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Leaf form

Leaves may be simple or compound ( [link] ). In a simple leaf    , the blade is either completely undivided—as in the banana leaf—or it has lobes, but the separation does not reach the midrib, as in the maple leaf. In a compound leaf    , the leaf blade is completely divided, forming leaflets, as in the locust tree. Each leaflet may have its own stalk, but is attached to the rachis. A palmately compound leaf    resembles the palm of a hand, with leaflets radiating outwards from one point Examples include the leaves of poison ivy, the buckeye tree, or the familiar houseplant Schefflera sp. (common name “umbrella plant”). Pinnately compound leaves take their name from their feather-like appearance; the leaflets are arranged along the midrib, as in rose leaves ( Rosa sp.), or the leaves of hickory, pecan, ash, or walnut trees.

Photo (a) shows the large-leaves of a potted banana plant growing from a single stem; (b) shows a horse chestnut plant, which has five leaves radiating from the petiole as fingers radiate from the palm of a hand; (c) shows a scrub hickory plant with feather-shaped leaves opposing each other along the stem, and a single leaf at the end of the stem. (d) shows a honey locust with five pairs of stem-like veins connected to the midrib. Tiny leaflets grow from the veins.
Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant ( Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut ( Aesculus hippocastanum ), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory ( Carya floridana ). The (d) honey locust has double compound leaves, in which leaflets branch from the veins. (credit a: modification of work by "BazzaDaRambler"/Flickr; credit b: modification of work by Roberto Verzo; credit c: modification of work by Eric Dion; credit d: modification of work by Valerie Lykes)

Leaf structure and function

The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata ( [link] ): openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing.

Photo (a) shows small oval-like stomata scattered on the bumpy surface of a leaf that is magnified 500 times; (b) is a close-up of a stoma showing the thick lip-like guard cells either side of an opening. Photo (a) and (b) are scanning electron micrographs. Photo (c) is a light micrograph of a leaf cross section that shows a large air space underneath two guard cells. The air space is surrounded by large oval and egg-shaped cells.
Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the surface of this sumac ( Rhus glabra ) leaf. At 5,000x magnification, the guard cells of (b) a single stoma from lyre-leaved sand cress ( Arabidopsis lyrata) have the appearance of lips that surround the opening. In this (c) light micrograph cross-section of an A. lyrata leaf, the guard cell pair is visible along with the large, sub-stomatal air space in the leaf. (credit: modification of work by Robert R. Wise; part c scale-bar data from Matt Russell)

The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the cuticle    covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface ( [link] ).

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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