THE flowering plant consists of a portion above ground, the shoot, and a portion below ground, the root, although this does not imply that any part of a plant below ground must be a root. The shoot is usually made up of a stem, bearing leaves, buds and tlowers.


   General characteristics. A stem has leaves at regular intervals and a terminal bud at the growing point. The region of the stem from which the leaf springs is called the node, and the length of stem between the nodes, the internode.

Commonly, the stem is erect, but it may be horizontal as in runners; underground as in rhizomes; very short and never showing above ground as in bulbs and corms; long, thin and weak as in climbing plants; or stout and thick as in trees. Young stems are usually green and contain chlorophyll.

The cells in young stems are living and obtain a supply of Oxygen from the air through openings, stomata or lenticels (described below), in their epidermis. Older stems are supported by woody and fibrous tissues which are added layer by layer, so increasing their thickness. Young stems depend for their rigidity on the turgidity of their cells, the cylindrical distribution of their conducting tissues and the opposing stresses of the pith and epidermis. Running through the stem are tubes which conduct water from the soil up to the leaves and food from the leaves to various parts of the plant.

   Functions of the stem. It (a) supports the structures of the shoot; (b) spaces out the leaves so that they receive adequate air and sunlight; (c) allows conduction of water from soil to leaves, and food from leaves to other parts of the plant; (d) holds flowers above ground, thus assisting pollination by insects or wind. (e) If the stem is green, photosynthesis may occur 1n it.

    Detailed structure A fairly typical stem, such as that of a sunflower, is in the form of a cylinder. The outer layer of cells forms a skin, the epidermis, the inner cells make up the cortex and pith. Between the cortex and pith lie a number of vascular bundles containing specialized cells which carry food and water.

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    EPIDERMIS. The single layer of closely fitting cells is effective in holding the inner cells in shape, preventing loss of water, affording protection from damage and preventing the entry of fungi, bacteria and dust. This layer is relatively impermeable to liquids and gases, and oxygen can enter, and carbon dioxide scale, only through stomata (described below) in young stems. The epidermis is usually in a state of strain, in which it tends to shrink along its length. This shrinking effect contributes to the rigidity of the stem.

     CORTEX AND PITH. These are tissues consisting of fairly large, thin-walled cells with air spaces between them. This a space system is continuous throughout the living tissues and allows air to circulate from the stomata or lenticels to all Iiving regions of the stem. The cortex and pith contribute to the rigidity of the stem by pressing out against the epidermis and by tending to increase in length against the shrinking tendency or the epidermis. These tissues also space out the vascular bundies and have a general value as packing. Many stems, however, are hollow with only a narrow band of pith within the cortex.

   VASCULAR BUNDLES, Sometimes called veins, are made up of vessels and sieve tubes, with fibrous and packing tissue between and around them.

    Vessels consists of long tubes a metre or so in length. They are formed from columns of cells whose walls have become impregnated with a woody substance and whose protoplasm has died. The horizontal cross-walls of these cells have broken down, thus forming a long continuous tube. In these vessels water is carried from the roots, through the stem and to the veins in the leaves.  Sieve tubes are formed from columns of living cells the horizontal walls of which are perforated. These perforations allow dissolved substances to flow from one cell to the next, so carrying food made in the leaves to other parts of the plant, e.g. to the ripening fruits, growing points or underground storage organs, according to the species of plant and the time of year.

Vessels and sieve tubes are surrounded by cells that space them out and support them. The tissue, consisting of vessels and the long fibre-like cells among them, is called xylem. The sieve tubes and their packing cells are called phloem.

   CAMBIUM. Between the xylem and the phloem is a layer of narrow, thin-walled cells called cambium.

   Once cells have been formed from the growing point and have grown to their full extent, they are no longer capable of dividing to make new cells. They may have become changed in structure and specialized to a particular function, as has, for example, a sieve tube. The cells in the cambium, however, do not lose their ability to divide and are able to multiply and make new cells.

Although at first the cambium is restricted to the vascular bundles, it later forms a continuous cylinder within the stem between the cortex and pith. Its cells divide in such a way as to make new xylem cells on the inside and new phloem cells on the outside. In woody plants like trees, this continues throughout their life-time, and as the cambium continues to divide and add new cells the stem increases in thickness, a process called secondary thickening. In such woody stems the epidermis is often replaced by a dead, corky layer, bark, which itself is made by a separate layer of cork cambium just beneath the epidermis.

The phloem becomes a thin layer oI living cells between the bark and the woody core of xylem.

