous as animal-pollinated ones. They are commonly grouped in long, loose clusters or catkins, and have reduced sepals and petals, so that the anthers and stigmas are exposed to the wind. It needs only a gentle rustle when the anthers are ripe for a cloud of free-floating pollen to be released. The pollen grains of these are typically small and smooth, enabling them to be blown easily, whereas those of insect-pollinated flowers are large and ornamental. The anthers in wind-pollinated flowers are frequently on the end of long filaments and are, therefore, shaken with every breeze, shedding pollen. The stigmas are often sticky or hairy to catch the pollen and are carried on the end of long styles. Like other plants, trees can be hermaphrodite (having male and female parts on the same flower), dioecious (with male and female flowers on different trees), or monoecious, the male and female flower occurring on the same tree.

The catkins of the yellow or silver birch shed pollen as the wind shakes them, releasing from each one 5 million or so grains of pollen. Because they do not rely on animals for pollination the catkins do not have bright colors.

Seeds and fruits

The fruits that are produced by trees also show great variety. Seeds and fruits that are wind-dispersed are quite common because they have the advantage of height from which to drop and be blown. Some trees have winged seeds, the wings of which may be single, as on the jacaranda [Jacaranda spp.), or

double, as found on some maples. Elms (Ul-mus spp.) have seeds rather like a flying saucer. Parachute seeds are also common and are found on trees as diverse as willows [Salix spp.) and kapok (Ceiba pentandra).

The brightly colored fleshy fruits produced by some trees attract animals that the trees rely on for the dispersal of their seeds. The animals eat the seeds contained in the fruits and later excrete them away from the parent plant.

Many trees produce edible fruits, which may be dry or succulent. These are adapted to be carried by animals away from the parent tree. Dry fruits, such as acorns, are often taken away by animals to be buried in the ground and stored for later consumption; there they frequently take root and grow. Succulent fruits usually have hard, resistant seeds, which often need to pass through a gut before they will germinate. The seeds are deposited with their own fertilizer as the animal (usually a bird) defecates. More rarely, tree seeds are designed to be carried by water; alders (Alnus spp.) have a float, as do coconuts (Cocos nucifera), and many others are capable of floating.

The way in which red mangrove trees shed their seeds is remarkable. The seeds of Rhizo-phora mangeli, for example, germinate on the parent tree and develop a long heavy cylindrical root. The seed then drops and spears itself into the mud, before the tide can wash it away.

Orange trees (Citrus spp.) attract insects with the smell and color of their flowers, which precede the fruit. After the flowers are pollinated the flower receptacles swell and become the fleshy fruit.

Classification

Most plant classification is based mainly on the structure of the flowers. The flowers of the family Magnoliaceae are among the most primitive of the angiosperms. The trees of this family, such as the tulip tree (Liriodendron tulipifera) and magnolias (Magnolia spp.), have large solitary flowers, the sepals and petals of which are in groups of three. The family has a wide distribution in North America and Asia and a number of species are popular as garden trees.

In contrast, the members of the family Lo-ganiaceae, for example, are considered to be some of the least primitive of the dicotyledonous trees. Whereas the petals of magnolias are separate, those of the loganias (such as Bud-dleia) are fused into a symmetrical, tubular, or bell-shaped flower. The family is distributed throughout the tropical and subtropical regions with a few temperate outliers, and includes trees of the genus Strychnos, which provide strychnine (from S. nux-vomica), curare (from the bark of S. toxifera), and edible fruit (from S. spinosa).

Many other plant families contain important tree species. The rose family (Rosaceae) comprises most of the commonly cultivated temperate fruit trees, including apple (Malus sp.), pear (Pyrus sp.), plum, peach, and almond (Prunus sp.) trees. In Rosaceae the flowers are usually hermaphrodite, and the sepals and petals are usually five in number. In pear and apple trees the sepals are green, and the petals are generally white or pink. After pollination, the receptacle of the flower becomes fleshy and swollen and encloses the carpels to form the edible fruit.

Whereas the loganias, magnolias, and rosaceous trees are animal-pollinated, those of the beech family (Fagaceae) are wind-pollinated. The male flowers grow in catkins, but the female flowers form a spike of up to three flowers. In both the male and the female flowers, the sepals and petals together form a perianth, which may be green-brown or colorless. The flower parts are simple and reduced because of their method of pollination, rather than their advanced stage of development. The pollen is light and copious.