The bay tree (Laurus nobilis) seems to have ordinary leaves. Their structure is similar to the leaves of other trees and shrubs, and they execute similar chemical reactions. But if you crush a leaf, you will detect an aromatic smell, which shows that the bay leaf is special.
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A short stem called a petiole attaches the bay leaf to its tree. The leaf has a lanceolate (lance-shaped) form and a pinnate (feather-like) venation, with the principal vein running down the center of the leaf, from its petiole to its tip, while secondary veins branch off to either side. The margin (edge) is smooth but sometimes wavy.
Epidermis and Cuticle
The bay leaf has a single layer of cells covering its top and bottom. Because it serves as a sort of skin for the leaf, this outer covering is called the epidermis (from a Greek word meaning "skin"). For extra protection, these epidermal cells secrete a cuticle (a waxy substance that coats the outside of the leaf).
Within the two epidermal layers is the mesophyll (meaning "the middle of the leaf"). The mesophyll includes two sections: the palisade cells on top, and the spongy parenchyma on the bottom. The palisade cells are packed closely together right below the upper epidermis. They form a sort of fence or hedge, and that is why botanists call them a palisade. The spongy parenchyma, a loosely packed tissue, lies below the palisade cells. It contains ample space for air and water.
Movement of Materials
In the lower epidermis, stomata (tiny holes or pores) allow carbon dioxide and oxygen to enter and exit the leaf. Water also makes an exit from the leaf through these stomata (a process called transpiration). The leaf also has water-conducting tissue called xylem; and other tissue, called phloem, which transports carbohydrates out of the leaf, to be stored in other parts of the plant.
Chemical reactions take place in the cells of the bay leaf. Many leaf cells have chloroplasts, which give the leaf its green color. These chloroplasts are instrumental in a chemical reaction called photosynthesis. In photosynthesis, the chloroplasts capture the sun's energy and use it to convert carbon dioxide and water into glucose and oxygen. Further reactions convert glucose into starches. The cells also engage in the chemical reaction of respiration, which converts the starches and sugars back into carbon dioxide and water, while liberating energy, which the plant needs for such activities as cell division.
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