YAPRAKLAR

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The principal function of leaves is to absorb sunlight to manufacture plant sugars through a process called photosynthesis. Leaf surfaces are flattened to present a large area for efficient light absorption. The blade is the expanded thin structure on either side of the midrib and usually is the largest, most conspicuous part of a leaf (Figure 11).

A leaf is held away from its stem by a stem-like appendage called a petiole (The stalk that attaches a leaf to a stem), and the base of the petiole is attached to the stem at a node(An area on a stem where a leaf, stem, or flower bud is located). Petioles vary in length or may be lacking entirely, in which case the leaf blade is described as sessile or stalkless.

The node where a petiole meets a stem is called a leaf axil. The axil contains single buds or bud clusters, referred to as axillary buds. They may be either active or dormant; under the right conditions, they will develop into stems or leaves.

A leaf blade is composed of several layers (Figure 12a and Figure 12b: click on images to display larger versions.). On the top and bottom is a layer of thick, tough cells called the epidermis(The outermost layer of cells on all the primary parts of a plant: stem, roots, leaves, flower, fruit, and seeds). Its primary function is to protect the other layers of leaf tissue. The arrangement of epidermal cells determines the leaf's surface texture. Some leaves, such as those of African violet, have hairs (pubescence), which are extensions of epidermal cells that make the leaves feel like velvet.

The cuticle(Part of the epidermis; produces cutin, a waxy substance that protects a leaf from dehydration and disease) is part of the epidermis. It produces a waxy layer called cutin((1) A waxy substance on plant surfaces that tends to make the surface waterproof and can protect leaves from dehydration and disease. (2) A waxy substance on an insect's cuticle that protects the insect from dehydration), which protects the leaf from dehydration and disease. The amount of cutin on a leaf increases with increasing light intensity. For this reason, when moving plants from shade into full sunlight, do so gradually over a period of a few weeks. This gradual exposure to sunlight allows the cutin layer to build up and protect the leaves from rapid water loss or sunscald.

The waxy cutin also repels water. For this reason, many pesticides contain a spray additive to help the product adhere to, or penetrate, the cutin layer.

Special epidermal cells called guard cells(Epidermal cells that open and close to let water, oxygen, and carbon dioxide pass through the stomata) open and close in response to environmental stimuli, such as changes in weather and light. They regulate the passage of water, oxygen, and carbon dioxide into and out of the leaf through tiny openings called stomata((singl. stoma) - Tiny openings in the epidermis that allow water, oxygen, and carbon dioxide to pass into and out of a plant). In most species, the majority of the stomata are located on the underside of leaves.

Conditions that would cause plants to lose a lot of water (high temperature, low humidity) stimulate guard cells to close. In mild weather, they remain open. Guard cells also close in the absence of light.

Located between the upper and lower epidermis is the mesophyll(The part of a leaf located between the upper and lower epidermis; contains the chloroplasts and other cell organelles). It is divided into a dense upper layer (palisade mesophyll) and a lower layer that contains lots of air space (spongy mesophyll). Located within the mesophyll cells are chloroplasts(A specialized component of certain cells; contains chlorophyll and is responsible for photosynthesis), where photosynthesis takes place.

Yaprak Tipleri

There are many kinds of plant leaves. The most common and conspicuous leaves are referred to as foliage and are the primary location of photosynthesis. However, there are many other types of modified leaves:

• Scale leaves (cataphylls) are found on rhizomes and buds, which they enclose and protect.
• Seed leaves (cotyledons=The first leaves that appears on a seedling. Also known as a seed leaf) are found on embryonic plants. They store food for the developing seedling.
• Spines and tendrils, such as those found on barberry and pea plants, protect a plant or help support its stems.
• Storage leaves, such as those on bulbous plants and succulents, store food.
• Bracts often are brightly colored. For example, the showy structures on dogwoods and poinsettias are bracts, not petals.

Damarlanma

The vascular bundles of xylem and phloem extend from the stem, through the petiole, and into the leaf blade as veins. The term venation refers to how veins are distributed in the blade. There are two principal types of venation: parallel-veined and net-veined (Figure 13).

In parallel-veined leaves, numerous veins run essentially parallel to each other and are connected laterally by minute, straight veinlets. Parallel-veined leaves occur most often on monocotyledonous plants. The most common type of parallel veining is found in plants of the grass family, whose veins run from the leaf's base to its apex. Another type of parallel venation is found in plants such as banana, calla, and pickerelweed, whose veins run laterally from the midrib.

In net-veined leaves (also called reticulate-veined), veins branch from the main rib or ribs and subdivide into finer veinlets. These veinlets then unite in a complicated network. This system of enmeshed veins makes the leaf more resistant to tearing than does a parallel vein structure. Net-veined leaves occur on dicotyledonous plants.

Net venation may be either pinnate or palmate. In pinnate venation, the veins extend laterally from the midrib to the edge (e.g., apples, cherries, and peaches). In palmate venation, the principal veins extend outward, like the ribs of a fan, from the base of the leaf blade (e.g., grapes and maples).

Yapraklar ve Bitkinin Tanımlanması Lanceolate - Longer than wide and tapering toward the apex and base. Linear - Narrow, several times longer than wide, and of approximately the same width throughout. Cordate (heart-shaped) - Broadly ovate, tapering to an acute apex, with the base turning in and forming a notch where the petiole is attached. Elliptical - About two or three times as long as wide, tapering to an acute or rounded apex and base. Ovate - Egg-shaped, basal portion wide, tapering toward the apex.

Entire - Having a smooth edge with no teeth or notches. Crenate - Having rounded teeth. Dentate - Having teeth ending in an acute angle pointing outward. Serrate - Having small, sharp teeth pointing toward the apex. Incised - Having a margin cut into sharp, deep, irregular teeth or incisions. Lobed - Having incisions that extend less than halfway to the midrib.

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