THE green plant, owing its colour to the possession of the mixture of pigments known as chlorophyll, is the primary source of food material for almost all the living creatures that inhabit the earth. With the exception of a few kinds of bacteria, green plants are the only organisms which can build up food from simple substances collected from the soil and from the air. These simple substances are not food; they are the raw materials from which the plant builds up food for its own needs. The green plant is eaten by many animals, which in their turn provide food for theanimals; moreover, alive or dead, the green plant nourishes hosts of plants which have no chlorophyll and therefore cannot make the food substances necessary for their sustenance.
As the activities of the green plant are so essential to the persistence of life on the earth, a study of them cannot fail to reveal many points of interest. The structure of the plant must also be considered, since a knowledge of the structure helps to an understanding of the manner in which the plant works; structure and function are closely interrelated.
We are acquainted with many thousands of kinds of green plants. Some of these are minute, so small that if five thousand were placed in a straight line, that line would measure only one inch; at the other extreme some of the largest trees rival St. Paul’s Cathedral in height, and weigh many tons. Green plants are equally diverse in the situations that they inhabit. Some seaweeds flourish in the Polar seas at a temperature little above freezing-point, and some of our common garden weeds are able to grow slowly and produce a fewat almost equally low temperatures. In contrast to these, some simply organised green plants have been observed in a flourishing condition in water much too hot for the hand, and some of the desert plants must pass long hours at comparatively high temperatures. A few plants exist on the surface of rocks under conditions of almost uninterrupted
drought, others spend their whole lives immersed in water. No green plants can live their whole lives in perpetual darkness, though for some ferns the amount of light necessary is small. However, our present concern is rather with the more ordinary plants. Of these we may briefly consider three, in order to establish a few general points.
THE LIFE OF AN ORDINARY PLANT
SEEDS of Virginia stock, planted in moist soil in spring, and provided with a moderate supply of water, soon germinate. First we have the, with a delicate upright axis bearing two small of simple outline, with a bud between them. From this bud an upright arises, producing , and presently, branches which are veiy like the parent . After the plant has spent a few weeks in the development of leaves and branches, begin to form at the ends of the branches. In due course the flowers wither and are replaced by fruits, the vessels in which the ripen. As this is going on the plant slowly dies. The fruits split, the seeds are set free, and the next generation is ready to begin its development. Thus, the Virginia stock has a short life and matures one crop of seeds; these remain quiescent during the colder part of the year, and provide the starting points for the next season’s growth.
The Snapdragon furnishes our second example. Up to the ripening and liberation of the seeds, development proceeds much as in the Virginia stock, except that the parent plant does not die as the seeds ripen. In the following spring, new branches arise from the bases of the old branches and bear flowers in their turn. Under favourable conditions the plant may live for several seasons and produce several crops of flowers and seeds.
HOW A TREE GROWS
THE seeds of the Sycamore, enclosed in their winged fruits, fall from the tree in autumn and lie dormant all winter. In spring, the seeds germinate, giving young plants with a delicate upright axis and two strap-shaped leaves with a bud between them. A leafy stem grows from the bud and may reach a length of several inches during the first season of growth. It seldom branches in the first year and never flowers. In autumn, growth stops, the leaves drop, and the stem passes
the winter crowned by a large terminal bud, and with a number of smaller lateral buds projecting just above the scars left by the fallen leaves. In the second spring, the terminal bud becomes active and yields a few more inches of stem and several pairs of leaves.
As this is going on, the portion of stem formed in the first year increases slightly in thickness but does not increase in length; its lateral buds remain inactive. In the second autumn, the leaves fall, and the stem, now consisting of two portions formed in successive years, again rests for the winter. In the third and subsequent years the story is repeated. Each season the older parts of the stem become more woody and increase steadily in thickness, but they do not elongate. The elongation of the plant is due to the addition of a portion during each season of growth. It is worth notice that as the plant increases in size, theincrements for some years become longer and longer.
