Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section One:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8

Biology: Plant Organs, Tissues, and Cells: Part Six Epidermal Cells The epidermal tissue functions in the prevention of water loss. It also acts as a barrier to fungi and other invaders. Epidermal cells are closely packed, with little intercellular space, and serve a function much like that of human skin. To further cut down on water loss, many plants have a waxy cuticle layer deposited on top of the epidermal cells. The thickness of the cuticle varies with the plant species. Elodea (a common aquarium plant often observed in beginning biology labs) has no appreciable cuticle. Since the aquarium plant has no need to conserve water, it has little need for a cuticle. In contrast, the Indian rubber plant has a thick cuticle. It grows in hot areas with water and invading organisms. A series of openings, known as stomata (singular stoma), allow a plant to exchange gases with its surroundings. The stoma is an opening through the cuticle and epidermal cells that connects the inner parts of leaves, stems, and fruits to the plant’s external environment. The term stoma is derived from the Greek word for “hole.” Oxygen, carbon dioxide, and water vapor (gaseous water) pass through this hole by the process of simple diffusion from areas of higher concentration to areas of lower concentration. The stomata allow gas exchange, but they do so at a cost of water loss. Guard cells are bean-shaped cells that cover and regulate the size of the stoma. Guard cells and epidermal cells. Note the chloroplasts inside the guard cells. Scanning electron micrograph of an oblique view of epidermis and cortex cells of Juglans nigra.  Note the close packed epidermal cells on the outside of the stem. Scanning electron microscope "hi-mag" (magnification) image of Helianthus annus upper leaf surface, showing trichomes and a stomate. Some plant leaves have extensions from their upper epidermal surfaces. These tiny extensions are known as trichomes, or leaf hairs. Trichomes may serve many purposes, including shading the plant leaf from intense sunlight and defending it against an animal that might want to eat the plant’s leaves. Many desert plants, such as the brittle bush, have a veritable forest of trichomes that shade the small leaves of these hardy desert plants. The stinging nettle has trichomes containing chemicals that cause pain to animals that eat its leaves. Scanning electron microscope image of Juglans nigra stem, showing part of the cross section and a variety of tricomes. Now go on to the next part.

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