Up to this point we have considered only those parts of the plant that are exposed to light and air. Plants usually possess below ground a branching system ofwhich anchor the plant to the soil and collect material from it. If young plants are carefully dug up and washed free from soil, they show commonly, but not always, a system composed of a main which is a direct downward prolongation of the main , surrounded by a number of branch roots. The branches are very like the main root in everything but size, and, if they in turn divide, their branches are smaller versions of the parent. The root system is much less complex than the parts of the plant above soil level, and there is a much greater likeness between the roots of one plant and those of another than between the shoot systems of different plants.
Although the young root systems of many kinds of plants are clearly arranged around a main root, the arrangement may not remain clearly visible in the adult plant, especially if it lives for a number of years. In trees particularly, as the crown spreads, the roots keep pace with it, so that the tips of the youngest branches of the root system lie in the earth beneath the outermost ends of the leafy branches, where most rain drains off from the tree; these active tips may be many feet away from the trunk of an old tree. Now, the downward growth of the main root is often checked by mechanical obstacles, and also by the further circumstance that, as deeper and deeper levels are reached in the soil, the supplies of oxygen essential to the young root become more and more scanty, and finally cease to be sufficient; this brings downward growth to an end. Root systems can usually spread sideways better than they can go downwards, and therefore, in old trees, the main root is usually smaller than some of its branches.
Roots branch freely and occupy the ground very thoroughly; for example, the roots of an average plant of the large sunflower search closely rather more than a cubic yard of soil. In deep, dry soils, which are usually well supplied with air,
roots may penetrate to a depth of ten feet or more, and under special conditions, the root system of a plant may be larger than the shoot system. Some of our coastal plants, growing in loose sand, show a few smalland , but their roots run several feet in all directions.
In limy soils most plants root freely, but in acid, peaty soils root development is often poor, and the plants may have at their disposal some other device which compensates for the disadvantages arising from small roots. Some, like the sundew and the butterwort, supplement the scanty ration of combined nitrogen they obtain from the soil, by catching and digesting insects; others, such as heather, appear to live in association with a fungus, which may provide them with material taken from the peat.
In general structure stems and roots are much alike. Young roots, like young stems, contain a number of conducting strands embedded in a mass of soft cells; as the root ages, the centre is occupied by a solid core of dead wood surrounded by a thin sheath of living cells, protected by an outer layer of cork. Roots have to meet pulls rather than bends, and in young roots we find that the strongest material is nearer to the centre of the organ than it is in young stems; the central disposition of the strongest material is the arrangement which offers the most resistance to pulls.
TUBERS AND BULBS AS UNDERGROUND LARDERS ALL subterranean parts of plants are not roots. Many plants have their main stem underground, and each spring this stem, which may be much branched, sends up leafy flowering stems which die down at the end of the season. Michaelmas daisies and other familiar herbaceous perennials have this sort of organisation. During the growing season, the food made by theand not used for the current needs of the plant is passed down and stored below ground, being utilised in the next spring in the preliminary stages of growth. Large storage organs are formed by the underground stems of some plants; the bulbs of and tulips, the corms of the crocus, and the tubers of the potato are familiar underground stems which act as food reservoirs. All these plants pass at least a part of the year in an inactive condition. Such plants are usually natives of situations where, for some reason or another, there are special hindrances to growth during part of the year. Some are woodland plants—the blue-
bell is a good example—others, including many of the bulbous plants in cultivation, come from places where there are sharp alternations of wet and dry seasons. The woodland plants, having stored food at their disposal, are able to start growth early in the year, and to produce their, set their and lay in renewed stores before the shade cast by the trees brings activity to an end. Those which live in droughty places can make a rapid start as soon as water is available and complete their work before water scarcity again becomes acute.
THE LEAF: A FACTORY FOR MAKING THE FOOD SUPPLY FINALLY, in our general survey of the plant, we come to the. The leaves of most plants are thin, flattened objects, usually so arranged on the plant that they present one broad face to the sky. This posture is directly related to
the need of the leaf for good light which plays an essential part in the manufacture of food, the essential function of the leaf.
Active leaves always contain a great deal of water, and to this they owe much of their stiffness; the crisp lettuce leaves so welcome in a salad contain about 95 per cent by weight of water; the tougher leaves of the trees have about 80 per cent of water. Leaves have the same general arrangement as stems in relation to rigidity, the strong epidermis resisting the expansive tendencies of the softer cells inside. The mass of soft cells inside the leaf is freely permeated by veins; these have some mechanical strength, and help in keeping the leaf spread out, but their main purpose is the conduction of watery solutions into and out of the leaf. Everyone has probably seen a leaf skeleton; one is easily made by rotting a leaf for a time in water, and then carefully washing away the soft rotten material. Such preparations show perfectly the complicated conducting system of the leaf and its intimate relation to all parts of the leaf.
