The blade of a leaf is the expanded, thin structure on either side of the midrib. The blade is usually the largest and most conspicuous part of a leaf. The petiole is the stalk which supports the leaf blade. It varies in length and may be lacking entirely in some cases where the leaf blade is described as sessile or stalkless.
The principal function of leaves is to absorb sunlight for the manufacturing of plant sugars in a process called photosynthesis. Leaves develop as a flattened surface in order to present a large area for efficient absorption of light energy. The leaf is supported away from the stem by a stem-like appendage called a petiole. The base of the petiole is attached to the stem at the node. The small angle formed between the petiole and the stem is called the leaf axil. An active or dormant bud or cluster of buds is usually located in the axil.
The leaf blade is composed of several layers. On the top and bottom is a layer of thickened, tough cells called the epidermis. The primary function of the epidermis is protection of leaf tissue. The way in which the cells in the epidermis are arranged determines the texture of the leaf surface. Some leaves have hairs that are an extension of certain cells of the epidermis. The African violet has so many hairs that the leaf feels like velvet.
Part of the epidermis is the cuticle, which is composed of a waxy substance called cutin that protects the leaf from dehydration and prevents penetration of some diseases. The amount of cutin is a direct response to sunlight, increasing with increasing light intensity. For this reason, plants grown in the shade should be moved into full sunlight gradually, over a period of a few weeks, to allow the cutin layer to increase and to protect the leaves from the shock of rapid water loss or sun scald. The waxy cutin also repels water and can shed pesticides if spreader-sticker agents or soaps are not used. This is the reason many pesticide manufacturers include some sort of spray additive to adhere to or penetrate the cuticle.
Some epidermal cells are capable of opening and closing. These cells guard the interior of the leaf and regulate the passage of water, oxygen, and carbon dioxide through the leaf. These regulatory cells are called guard cells. They protect openings in the leaf surface called stoma. The opening and closing of the cells are determined by the weather. Conditions that would cause large water losses from plants (high temperature, low humidity) stimulate guard cells to close. Mild weather conditions leave guard cells in an open condition. Guard cells will close in the absence of light. A large percentage of stomata occur in the lower epidermis.
The middle layer of the leaf is the mesophyll and is located between the upper and lower epidermis. This is the layer in which photosynthesis occurs. The mesophyll is divided into a dense upper layer, called the palisade layer, and a spongy lower layer that contains a great deal of air space, called the spongy mesophyll. The cells in these two layers contain chloroplasts which are the actual sites of the photosynthetic process.
Types of Leaves
Conifers, (pines, firs, spruce, laurel, etc.) have "needles" as leaves. They normally have waxy cuticles with sunken stomata to help deter desiccation. Also, most have resin canals on either side of the vascular system. The resin is thought to help deter and guard against insect damage.
Venation of Leaves
Parallel-veined leaves are those in which there are numerous veins which run essentially parallel to each other and are connected laterally by minute, straight veinlets. Possibly the most common type of parallel-veining is that found in plants of the grass family where the veins run from the base to the apex of the leaf. Another type of parallel-venation is found in plants such as banana, calla, and pickerelweed, where the parallel veins run laterally from the midrib. Parallel-veined leaves occur on plants which are part of the monocotyledon group.
Net-veined leaves, also called reticulate-veined, have veins which branch from the main midrib(s) and then subdivide into finer veinlets which then unite in a complicated network. This system of enmeshed veins gives the leaf more resistance to tearing than most parallel-veined leaves. Net-venation may be either pinnate or palmate. In pinnate venation, the veins extend laterally from the midrib to the edge, as in apple, cherry and peach. Palmate venation occurs in grape and maple leaves, where the principal veins extend outward, like the ribs of a fan, from the petiole near the base of the leaf blade. Net-veined leaves occur on plants which are part of the dicotyledon group.
Leaves as a Means of Identifying Plants
Shape of the Leaf Blade
Shape of the Leaf Apex and Base
Leaf Margins Studying leaf margins is especially useful in the identification of certain varieties of fruit plants.
Leaf Arrangement along a Stem
Leaves as Food