B. Organization of Plant Tissues into an Organ
Plant tissues are organized to form three different vegetative plant organs: the root, the stem, and the leaves. In this laboratory, you will study the stem. The stem is an above ground part of the plant. It has three functions: 1) provides mechanical support for the leaves, flowers and fruits; 2) furnishes a path of conduction between these organs and the roots; 3) acts as a storage organ for water and products of photosynthesis (to a limited extent, depending on the plant).
The internal anatomy of the stem is well suited to its function. The stem is covered by a single layer of cells, known as epidermis (Figures 1 and 2). These epidermal cells have thin inner walls, while the outer walls, which are exposed to air, are thicker and often covered with a cuticle (a waxy substance to prevent water loss). The epidermis may also develop spines for protection, glandular extensions to attract insects or hairs to reduce the rate of transpiration. In addition, stems possessing photosynthetic tissue (i.e they are green) have stomata in their epidermis. These are openings which facilitate gas exchange and limit water loss.
Stems are characterized by having a highly developed vascular system which is critical in their transport and support functions. Stems also possess varying amounts of fundamental tissues (parenchyma, sclerenchyma, etc.) to perform the storage and support functions
The angiosperms or flowering plants are subdivided into two broad classes -- the Dicotyledonae and the Monocotyledonae. They are often referred to as dicots and monocots respectively. Dicots are flowering plants with two cotyledons (seed leaves) and monocots are flowering plants with only one cotyledon. The stems of these two classes of plants differ in a number of structural characteristics. You will examine the stems of plants from both of these groups. Be sure to note their anatomical differences.