In computer science, a node in the lowest level of a tree data stucture is sometimes called a leaf.

Most nodes in a tree contain data and also refer to other nodes (via some kind of pointer). These nodes, in turn, point to still more nodes, etc, etc, creating a "tree." A leaf is a node, at the end of a path through the tree, that has no pointers to other data nodes (or, perhaps, has null pointers).

A leaf is a node that has no children and is the opposite of the root (the node that has no parent).

Note: the root can also be a leaf, but only in a tree of exactly one node.

A leaf can be generally defined as flattened extension from the stem of a plant whose primary purpose is photosynthesis and respiration. They are incredibly varied in size and shape, and can show some amazing adaptations (see Modifications).

General description and appearance

Despite their incredible diversity in appearance, can be loosely organized into three groups1, 2: the microphylls, the angiosperm leaves, and the sheath leaves. Microphylls are seen in relatively few species in nature, and are considered to be evolutionary relics. They are characterized by their extremely small size and lack of special form or differentiation. They are generally not photosynthetic. The most prevalent and common example are the leaves of Equisetum sp., the horsetails. Their leaves are little black triangles which lie around the circumference of the stem, and look like a pointy collar around each nodal joint.

The angiosperm leaf, on the other hand, is the classical leaf we all know and love. Examples include the maple, oak, chestnut etc. These leaves are generally quite large, vascularized and differentiated (their vascular organization is called netted veination, see Morphology and histology below). They attach to the stem by way of the petiole (the stalk-like portion of the leaf). The remainder of the leaf is called the blade, or lamina. In many cases, the leaves are not simple (as in the maple) but are instead compound, where many smaller blades are attached to the petiole (as in ash or hickory).

Finally, there are the sheath leaves of the monocotyledons. These leaves are flattened and simple in construction. They are generally shaped like very high triangles, and are far wider than they are thick. The veinous structures in these leaves lie nearly parallel to the long axis of the triangle, and do not cross (this pattern is called parallel veination, see Morphology and histology below). The sheath leaves generally join to the stalk not with a petiole, but surround the stalk like a scarf. Perhaps the easiest example of this leaf type is the corn, where the leaves surround the ear and overlap one another.

Several plant species have two distinct kinds of leaves. These different leaves are either produced by environmental or genetic factors. In many tree species, for example, the leave in the upper part of the crown are thicker and smaller, and are adapted to high light conditions (they are hence called sun leaves). Further down on the plant are the shade leaves, which are far larger and thinner, and have maximal photosynthetic performance under the low light conditions of the understory. These two leaf types are produced as a result of differing light conditions, and the control of their form is not purely genetic. In some aquatic plants, on the other hand, the two leaf morphologies are also observed and their formation is directly controlled by genetic factors. For example, the common bassweed (Potamogeton sp.) has small, paddle-shaped leaves which float along the surface while the leaves submerged under water are elongated and shaped like grass shoots.

Development

In almost all plants, the first leaves appear well after the stem and root systems have begun growth. They are produced by the shoot apex, the region of greatest cell production, at the tip of a stem. They appear, at first, to be no more than little buds growing off the stem of a plant (called leaf buttresses). As the stem continues to grow upwards, the buttress develops into the leaf primordium, which has a distinct peg-like appearance. Further development at this point is mostly in cell division, ageing and growth, called intercalary growth, rather than new cell development from the leaf meristem.

This general pattern does not always hold true, however. In the case of monocotyledon plants, such as the grasses (including corn and barley), the sheathing leaf base is developped quite early, and the initial growth occurs both laterally and longitunidally (meaning the leaf both wides and lengthens as it grows). As the leaf grows, is changes from a small collar-shaped protuberance, to a down-turned hood and finally the linear blade as in common grasses.

In most plant species common to temperate climates, the leaves are shed as the climate turns inhospitable in the winter months. The process of shedding leaves is called abscission, and is accomplished by structural and chemical changes at the base of the petiole. The abscission zone, where the petiole joins the stem, is comprised of two sections: the separation layer and the protective layer. The separation layer is comprised of small, weak-walled cells which are very effective at transporting water and nutrients. As the temperature cools and the daylight hours shorten, water and nutrients are transported away from the leaves back to the stem for storage. During this transporting process, the chloroplasts break down, eliminating the green colour of the leaf. The yellow and red pigments, however, are not degraded thus explaining why leaves change from green to yellow, red or orange in the fall. After this has been accomplished, specialized enzymes destroy the cells in the separation layer, causing the leaf to fall from the plant. The protective layer, during this time, develops highly dense cells with large cell walls, so as to protect the plant from the environment. After the leaf falls, this layer is clearly visible as the leaf scar.

