Plant Life Structure Function

Overview 300 million years ago, the earth’s landscape contained a variety of plants; mosses, ferns, as well as cone-bearing and flower-bearing plants. Today, there are more than 250,000 different plant species- all of which have evolved from green algae. Plants have evolved specialized physical and chemical adaptations that enable them to thrive in various environments.

There are two classifications of plants: I. Nonvascular (Nonconductors) A. Bryophytes Mosses, liverworts, and hornworts. Do not posses true leaves, roots, and systems. Have leaf like and stem like structures.

*Nonvascular Bryophytes Don’t posses true leaves Usually covered with a waxy cuticle to prevent water loss. Root like structures called rhizoids that anchor them to the soil. Water is absorbed through epidermal cells. Small, live by water. *Nonvascular Bryophytes Don’t posses true leaves

II. Vascular (Conductors) B. Tracheophytes Comes from the word tracheids, which are thick walled, tubelike cells that make up the water-conducting tissue called xylem. Most familiar and common plants are vascular. Possess true leaves, roots, and stems. Ferns and all seed-bearing plants. Seed-bearing plants are classified either gymnosperms or angiosperms. Gymnosperms are cone-bearing plants (confiers) such as pines, firs and spruces.

*Vascular Tracheophytes True Leaves Angiosperms are flowering plants such as apple trees and rose shrubs- all angiosperms have true roots, stems, leaves, and well-developed vascular systems. *Vascular Tracheophytes True Leaves Flowering plants are divided into two groups: monocots and dicots. Monocot: plant whose embryo has one cotyledon, or seed leaf. Dicot: plants have embryos with two cotyledons.

C. Leaves- Structure Leaves possess a flat, broad blade and a narrow stalk or petiole,which connects the blade to the stem. Tissue within the petiole extends into the blade as the veins of a leaf. That tissue extends into the stem of the plant, where it joins the conducting tissue of the stem. Epidermal layers of flat cells are on top and bottom of the leaves. In the epidermal layers contains a layer of chloroplast-bearing cells called the mesophyll or chlorenchyma. The upper epidermis of most leaves is covered with a then coat of wax called a cuticle. The cuticle prevents the evaporation of water and the blockage of disease-causing organisms.

The lower epidermis has tiny openings called stomates The lower epidermis has tiny openings called stomates. Each stomate lies between two guard cells that contain chloroplasts. Guard cells open and close stomates, regulating the exchange of gases between the leaf and the atmosphere, and the evaporation of water from inside the leaf . Guard cells swell because of an increase in turgor pressure, or internal pressure. During photosynthesis the carbon dioxide level within a leaf is low. The low level of carbon dioxide triggers potassium ions into the cells.

Potassium enters guard cells. Water then enters guard cells. Turgor pressure causes stomates to open.

Veins (conducting tissue) consists of xylem and phloem tubes. The mesophyll is composed of the palisade layer, spongy layer, and veins. Palisade layer, boxlike cells, the location and arrangement of the cells enable them to receive maximum light. Because they contain many chloroplasts and absorb light, most photosynthesis occurs here. The spongy layer, located under the palisade layer, is loose rounded cells. Air spaces store gases, released by leaf cells and that enter the leaf from the atmosphere. Veins (conducting tissue) consists of xylem and phloem tubes. Xylem transports water and dissolved minerals to leaf cells from the stem and roots. Phloem transports materials manufactured in leaf cells to the stem and roots.

D. Functions Leaves regulate the water content of a plant by eliminated excess water and by conserving water. It is also a chemical factory that changes inorganic substances to organic substances by photosynthesis. Photosynthesis is when chloroplasts absorb light energy, which is used to combine carbon dioxide and water to synthesize carbohydrates. Oxygen is a by-product and is either released into the air through the stomates, remains in the air spaces, or is used by a plant’s cells during respiration.

Transpiration is the escape of water vapor from the stomates. Respiration, in daylight, during photosynthesis the plant produces more oxygen than its cells can use, and takes in more carbon dioxide than its cells produce in respiration. At dawn, carbon dioxide is first used to carry on photosynthesis during the night, this reduces CO2 in the air spaces and creates more oxygen inside the leaf and the oxygen in the air outside the leaf, resulting in the diffusion of oxygen out of the leaf into the air. Transpiration is the escape of water vapor from the stomates.