HOW THE LEAF FACTORY IS BUILT
THE leaf is covered completely by a well-organised epidermis enclosing a thin layer of softer cells. These are not all of the same kind. Beneath the upper surface, and therefore in the best lit, lie one, two or more rows of cells which are several times longer than they are broad. The cells are grouped into tightly packed layers with one narrow end of each cell turned towards the upper surface of the leaf. The microscope reveals the presence, inside each cell, of large numbers of small green granules; these are the chloroplasts and they contain the chlorophyll. The closely packed layers, with the ends of the cells turned towards the light, form the palisade tissue of the leaf, and the lower ends of the cells are often close to the veins.
Between the palisade tissue and the under epidermis, there is a region occupied by cells of rather irregular form. They contain chloroplasts, but they are not packed closely together, for although they touch one another here and there, and although some of them may make contact with the lower ends of some of the palisade cells, there are in this region many air-filled spaces, so that there is a general spongy texture. The spaces between the cells are in communication and they unite also with some rather large air spaces just
inside the epidermis, and communicating with the air outside the leaf through pores in the epidermis.
As a rule, these pores—the stomata—which are not mere holes, but have an organisation by means of which they can open and close, are most abundant on the lower surface of the leaf. The number of stomata in a leaf is not uniform for all plants, but, as an indication of the numbers in which they may occur, we may note that a large sunflower leaf has about 13 million stomata, and that, as an average value, every square millimetre of the under surface of an oak leaf bears 346 stomata. The many and relatively large spaces between the cells inside the leaf provide room for a considerable amount of air, and the abundant stomata allow of the ready exchange of gases between this internal atmosphere and the air outside.
THE CELL: CRADLE OF THE LIFE FORCE
Up to this point we have taken for granted that the plant is made up of cells or of structures derived from cells. We must now pass to a somewhat more detailed treatment of the cell, since we need some knowledge of this before we can study the inner workings of the plant.
Cells are commonly very small objects, though some are large enough to be seen with the naked eye. If we take a ripe tomato which is still in good firm condition, and tear it in halves, we can see without the use of any magnification that the whitish material running into the fruit from the junction of the stalk, and the reddish material of the fleshy wall around the fruit have a finely granular structure. A hand lens of moderate power shows that the granular condition is due to the presence in those parts of many small globules which do not fit very closely together. The globules are the cells. They have a shiny, swollen aspect, since they are distended with water.
Full details can be obtained only by the use of the higher powers of the compound microscope, assisted by the use of fluids which kill the cells without causing them to shrink, and by the application of stains to bring out details. By such methods we find that each cell is surrounded by a thin, firm wall. Inside this, and in intimate contact with the wall, there is a layer of a viscous fluid full of tiny, nearly colourless granules; the fluid is protoplasm saturated with water. It is not unlike white of egg in its general characters, and, though its exact nature is unknown, it appears to be a very complicated system of complicated substances, and it is the part of the cell in which the mysterious thing we call life has its being. The layer of protoplasm forming the protoplast of the cell completely surrounds a cavity—the vacuole— filled with a watery fluid containing sugars, mineral salts and many other substances in solution. In properly prepared material, stains show that the protoplast includes a rounded object—the nucleus—and that this often includes a smaller rounded body—the nucleolus.
The nucleus is a most important part of the protoplast, for it presides over the activities of the cell, and, though we are not concerned with the subject here, the nucleus appears to be the chief agent in the transmission of inheritable char_ acters. In most cells the nucleus usually forms but a small
part of the protoplast, the remaining and larger part being known as the cytoplasm. When chloroplasts are present they are embedded in the cytoplasm and are therefore in intimate contact with the active material of the cell. Cells vary so much in size that it is not possible to give even average figures of their dimensions, but, as some indication of the size of nuclei, we may note that if 2500 nuclei of average size were strung like beads, the string would be an inch long.
Plant bodies are made up chiefly of cells, which may have the simple form just described or which may be much more complex. The complications affect mainly the shape and thickness of the walls. The walls consist of dead material formed by the protoplast, and provide a mechanical framework in which the protoplast lives and works. The walls are pierced by fine pores through which the protoplasts of neighbouring cells remain in communication.
THE CELLS THAT HOLD OR CONDUCT THE MATERIAL fipHE conducting strands of the plants, and in particular JL those of the wood, are not formed of simple cells. The tubular wood vessels are formed by the union, end to end, of several thin-walled cells, with subsequent break-down of the walls between the cells, and thickening of the walls along the sides; one can get a rough idea of the process by imagining several cylindrical tins piled one on top of another, and then all the lids and bottoms disappearing except those at the two ends of the pile.
As the vessels form, the living contents of the uniting cells disappear so that the vessels contain no living material when they are fully formed. They can conduct material only as long as living cells are close by. This explains how it is that most of the wood of a large tree is dead and unable to conduct fluids. In the tree living cells exist only in the surface layers of the wood, and they die as new cells are laid down outside them. When they die, the vessels in their vicinity lose their power of conduction.
The sieve tubes which are concerned in the movement of the more complicated food substances inside the plant are also formed from elongated cells; mature sieve tubes contain living cytoplasm but no nucleus. They usually have in close association one or more small cells with specially large nuclei, and it seems probable that these nuclei control the activities of the cytoplasm in the neighbouring sieve tubes.