V.

Life Processes

Most of the physiological processes undergone by trees are common to all higher plants. Because the structure of all trees is basically similar, many processes are carried on in uniform ways by most trees.

A.

Basic Structure

Growth of trees, like that of shrubs, requires the successive addition of many layers of woody tissue to the stem of the original young seedling. The axis, or root and stem, of a tree seedling is divided into three main layers. The outermost layer, called the epidermis, is composed of thin-walled cells and protects the inner layers of the axis. The middle layer, called the cortex, is composed of larger, thin-walled cells, which function temporarily as storage cells. The innermost layer, or stele, is composed of a layer of tough pericyclic (outer vascular) cells, a multicellular layer of phloem cells, which conducts food materials, a multicellular layer of xylem, or wood, cells, which conducts water, and an inner core of thin-walled cells that is called pith.

B.

Embryonic Growth

Early in the development of the plant, an embryonic layer of cells, called the cambium, develops between the phloem and xylem layers. Throughout the plant's life the cambium layer alternately produces additional phloem and xylem cells by constant division. After one division, the inner of the two cells produced develops into a xylem cell. The outer cell then divides, and the outer one of the resulting cells develops into a phloem cell, while the inner cell continues to function as cambium.

C.

Development

The constant divisions of the cambium gradually increase the circumference of the axis. The cambium continually increases in circumference as the area of wood enlarges because of the increase in xylem cells. The tissues outside the cambium—namely, phloem, pericycle, cortex, and epidermis—soon rupture, form deep fissures, and eventually split off. A new cambium, called the cork cambium, or phelloderm, develops outside the phloem and produces successive layers of cork cells that protect the cambium. As the axis continues its expansion, the layers of cork frequently develop characteristic fissures at the surface, and as each cork cambium is split by the expansion of wood, a new cork cambium develops to replace it.

D.

Maturity

At maturity, the tree axis normally consists of several layers of cork cells, the outer part of which is fissured: the cork cambium, a few layers of crushed phloem, a few layers of functioning phloem, the cambium, and many layers of xylem. The xylem layers usually constitute more than 95 per cent of the diameter of the axis. The xylem layers are collectively called wood, and the layers outside the cambium are collectively called bark. The cork cambium divides the bark into outer bark and inner bark.

E.

Annual Rings

Because the xylem cells produced in the spring are larger than those produced later in the year, and because an interruption of growth occurs during the winter, the growth of wood for each year appears as a distinct ring, called an annual, or growth, ring. The width of each ring is affected by climate and other variables, and archaeologists have studied tree rings to determine the climatic conditions and variations in environment of former times. By starting with trees of known ages and comparing their rings with those of trees of unknown age, archaeologists have worked out a chronology extending back some 4,000 years. This tree-dating method, called dendrochronology, has been used to date ancient structures and buildings whose wooden beams have been preserved (see Dating Methods). Older annual rings are usually darkened and non-functional, and are collectively called heartwood; younger layers are lighter in colour and transport sap, and are therefore called sapwood. Tropical trees tend not to have a regular ring pattern as the climate is more uniform.

F.

Nutrition

The sapwood (xylem) of the axis carries water and dissolved mineral nutrients upward from the soil to the leaves (see Transpiration). In the leaves the water is used, in combination with carbon dioxide taken in through the stomata (leaf pores), in a process of food manufacture called photosynthesis. The sapwood also transports the gaseous products of respiration, which occurs in all living cells of the plant, to the leaves, from which the gases are released into the atmosphere. Food, manufactured by photosynthesis, and oxygen, absorbed from the air and used in respiration, are transported downward to the roots by the phloem.

G.

Reproduction

The reproduction of trees is carried out by seeds. As in almost all other plants, reproduction involves the alternation of generations. Ovules and pollen may be borne in the same flower or the same inflorescence of a single tree. In most hollies and poplars, and in yew, juniper, and ginkgo, however, the trees are either “male” or “female”. Trees are usually wind- or insect-pollinated, but several species produce fertile seeds without pollination. The seeds of most broadleaved trees have protective coverings, known as fruit, while most of the seeds of needle-leaf trees lie in cones. The seeds inside the cones develop “wings” and are scattered by the wind.

H.

Lifespan

The normal age span of trees is different for each species. Some birches, for example, normally die after about 40 years of life. The sugar maple, on the other hand, frequently lives as long as 500 years, some oaks as long as 1,500 years, some junipers as long as 2,000 years, and some giant sequoias as long as 4,000 years. Bristlecone pines, some between 4,000 and 5,000 years old, are among the oldest living things.