[急急急] 聽日考試:: 中三BIO Structure of a leaf [[[15分]]]

Structure of a leaf: chlorophyll containing outgrowth of a stem or branch; the site of photosynthesis.
Blade (lamina): main part of the leaf.
Vein: line dividing, but not completely, the leaf into sections.
Midrib: line dividing the leaf in two.
Leafstalk (petiole): part of the leaf that is attached to the stem.
Stem: part of the plant that carries the leaves.
Sheath: part of the leaf that attaches the petiole to the stem.
Stipules: small appendage at the base of the petiole.

Leaves are a plant's main photosynthetic organs. Leaf structure is closely associated with its photosynthetic function. Leaves must permit carbon dioxide access to the photosynthetic cells but impede water from diffusing out. The oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf. Demonstrate your knowledge of leaf structure by placing the labels in their correct locations.

1 The most numerous parts on most plants are their leaves. Leaves are considered to be a plant organ. An organ is a group of tissues that performs a specialized task. Leaves capture the sun's energy and carry out the food-making process of photosynthesis.

2 Plant leaves come in all sizes and shapes. Pine trees, for example, have needle-shaped leaves. Yellow skunk cabbage, a plant that grows in the northwestern United States, has oval leaves that can be more than one meter wide. No matter what their size or shape, leaves play an important role in a plant.

3 If you were to cut through a leaf and look at the edge under a microscope, you would see different structures. Leaves have three main parts. They are the epidermis, the mesophyll, and the vascular tissue.

4 The epidermis is the outer layers of cells covering the leaf. People also have an epidermis: their skin. The epidermis is transparent (not green). There is a waxy, waterproof coating covering the surface of the epidermis. This covering is called the cuticle. The cuticle is usually thicker on the upper surface of the leaf than on the underside. Cuticles on leaves are usually thicker in dry or windy climates than in wet or calm environments. For example, plants that grow near the ocean often have thicker cuticles to keep the ocean breezes from drying the plant out.

5 The epidermis has small openings or pores called stomata (singular stoma). The Greek word stoma means "mouth," and stomata do look like small mouths. The stomata have guard cells that control their opening and closing. The stomata open and close to control when gases enter and leave the leaf. When the stomata are open, carbon dioxide enters the leaf, and oxygen and water vapor exit. There are usually more stomata on the underside of a leaf than on the upper surface.