Morphology and histology

Leaves are comprised of several tissue types, and have a remarkable design. The upper layer of the leaf (that facing the sun) is a one-celled layer called the epidermis. Its primary purpose is protection of the leaf from harsh environmental conditions and insect pests. The cells of the epidermis are tightly packed and have a waxy cuticle which reduces water loss due to evapotranspiration.

Beneath the epidermis lies the mesophyll, which has a large volume of loosely packed cells. It is in this area that photosynthesis occurs. The large air spaces permit the leaf to easily exchange gases required for photosynthesis and respiration. In most plants, the mesophyll is composed of two layers: the palisade parenchyma and the spongy parenchyma. The palisade cells are column-shaped, with the longer axis at right angles to the epidermis. These cells have up to four times as many chloroplasts than those in the spongy parenchyma, and thus perform the majority of the energy fixation. The spongy parenchyma is more useful in gaseous, water and nutrient transport. Note that in some species, particularly the grasses, the distinction between these two parenchymal types is either very subtle or non-existent. The primary vascular tissues of the leaf are also found in the mesophyll, and are bundled into veins (these are clearly seen in the veinous structure of the maple leaf, for example). These veins contain both the xylem and the phloem. These vascular tissues are never exposed directly to the environment, instead being sheathed by parenchymal cells. These cells have very few chloroplasts, and are adapted for water and nutrient transport.

The lower layer of each leaf is also covered by a single layer of epidermal cells, similar in construction to the upper epidermis. However, the lower layer is also covered with stomata, which are small cells used to promote gas exchange (note that the stomata are found on the upper epidermis in aquatic plants, as the atmosphere is only in contact with the upper surface of the leaf). This lower layer may also be covered in leaf hairs, or trichomes, which are used to reduce the air flow along the bottom of the leaf, so as to give the leaf more control over gaseous exchange. These hairs are often used in semi-arid climates to reduce water loss.

Modifications

There are many leaf modifications that can be observed in nature. Already mentioned are those plants which have dimorphic leaves, but there are far more unusual and fascinating adaptations. Consider the leaves of celery or rhubarb, where the largest part of the plant is the petiole, swollen and elongated to store water and nutrients. Even more fascinating is the example of the venus flytrap. These plants have a single pair of leaves at the upper apex of each stem. Each pair of leaves is shaped like a half-circle, and each leaf has numerous toothy projections. These leaves are used to capture insect prey. The leaves secrete a sweet nectary-substance, which attracts insects like flies. Once the insect lands on one or both leaves, one or more of the three sensitive hairs on the inner surface of each leaf is triggered, and the leaves close shut capturing the insect. Digestive enzymes then liquify the prey item, and the nitrogen and phosphorus contained in the insect's body is absorbed by the plant. These leaves are so highly adapted that the plant can differentiate between insect prey and inanimate matter. If you poke one with a blade of grass of a small stick, it will likely not close.

1 While they are biologically speaking leaves, I have chosen not to include the needles of gymnosperms (evergreens) in this write-up. They are quite different from the leaves discussed in this write-up, and will be treated more fully in their own node.
2 Again, while the byrophytes (mosses and liverworts) also have leaves, botanically speaking, their formation is quite distinct from those of the higher plants. Some discussion of their body form may be found in their own node.


Composed with help from Raven, P. H., Evert, R. F. and S. E. Eichorn. (1992) Biology of Plants, 5th Ed. Worth Publishers.

Leaf (?), n.; pl. Leaves (#). [OE. leef, lef, leaf, AS. le�xa0;f; akin to S. lf, OFries. laf, D. loof foliage, G. laub,OHG. loub leaf, foliage, Icel. lauf, Sw. lof, Dan. lov, Goth. laufs; cf. Lith. lapas. Cf. Lodge.]

1. Bot.

A colored, usually green, expansion growing from the side of a stem or rootstock, in which the sap for the use of the plant is elaborated under the influence of light; one of the parts of a plant which collectively constitute its foliage.

⇒ Such leaves usually consist of a blade, or lamina , supported upon a leafstalk or petiole, which, continued through the blade as the midrib, gives off woody ribs and veins that support the cellular texture. The petiole has usually some sort of an appendage on each side of its base, which is called the stipule. The green parenchyma of the leaf is covered with a thin epiderm pierced with closable microscopic openings, known as stomata.

