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 A number of rather distinct types of leaves occur on plants. Leaves commonly referred to as foliage are the most common and conspicuous, and as previously stated, serve as the manufacturing centers where the photosynthetic activity of the plant occurs. Scale leaves or cataphylls are found on rhizomes and are also the small, leathery, protective leaves which enclose and protect buds. Seed leaves, or cotyledons, are modified leaves which are found on the embryonic plant and commonly serve as storage organs. Spines and tendrils, as found on barberry and pea, are specialized modified leaves which protect the plant or assist in supporting the stems. Storage leaves, as are found in bulbous plants and succulents, serve as food storage organs. Other specialized leaves include bracts, which are often brightly colored. The showy structures on dogwoods and poinsettias are bracts, not petals.
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 The vascular bundles from the stem extend through the petiole and spread out into the blade. The term venation refers to the patterns in which the veins are distributed in the blade. Two principal types of venation are parallel-veined and net-veined.
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 Leaves are useful in identifying species and varieties of horticultural plants. The shape of the leaf blade and the type of margin are of major importance as identifying characteristics. Simpleleaves are those in which the leaf blade is a single continuous unit. A compound leaf is composed of several separate leaflets arising from the same petiole. A deeply lobed leaf may appear similar to a compound leaf, but if the leaflets are connected by narrow bands of blade tissue it may be classified as a simple leaf. If the leaflets have separate stalks and if these stalks are jointed at the point of union with the main leafstalk, the leaf is considered to be compound. Some leaves may be doubly compound, having divisions of the leaflets.
Shape of the Leaf Blade The following are some common shapes which are found in leaves and leaflets.
Narrow, several times longer than wide; approximately the same width.
2 or 3 times longer than wide; tapering to an acute or rounded apex and base.
Egg-shaped, basal portion wide; tapering toward the apex.
Longer than wide; tapering toward the apex and base.
Heart-shaped, broadly ovate; tapering to an acute apex, with the base turning in and forming a notch where the petiole is attached
Shape of the Leaf Apex and Base The following are common shapes found in leaves.
Tapering to a long, narrow point.
Ending in an acute angle, with a sharp, but not acuminate, point.
Tapering to a rounded edge.
Arrowhead-shaped, with two pointed lower lobes.
Having a relatively square end.
Leaf Margins Studying leaf margins is especially useful in the identification of certain varieties of fruit plants.
A smooth edge with no teeth or notches.
Having a pronounced sinuous or wavy margin.
Having rounded teeth.
Having teeth ending in an acute angle, pointing outward.
Having small, sharp teeth pointing toward the apex.
Margin cut into sharp, deep, irregular teeth or incisions.
Incisions extend less than halfway to the midrib.
Incisions extend more than halfway to the midrib.
Leaf Arrangement along a Stem The various ways leaves are arranged along a stem are also used to help identify plants.Rosulate arrangement is one in which the basal leaves form a rosette around the stem with extremely short nodes. Opposite leaves are positioned across the stem from each other, two leaves at each node. Alternate or spiral leaves are arranged in alternate steps along the stem with only one leaf at each node. Whorled leaves are arranged in circles along the stem.
Leaves as Food The leaf blade is the principal edible part of several horticultural crops including chive, collard, dandelion, endive, kale, leaf lettuce, mustard, parsley, spinach, and Swiss chard. The edible part of leek, onion, and Florence fennel is a cluster of fleshy leaf bases. The petiole of the leaf is the edible product in celery and rhubarb. In plants like Brussels sprouts, cabbage, and head lettuce, the leaves form a large, naked bud and are the edible product.