Leaves
...ary considerably and can be long, short, rounded or flat. Some leaves have no petioles in which case they are said to be sessile. At the base of the petiole in many leaves are small leaf-like structures called stipules e.g. in peas, beans and roses. Between the petiole and the stem is a bud of a potential branch (an axial bud). Leaves may be arranged on the stem in a variety of ways. The place on the stem from where the leaves grow is called a node and the part between the nodes is the internodes. If only one leaf arises at a node the leaves are said to be alternate, if there are two leaves they are opposite and if there are more than two they are whorled. A leaf may have one or more blades. If a leaf has only one blade. A leaf with more than one blade. Few plants have double compound leaves, in which each leafier in further divided into still smaller leafier. Inside A Plant In the leaves of most trees, three distinct tissue layers can be discerned in their leaves. These are: 1. The epidermis with its cuticle and stomata 2. The mesophyll where most of the chloroplasts are found and photosynthesis takes place 3. The leaf veins which transport water and inorganic compounds into the leaf, and organic compounds produced by photosynthesis away from the leaf, to other parts of the plant. I. Leaf Structure and Anatomy A. Structural Features of the Leaf Question: How do plants respire? Plants need to take in CO2 from the atmosphere in order to photosynthesize. How does the CO2 get into the leaf? Answer: Leaves have pores called stomata on the epidermal layer of the leaf. Stomata are the openings through which plants respire. The stomata are flanked by two guard cells, which control the size of stomata openings. Guard cells thus regulate the flow of gas and water between the leaf and its environment. Plants typically close their stomata at night to avoid too much water loss. Stomata are usually found on the underside of the leaf in terrestrial plants. Some floating aquatic plants, like water lilies, have the stomata located on the upper side of the leaf. Submerged aquatic plants do not have stomata. B. Internal Leaf Structure Even though leaves are very thin to the naked eye, if you looked at a cross section of a leaf under a microscope you would see several cell layers. The top most layer of a leaf is called the upper epidermis. This protects the leaf and may be covered by a waxy cuticle. The next layer is the palisade mesophyll, which is a layer of closely packed cells that perform photosynthesis. The third layer is the spongy mesophyll, a layer of loosely packed photosynthetic cells. Finally, the bottom layer is called the lower epidermis and contains the guard cells with stomata openin...