Plant Stems and Their Structure
The downward growth of a plant’s primary root from its seed is swiftly followed by the upward growth of its primary shoot — the stem or plumule. Both grow from a common beginning — the same single cell — but what causes their subsequent growth to differ is an unsolved riddle. Although in structure and growth they have much in common, the embryo stem grows to the light as eagerly as the embryo root grows down into the soil.
When a seedling stem makes its way above ground it is white or yellowish with a tapering budding tip. The seed leaf or leaves (cotyledons) may remain below ground, as in the case of the broad bean andseedlings, or be carried above ground on the stem, as with runner bean and onion seedlings, but the true leaves develop quickly.
Once exposed to light, the young stem tissues quickly turn green, as the light energizes within them the formation of a green pigment, chlorophyll. From this moment, the new plant can synthesize its own food, and feed its growing tissues. Roots do not have chlorophyll.
The stem has two chief functions: to bear and support the leaves and flowers where they can best play their part in food manufacture and reproduction, and to provide thethrough which the water solution absorbed by the roots may be carried to the leaves, and the food then produced in the leaves circulated to all other parts of the plant.
Although stems vary considerably in form, shape and structure, from the tall trunks of trees to the thin basal plates of undergroundor the squat crowns of some , they possess features in common.
A broad distinction is, however, made between the stems of dicotyledonous and monocotyledonous plants.
In dicotyledons, which include many flowering plants, hardwood trees and shrubs, the growing point (or meristem) of the stem consists of closely packed cells with thin walls. These cells elongate and develop into different kinds of cells with different jobs to do. The outer cells divide to add new cells to the outer covering of the stem, the inner ones to form the tissue of the stem, while at the tip cells form the embryo buds. As the plant develops, these tip buds form leaves and bend to one side, while the stem continues to elongate beyond. At each point where a leaf is formed a tiny group of cells within the axil (the angle formed by the leaf and the stem) do not elongate but remain to form another bud or buds. They behave like the original growing point of the main stem. In some plants they form lateral stems or branches, in others they remain as dormant buds for a season or more, or they may form leaves or flowers.
The point at which a leaf joins a stem is called a node, and the length of stem between one node and the next is called an internode.
Meanwhile, the stem itself builds up its substance. A cross section of the stem shows cells in concentric rings. On the outside is a single layer of cells with their outer walls covered with a tough varnish-like cuticle, impervious to moisture. This is the skin or epidermis. In perennial stems such as those of trees, this outer layer becomes one of bark or cork which consists of dead cells formed and pushed out by a layer of active cells behind it, called the cork cambium.
Within the epidermis is a broader ring of softer cells with large cavities inside them. This layer is called the cortex, and in it are the most important structures of the stem, the vascular bundles. These are long, tough strands of cells which run from the roots to the leaves. Each vascular bundle is made up of different kinds of cells which run alongside each other. Towards the inside of the vascular bundle are the long cells with firm woody walls making up the tissue known as xylem (wood). These cells have lost their protoplasm but are very permeable to water and, apart from strengthening the stem, are the tubes through which the water solution of salts absorbed by the roots moves to connecting cells in the leaf veins.
On the outer side of a vascular bundle is a ring of long cells, called phloem. Their chief function is to convey the sugars and other foods manufactured in the leaves to all other parts of the plant. These cells contain protoplasm and are soft, and the walls between them are full of holes. Hence the older name for phloem, sieve cells.
Between the xylem and phloem is a ring of narrow young cells called the cambium. As these cells divide and mature, those on the inner side of the ring change to xylem (woody) cells and those on the outer side to phloem (softer) cells. In persistent stems such as branches and trunks of trees the formation of xylem pushes the cambium and phloem farther out and the stems thus grow thicker.
The xylem cells formed in spring and early summer, when growth throughout the plant is most active, are larger than those of late summer and autumn. The result is a ring of annual growth that can be seen when the stem is cut across. The width of the growth ring reflects the growing conditions of the year in which it was formed, and by counting these rings the age of a tree can be assessed.
In the centre of a young stem is a mass of large, thin-walled, soft cells, called the pith, that help in water conduction. Later these cells die or dry out to form a light dry mass such as is found at the centre of a willow stem, or they disappear to leave a hollow centre as in the delphinium, or harden as at the heart of some tree trunks. Connecting the pith with the cortex are thin sections of similar cells running between the vascular bundles and called medullary rays. Collectively, these are known as parenchyma, and are sometimes called ground tissue. They hold water and food reserves, and help to keep the plant turgid and firm.
The stems of monocotyledonous plants are less complicated. The vascular bundles are scattered throughout the stem among pith and ground tissue cells and do not form a ring as they do in the dicotyledonous stems. There is no cambium, so the stems do not enlarge or become stronger each year. Consequently, most monocotyledonous plants are herbaceous; that is, the parts above ground die down each year. They include theand cereals, sedges and rushes, , and other bulbs.
Modifications in Stems
The purpose of stems is to carry the leaves and flowers into the air and light, and therefore the stems of most plants grow erectly and have strong woody cells to make them self-supporting. But in others the stems are modified for climbing. These stems grow through other plants or up supports, either by twining as in the honeysuckle, by tendrils as in the pea, by adhering by suckers or aerial roots as in the ivy, or by hooking by prickles as in the. Such climbing stems are very supple, devote less energy to forming rigid woody tissue and so grow more quickly.
In certain plants the stem is modified and grows underground. The tubers of the potato are really modified stems. Other plants with underground stems, known as rhizomes, areof the valley and .
At the base of a plant the stem merges into roots, the channels of xylem and phloem are continuous, and the root structure is made up of similar cells. These cells are, however, differently arranged. Vascular bundles are concentrated at the core of the root, with the xylem strands alternating with the phloem. In older roots a cambium forms between the two tissues, thus enabling the root to grow and enlarge. The core is surrounded by ground tissue and then an outer skin through which branch roots may grow or, at the root ends, the root hairs. The roots of monocotyledonous plants differ from those of dicotyledonous plants in that they have a little pith at the centre.
In the stalks and veins of leaves the construction and arrangement of cells are similar to those of the roots.