Leaves

1. A usually green, flattened, lateral structure attached to a stem and functioning as a principal organ of photosynthesis and transpiration in most plants.

2. A leaflike organ or structure.

         1. Leaves considered as a group; foliage.

         2. The state or time of having or showing leaves: trees in full leaf.

3. The leaves of a plant used or processed for a specific purpose: large supplies of tobacco leaf.

4. Any of the sheets of paper bound in a book, each side of which constitutes a page.

         1. A very thin sheet of material, especially metal.

         2. Such leaves considered as a group: covered in gold leaf.

5. A hinged or removable section for a table top.

6. A hinged or otherwise movable section of a folding door, shutter, or gate.

7. One of several metal strips forming a leaf spring

The foliage leaf is the chief photosynthetic organ of most vascular plants. Although leaves vary greatly in size and form, they share the same basic organization of internal tissues and have similar developmental pathways. Like the stem and root, leaves consist of three basic tissue systems: thedermal tissue system, the vascular tissue system, and the ground tissue system. However, unlike stems and roots which usually have radial symmetry, the leaf blade usually shows dorsiventral symmetry, with vascular and other tissues being arranged in a flat plane.

Stems and roots have apical meristems and are thus characterized by indeterminate growth; leaves lack apical meristems, and therefore have determinate growth. Because leaves are more or lessephemeral organs and do not function in the structural support of the plant, they usually lack secondary growth and are composed largely of primary tissue only. See also Apical meristem; Root (botany); Stem.

The internal organization of the leaf is well adapted for its major functions of photosynthesis, gas exchange, and transpiration. The photosynthetic cells, or chlorenchyma tissue, are normally arranged in horizontal layers, which facilitates maximum interception of the Sun’s radiation. The vascular tissues form an extensive network throughout the leaf so that no photosynthetic cell is far from a source of water, and carbohydrates produced by the chlorenchyma cells need travel only a short distance to reach the phloem in order to be transported out of the leaf (Fig. 7). The epidermal tissue forms a continuous covering over the leaf so that undue water loss is reduced, while at the same time the exchange of carbon dioxide and oxygen is controlled.

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