Strength of stems. Vertical stems are likely to experience sideways forces when the wind blows against them. The turgor of the cells, the opposing forces of the epidermis tending to shrink, and the pith tending to extend, all contribute to the stem's resilience. The vascular bundies usually contain the toughest structures in the stem, the woody vessels and, often, long stringy fibrous cells running alongside them. When the vascular bundles are arranged in a cylinder near the outside of the stem they add to its strength, a cylindrical structure being much more resistant to bending than a solid structure of the same weight. In many other stems the strengthening tissue is distributed in such a way as to make the stem resistant to bending stresses; for example, the ""square-sectioned stem of the Labiatae family with the vascular bundles and strengthening strands of cells in the corners.


General structure. A typical leaf is a flat, green lamina or blade made from a soft tissue of thin-walled cells, supported by a stronger network of veins. Leaves are sometimes joined to the stem by a stalk, petiole, which continues into its midrib (or the main vein). Sometimes there is no leaf stalk.

 The EPIDERMIS is a single layer of cells fitting closely together with no air spaces between them except at the stomata. The epidermis may secrete a continuous waxy layer, cuticle, which reduces evaporation.

The epidermis helps to maintain the shape of the leaf, protects the inner cells from bacteria, fungi and mechanical damage, and reduces evaporation. The epidermal cells, except the guard cells (see below) of the stomata, do not usually contain chloroplasts and are transparent. In consequence, sunlight can pass through to the cells below, which do contain chloroplasts.

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PALISADE LAY ERS. In the one or more rows of tall cylindrical cells, with narrow air spaces between them, which comprise the palisade layer, most of the photosynthesis (carbohydrate formation) occurs. There are many chloroplasts in the cytoplasm lining the walls. Lying immediately below the epidermis, the palisade cells receive and absorb most of the sunlight. The chloroplasts arranged along the side walls are not far from the supplies of carbon dioxide in the air spaces, and they can move up or down the cell according to the intensity or the sunlight. The elongated cells result in very little sunlight being absorbed by horizontal cross-walls before it reaches the chloroplasts.

CHLOROPLASTS are small, often discoid (discus-like) bodies made of protein. They contain chlorophyl, the green pigment which gives green plants their characteristic colour, and which can absorb energy from sunlight and use it in the chemical build-up of sugars and starch. This chemical activity is believed to take place in the chloroplast when it is receiving light.

SPONGY LAYER. The cells in this region do not fit closely together, and large air spaces are left between them. The air spaces communicate with each other and, through the stomata, with the atmosphere, thus allowing air to circulate in them and reach most of the internal cells of the leaf. The cells of the spongy layer can photosynthesize, but they receive less sunlight than do the palisade cells, and contain fewer chloroplasts. The palisade and spongy layers are known collectively as mesophyll.

MIDRIB AND EINS support the leaf, conducting water into 1t and food away from it. They contain vascular bundles surrounded by other fibrous and strengthening cells. Each cell of the leaf is not supplied with its own vein, but the network of veins is very fine, and water has to pass from a vein through only a few cells to reach, say, a palisade cell.

STOMATA. Usually more abundant on the lower Side of the leaf, stomata are openings in the epidermis. They are formed between two guard cells which, according to their internal pressure, or turgor, can increase or reduce the size of the stoma or close it completely. The conditions which affect the opening or closure of the stomata are thought to be connected principally with light intensity and, in some cases, with the loss of water. The mechanism by which they open is a chain of events leading to an increase in the concentration of sugars in the cell sap in the vacuoles of the guard cells. When this happens, the osmotie potential  of the cell sap falls and water enters the guard cells from their neighbours. This nereases the turgor pressure in the guard cell, which tends to swell. The wall of the cell is thickest along its inner border so that it does not readily stretch. The stretching of the outer walls, however, causes the guard cells to curve away from each other and so increases the gap between them.

    Functions of leaves. The important function of leaves is to make food, in the form of carbohydrates, by photosynthests.

The water necessary for this process is conveyed through the vessels which run in the vascular bundles branching from the stem, and through the petiole and midrib, dividing repeatedly to form a network of tiny veins throughout the lamina. In addition, for photosynthesis, the leaf needs a supply of carbon dioxide from the air. This diffuses in through the stomata in one or both of its surfaces. For respiration, all living cells need its  Supply of oxygen, which also enters through the stomata.

The broad, flat shape of the leaf presents a large surface area to the are, facilitating rapid absorption of oxygen and carbon dioxide and allowing the maximum sunlight to fall on its exposed surface. Most leaves are thin in section and, in consequence, the distance through which the gases have to diffuse, from the atmosphere to the cells inside, is small, and gaseous exchange can be fairly rapid. The permeability to gases and the large surface area of the leaf are also characteristics which encourage rapid evaporation of water vapour.


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