Roots

Often roots are overlooked, probably because they are less visible than the rest of the plant. However, it's important to understand plant root systems because they have a pronounced effect on a plant's size and vigor, method of propagation, adaptation to soil types, and response to cultural practices and irrigation.

Roots typically originate from the lower portion of a plant or cutting. They have a root cap, but lack nodes and never bear leaves or flowers directly. Their principal functions are to absorb nutrients and moisture, anchor the plant in the soil, support the stem, and store food. In some plants, they can be used for propagation.

Structure

Internally, there are three major parts of a root (Figure 2):

• The meristem is at the tip and manufactures new cells; it is an area of cell division and growth.
• Behind the meristem is the zone of elongation. In this area, cells increase in size through food and water absorption. As they grow, they push the root through the soil.
• The zone of maturation is directly beneath the stem. Here, cells become specific tissues such as epidermis, cortex, or vascular tissue.

A root's epidermis is its outermost layer of cells (Figure 3). These cells are responsible for absorbing water and minerals dissolved in water. Cortex cells are involved in moving water from the epidermis to the vascular tissue (xylem and phloem) and in storing food. Vascular tissue is located in the center of the root and conducts food and water.

Externally, there are two areas of importance: the root cap and the root hairs (Figure 2). The root cap is the root's outermost tip. It consists of cells that are sloughed off as the root grows through the soil. Its function is to protect the root meristem.

Root hairs are delicate, elongated epidermal cells that occur in a small zone just behind the root's growing tip. They generally appear as fine down to the naked eye. Their function is to increase the root's surface area and absorptive capacity. Root hairs usually live 1 or 2 days. When a plant is transplanted, they are easily torn off or may dry out in the sun.

Many roots have a naturally occurring symbiotic (mutually beneficial) relationship with certain fungi, which improves the plant's ability to absorb water and nutrients. This beneficial association is called mycorrhizae (fungus + root).

Stems

Stems support buds and leaves and serve as conduits for carrying water, minerals, and food. The vascular system inside the stem forms a continuous pathway from the root, through the stem, and finally to the leaves. It is through this system that water and food products move.

Leaves

The most obvious function of leaves is photosynthesis, but other functions often taken for granted are just as important. Leaves must not lose excessive amounts of water; they must not allow entry of fungi, bacteria, or other perpetrators; they must not be so nutritious and delicious to animals that they are a liability to the plant; they must not be such effective sails that the plant is blown over in a mild wind; and they must be cheap enough that the plant does not waste energy build them (leaves) rather than producing sugars for the rest of the plant.

The flat light harvesting portion of a leaf is the blade (also called the lamina). Most leaves are attached by a stalk called the petiole that holds the blade out into the light. The petiole allows the leaf to flutter in the wind which cools the leaf and helps prevent insects from landing. Leaves without a petiole are called sessile leaves. Leaves that are sessile tend to be tightly packed which helps prevent water loss by transpiration. A leaf blade may be either simple or compound. A simple leaf has a blade of just one part, whereas a compound leaf has a blade that is divided into several individual parts called leaflets. The leaflets are attached to an extension of the petiole, called a petiolule, by the rachis. Leaves may be palmately compound, with all leaflets attached at the same point, or pinnately compound, with leaflets attached individually along the rachis. Compound leaves have several advantages over simple leaves. When a mild breeze blows over two leaves of equal size, one simple and the other compound, it tends to flow smoothly over the simple leaf but turbulently over the complex leaf. Turbulence brings in carbon dioxide and removes excess heat. Also, an insect can crawl or a fungus can spread across the entire blade of a simple leaf, but cannot do so over the many leaflets of a compound leaf.

Within a leaf are veins or bundles of vascular tissue; in a dicot they occur in a netted pattern called reticulate venation. In monocots with long. strap-shaped leaves, the larger veins run side by side with few obvious interconnections: this is parallel venation. Veins distribute water from the stem into the leaf and simultaneously collect sugars produced by photosyntheisis and carry them to the stem for use or storage.

Look at the diagram of the internal anatomy of a leaf. Each structure has a particular function in the overall photosynthetic activities of the leaf. The leaf is covered with a waxy layer called the cuticle. The cuticle is important in helping to prevent transpiration (water loss). The outer layer of cells form the epidermis. Embedded in the epidermis are opening known as stomata. Stomata allow for air flow (oxygen and carbon dioxide) which is important for the process of photosynthesis, but stomata present a problem with transpiration. When the sun strikes the leaf blade, water is evaporated out of the leaf. So the opening and closing of the stomata is an important function of the stomata.