The Stem

Lab -6- The Stem
The stems of a plant represent elongate axes composed of nodes and internodes. Stems function as the principal supporting organ of the plant body, holding up and separating the leaves to increase their exposure to sunlight for photosynthesis, supporting the flowers for pollination, and holding up the fruits to increase their chances for effective dispersal. Stems contain vascular tissue for the conduction of materials throughout the plant body, and they produce new living tissue at apical meristems. Stems also can be modified for the storage of water or food,
Modifications of the Stem
In most plants, the stem is an upright structure that provides support for other parts of the plant body. Various types of stem modifications occur, however. You should be familiar with the following kinds of modified stems:
Stolons. Stems that run horizontally above ground are termed stolons or runners.
Rhizomes. Stems that run horizontally below the ground surface are termed rhizomes.
Tubers. The enlarged, fleshy tips of certain rhizomes (such as those of the Irish potato) are modified for food storage and are temed tubers.
Bulbs. These are unusual below ground structures that consist of a shortened, upright stem with modified, fleshy leaves attached to it.
Herbaceous Dicot Stems
The stems of most herbaceous dicots are composed of several different kinds of simple and complex tissues and exhibit a very characteristic structure when viewed in cross section. You should also be able to identify (and be familiar with the functions of) the following parts of the stem:
Epidermis. The outer layer of cells surrounding the stem.
Cortex. The cells lying between the epidermis and the vascular tissues constitute the cortex of the stem. Most of the cortical cells are parenchyma, but there may be one or more layers of collenchyma lying immediately beneath the epidermis.
Pith. The central portion of the stem is filled with parenchyma cells that have very distinct intercellular spaces; this region is collectively known as the pith.
Vascular Bundles. Separating the pith from the cortex is a ring-shaped arrangement of structures known as vascular bundles. Each bundle represents a longitudinal strand running the length of the stem and containing two different kinds of complex tissues, the vascular, which are responsible for the transport of materials throughout the plant body.
Xylem. A complex tissue composed of parenchyma cells, sclerenchyma fibers, and two kinds of specialized water-transport cells, tracheids and vessel elements.