THE ESSENTIALS OF THE PLANT BODY

STEMS

A stem consists of nodes where the leaves are attached and internodes which are between the nodes. Usually stems get long in the internodal segments. In monocots (example corn), vascular bundles containing xylem and phloem are scattered throughout the ground tissue. In dicots (example oak tree) xylem and phloem are in a single ring of vascular bundles near the edges of the stem.

Special types of stems include the following: stolons - runners that spread the plant, such as Bermuda grass; thorns - for protection such as roses; succulents - store water such as cactus; tendrils - coil around objects to let a plant climb as in running beans; and tubers - swollen underground stems for storage such as Irish potatoes.

LEAVES

Leaves generally consist of a blade and a petiole and give a large surface area for photosynthesis. According to patterns on stems, leaves are alternate - one leaf per node (example oaks); opposite - two leaves per node (example maple); and whorled - three or more leaves per node (example horsetails).

There are two main types of leaves, simple and compound. Simple leaves have flat, undivided blades such as elm leaves; compounds leaves are divided into leaflets such as mimosa. Compound leaves are further classified into pinnate with leaflets paired along the central axis (example pecan leaves) and palmate with leaflets all attached to one point (example clover).

There is some importance in being aware of the general structure of a leaf under the microscope when viewed in cross-section. Look at the picture on page 545 in the text and we will also view a slide on this. Identify epidermis, palisade mesophyll, spongy mesophyll, air spaces, veins, and stomata.

ROOTS

Roots account for a substantial portion of the body of a plant. They anchor, absorp, transport, and store.

There are two types of root systems. A taproot system (example pine tree) grows deep and fast to anchor the plant and let it use materials deep in the soil. A fibrous root systemn (example grass) has many adventitious roots and they are shallow so as to absorb materials near the surface and prevent soil erosion.

The features of a growing root include the root cap which covers and protects the end of the root; the quiescent center where the cells seem to be inactive but actually is a reserve to replace damaged cells in the apical meristem (dividing cells); the zone of cell division is undergoing rapid mitosis; the zone of cell elongation where the cells grow rapidly in length and push the root forward through the soil; and the zone of cell differentation where root hairs develop from the epidermal cells to increase surface area for absorption.

Roots with special functions include storage roots (examples beets and carrots); pneumatophores which grow up from trees growing in stand water to absorb oxygen (example cypress "knees"); aerial roots that grown in the air (banyan tree and mistletoe are examples); buttress roots which support the base of a tree (example tropic fig trees); and prop roots that provide support from the stem (example corn).