BOTANY 101 � BASIC PLANT STRUCTURE
By Wayne L. Handlos, Ph.D.
The typical angiosperm (seed bearing plant) consists of three parts: namely - stems, roots and leaves.
STEM. The stem forms the main axis of the plant. It carries the leaves and is anchored to the ground by the root. If the stem is strong and woody, as in trees, we would call it the trunk. The principle function of the stem is to support the leaves, flowers and fruits. If the stem is elongate, it distributes the leaves through space. The place on a stem where a leaf is attached is called a node (white arrow) and the section of stem between nodes is called the internode (yellow arrow). In some species (Pelargonium grossularioides, P. aciculatum), the stem is very short (sometimes described as �absent�), in which case the leaves grow close to the ground. In species like P. gibbosum the stem is swollen at intervals giving the common name of �gouty geranium.� Many of the species which grow in seasonally dry or desert areas have succulent or semi-succulent stems. Such stems store water internally. Some species (P. echinatum) have spiny stems with prickly, hardened projections from the surface. Such adaptations discourage animal grazers from eating the plants.
^ Nodes and internodes of stems ^
< Pelargonium leaf surface (Scanning Electron Microscope)
LEAF. The most conspicuous organs of plants in the geranium family (Geraniaceae) are the leaves. Leaves are the green food-making parts of the plant. The leaves consist of two major parts: the petiole and the blade or lamina. The petiole is the stalk at the base of the leaf where the leaf is attached to the stem or node. The leaf blade is the thin, flat, wide portion of the leaf. Here is the greatest concentration of food-making structures in the plant. At each node, small flaps of tissue called stipules may be found. These are usually considered part of the leaf and vary in size, shape and structure from species to species. In some plants these stipules are hardened and spiny (P. echinatum). In others, they are large and green (especially in P. x hortorum). In many species of the Geraniaceae, the leaves persist from season to season and the plants are considered evergreen; but other species from desert areas shed their leaves seasonally and are called deciduous. These are terms we are familiar with; coniferous (cone-bearing) plants are generally evergreen, while many trees shed their leaves in the fall and are leafless during the winter. Many species of Pelargonium from the Mediterranean climate of South Africa are deciduous, but their leaves are green during the winter (the wet season) and are shed during the summer (the dry season). Leaves vary considerably in size and shape and are important in the identification of the various species of Geraniaceae.
ROOT. At the base of the stem, the root or roots penetrate the soil and anchor the aerial plant body to the earth. The roots also absorb water and minerals from the soil. Most species of Geraniaceae have simple, highly branched fibrous roots. A few species have thick, fleshy roots which may store food and water (P. sidoides) and a few species produce small, swollen storage roots which are called tuberous roots. Again, these store food and water but in a more compact form than fleshy roots. Occasionally there is a main central root (like a carrot) which forms a tap root (Erodium trifolium) Single celled projections from the surface of the roots are called root hairs. It is through these structures that most of the absorption of water and nutrients occur. These materials are then conducted to the rest of the plant.
INTERNAL STRUCTURE. Stem. Inside the stem are the tissues and structures which allow the stem to perform its various functions. The transport of water and minerals from the roots occurs primarily through the xylem. The xylem contains specialized tube-like cells (vessels and tracheids) through which the water and dissolved minerals move. A different group of specialized cells move food substances within the plant. This tissue is called the phloem and the primary transport cells are called sieve cells with small associated cells called companion cells. Frequently the movement of food is from the leaves to the roots. Together the xylem and phloem are called the vascular tissue. Between these two tissues may be found a layer of cells capable of dividing and producing more xylem and phloem. This is called the cambium.
The outer surface of the stem consists of a layer of cells called the epidermis. In the Geraniaceae the epidermis may bear various types of hairs. Between the phloem and the epidermis, the stem tissue is called the cortex. It may contain green photosynthetic cells, colorless storage cells and thick walled cells called fibers. The very center of the stem usually consists of simple, undifferentiated cells called the pith.
< Stem cross sections >
< Longitudinal section of Pelargonium stem
Leaves. Leaves usually have a distinctive upper and a lower surface. The outermost layer of cells of the leaf forms the epidermis. Various types of hairs or trichomes may project from the epidermis. Between the upper and lower epidermis, most of the plant�s green, chlorophyll bearing cells are found. This is where most of the photosynthesis occurs in the plant. Here water and carbon dioxide are combined to make sugars and release oxygen. This tissue is called mesophyll and may be further differentiated into palisade mesophyll where the cells are elongated and densely packed and spongy mesophyll where the cells are irregularly shaped with lots of spaces between them. The epidermis is perforated with openings through which gases enter and exit the leaf. These holes are called stomates. Two cells (called guard cells) regulate the opening and closing of the stomates and thereby control the movement of carbon dioxide, oxygen and water vapor into and out of the leaf. The veins of the leaf are pathways for food and water movement and contain the xylem and phloem which connect to this same vascular tissue in the stem.
Roots. From the epidermis of the root, root hairs develop which absorb water and dissolved minerals. These substances move to a central vascular strand composed of xylem and phloem in the root and are transported to the vascular tissue of the stem (and ultimately the leaves).