WHEN A TREE REACHES MATURITY
FOR the first two or three years the stem may not branch, but with increasing age, branches begin to appear from the buds along the side of the stem. They elongate, thicken and branch in the same way as the main stem, but they do not become as strong as it. The lowermost branches die off as others develop above them so that the lower parts of the main stem and of its branches again become bare. In the young plant, for twenty years or more, the main stem remains clearly visible as the central axis around which the shoot system of the plant is arranged. As the Sycamore becomes a small tree, it begins to flower. Henceforth, each year, the branches increase in length, fresh branches are put out, a new covering of leaves is developed, and flowers, fruits and seeds are produced. Each year also, the older parts of the shoot system become thicker and thicker.
As the tree ages, the main stem loses its obvious superiority over the larger branches and the marked elongation characteristic of youth no longer occurs, since the material available for growth in length is shared among a large number of twigs. After many years, signs of debility appear, decay sets in, and the tree weakens and slowly dies. The causes of this degeneration are not fully understood, but it seems probable that disease caused by the attacks of enemies, together with changes in the living part of the plant, are largely responsible.
WHAT IS INSIDE THE TREE
ALL trees contain a massive central core of dead wood, running through, trunk and branches, and reaching into the bases of the youngest twigs and youngest roots; these, and the leaves and flowers, consist for the most part of living cells. The dead core is surrounded by a thin sheath of living material, complicated in structure and function, and covered on its outer surface by a layer of dead cork. This forms the bark of the tree, protecting the delicate living sheath from drying up, from mechanical injury and from enemies. The bark is pierced by many openings loosely packed with corky cells; these openings—the lenticels—convey air to the living cells within and allow carbon dioxide to escape from them. Lenticels are usually most obvious on the thinner branches; they are clearly visible as small dots on young branches of apple trees, and as horizontal streaks on branches of cherry .
large elongated lenticels are often conspicuous on birch trees.
Both wood and cork are formed by the alteration of cells of the living sheaths just under the bark, and, as long as the tree continues to grow, additions are made to both wood and cork. The wood, lying within the sheath of living cells from which it is formed, accumulates steadily during the whole life of the plant, and since its development corresponds with the annual periods of active growth, the wood is laid down in a series of concentric cylinders. Thus, a trunk cut across shows a series of rings, known as annual rings, since, normally, one ring indicates the amount of wood formed in one year; by counting these the age of the tree may be determined.
HOW THE TREE MAKES USE OF ITS DEAD PARTS WE thus reach the surprising conclusion that most of the substance of a large tree is dead. The dead material is by no means useless. The woody central core has great mechanical strength, and forms a scaffolding on which the annual crop of leaves, flowers and fruit is exposed to light and air. The youngest parts of the core lie on its outer surface, where some living cells a.e mixed with theof tiny pipes through which watery solutions move about the plant.
The mass of wood increases as the weight of leaves and branches increases, and, with the steady increase in the diameter of the woody core, more and more space is provided on its outer surface for the increasing number of conducting channels needed for the supply of water and other substances to the leaves, and for the distribution of the food made in the leaves. Most of the material moves up and down in the plant, but there is some movement sideways, especially in the wood. This takes place chiefly in the medullary rays, thin sheets of elements set vertically in the wood and running radially in it and out into the living cells beyond it. In oak, the medullary rays are particularly large and cause the well-known silver grain of the timber, but in many trees they are small and difficult to see without magnification.
So long as the wood is surrounded by an unbroken sheath of living material, it is protected to some extent from invasion by the bacteria and moulds which cause rotting, but this protection seldom remains fully effective for long. Wind tears off branches and exposes portions of the dead and vulnerable wood, animals injure the bark and damage the living sheath
below, and this sheath is itself attacked by a number of creatures, which then attack the wood or open a way for others to do so.
Although the living cells of the stem add to the inner surface of the bark during the growing season, the bark does not increase in thickness as noticeably as does the wood. Additions are not made to the bark as freely as they are to the wood, and moreover, portions are frequently lost from the outer surface of the bark by a definite process of shedding. The process is very familiar in the plane tree, since the young bark exposed as the older material falls away is bright yellow and is very noticeable. The bark of the cork oak, which furnishes the commercial product, is exceptional in its thickness and uniformity of texture.