2. Bot.

A special organ of vegetation in the form of a lateral outgrowth from the stem, whether appearing as a part of the foliage, or as a cotyledon, a scale, a bract, a spine, or a tendril.

⇒ In this view every part of a plant, except the root and the stem, is either a leaf, or is composed of leaves more or less modified and transformed.

3.

Something which is like a leaf in being wide and thin and having a flat surface, or in being attached to a larger body by one edge or end; as : (a) A part of a book or folded sheet containing two pages upon its opposite sides. (b) A side, division, or part, that slides or is hinged, as of window shutters, folding doors, etc. (c) The movable side of a table. (d) A very thin plate; as, gold leaf. (e) A portion of fat lying in a separate fold or layer. (f) One of the teeth of a pinion, especially when small.

Leaf beetle Zool., any beetle which feeds upon leaves; esp., any species of the family Chrysomelidae, as the potato beetle and helmet beetle. -- Leaf bridge, a draw-bridge having a platform or leaf which swings vertically on hinges. -- Leaf bud Bot., a bud which develops into leaves or a leafy branch. -- Leaf butterfly Zool., any butterfly which, in the form and colors of its wings, resembles the leaves of plants upon which it rests; esp., butterflies of the genus Kallima, found in Southern Asia and the East Indies. -- Leaf crumpler Zool., a small moth (Phycis indigenella), the larva of which feeds upon leaves of the apple tree, and forms its nest by crumpling and fastening leaves together in clusters. -- Leaf cutter Zool. , any one of various species of wild bees of the genus Megachile, which cut rounded pieces from the edges of leaves, or the petals of flowers, to be used in the construction of their nests, which are made in holes and crevices, or in a leaf rolled up for the purpose. Among the common American species are M. brevis and M. centuncularis. Called also rose-cutting bee. -- Leaf fat, the fat which lies in leaves or layers within the body of an animal. -- Leaf flea Zool., a jumping plant louse of the family Psyllidae. -- Leaf frog Zool., any tree frog of the genus Phyllomedusa. -- Leaf green.Bot. See Chlorophyll. -- Leaf hopper Zool., any small jumping hemipterous insect of the genus Tettigonia, and allied genera. They live upon the leaves and twigs of plants. See Live hopper. -- Leaf insect Zool., any one of several genera and species of orthopterous insects, esp. of the genus Phyllium, in which the wings, and sometimes the legs, resemble leaves in color and form. They are common in Southern Asia and the East Indies. -- Leaf lard, lard from leaf fat. See under Lard. -- Leaf louse Zool., an aphid. -- Leaf metal, metal in thin leaves, as gold, silver, or tin. -- Leaf miner Zool., any one of various small lepidopterous and dipterous insects, which, in the larval stages, burrow in and eat the parenchyma of leaves; as, the pear-tree leaf miner (Lithocolletis geminatella). -- Leaf notcher Zool., a pale bluish green beetle (Artipus Floridanus), which, in Florida, eats the edges of the leaves of orange trees. -- Leaf roller Zool., the larva of any tortricid moth which makes a nest by rolling up the leaves of plants. See Tortrix. -- Leaf scar Bot., the cicatrix on a stem whence a leaf has fallen. -- Leaf sewer Zool., a tortricid moth, whose caterpillar makes a nest by rolling up a leaf and fastening the edges together with silk, as if sewn; esp., Phoxopteris nubeculana, which feeds upon the apple tree. -- Leaf sight, a hinges sight on a firearm, which can be raised or folded down. -- Leaf trace Bot., one or more fibrovascular bundles, which may be traced down an endogenous stem from the base of a leaf. -- Leaf tier Zool., a tortricid moth whose larva makes a nest by fastening the edges of a leaf together with silk; esp., Teras cinderella, found on the apple tree. -- Leaf valve, a valve which moves on a hinge. -- Leaf wasp Zool., a sawfiy. -- To turn over a new leaf, to make a radical change for the better in one's way of living or doing. [Colloq.]

They were both determined to turn over a new leaf. Richardson.

 

© Webster 1913.


Leaf (?), v. i. [imp. & p. p. Leafed (?); p. pr. & vb. n. Leafing.]

To shoot out leaves; to produce leaves; to leave; as, the trees leaf in May.

<-- = leaf out -->

Sir T. Browne.

 

© Webster 1913.

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