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leaf
Table of Contents
leaf

 * Introduction
   
 * 
   Leaf function
   
 * 
   Leaf morphology
   
 * 
   Leaf modifications
   
 * 
   Senescence
   

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Home Science Biology Cells, Organs & Tissues


LEAF

plant anatomy
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Table of Contents
leaves; beech
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Related Topics: cotyledon bulb stomate tendril pinnate leaf ...(Show more)
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leaf, in botany, any usually flattened green outgrowth from the stem of a
vascular plant. As the primary sites of photosynthesis, leaves manufacture food
for plants, which in turn ultimately nourish and sustain all land animals.
Botanically, leaves are an integral part of the stem system. They are attached
by a continuous vascular system to the rest of the plant so that free exchange
of nutrients, water, and end products of photosynthesis (oxygen and
carbohydrates in particular) can be carried to its various parts. Leaves are
initiated in the apical bud (growing tip of a stem) along with the tissues of
the stem itself. Certain organs that are superficially very different from the
usual green leaf are formed in the same manner and are actually modified leaves;
among these are the sharp spines of cacti, the needles of pines and other
conifers, and the scales of an asparagus stalk or a lily bulb.




LEAF FUNCTION

Understand how a leaf's guard cells, stomata, epidermis, and mesophyll regulate
transpiration
Learn about how the structure of leaves affects their function.
Encyclopædia Britannica, Inc.See all videos for this article

The main function of a leaf is to produce food for the plant by photosynthesis.
Chlorophyll, the substance that gives plants their characteristic green colour,
absorbs light energy. The internal structure of the leaf is protected by the
leaf epidermis, which is continuous with the stem epidermis. The central leaf,
or mesophyll, consists of soft-walled, unspecialized cells of the type known as
parenchyma. As much as one-fifth of the mesophyll is composed of
chlorophyll-containing chloroplasts, which absorb sunlight and, in conjunction
with certain enzymes, use the radiant energy in decomposing water into its
elements, hydrogen and oxygen. The oxygen liberated from green leaves replaces
the oxygen removed from the atmosphere by plant and animal respiration and by
combustion. The hydrogen obtained from water is combined with carbon dioxide in
the enzymatic processes of photosynthesis to form the sugars that are the basis
of both plant and animal life. Oxygen is passed into the atmosphere through
stomata—pores in the leaf surface.


photosynthesis
Green plants such as trees use carbon dioxide, sunlight, and water to create
sugars. Sugars provide the energy that makes plants grow. The process creates
oxygen, which people and other animals breathe.
Encyclopædia Britannica, Inc.
Read More on This Topic
Why Do Leaves Change Colour in the Fall?
A number of pigments are involved in producing beautiful autumn foliage.



LEAF MORPHOLOGY

Typically, a leaf consists of a broad expanded blade (the lamina), attached to
the plant stem by a stalklike petiole. In angiosperms leaves commonly have a
pair of structures known as stipules, which are located on each side of the leaf
base and may resemble scales, spines, glands, or leaflike structures. Leaves
are, however, quite diverse in size, shape, and various other characteristics,
including the nature of the blade margin and the type of venation (arrangement
of veins). When only a single blade is inserted directly on the petiole, the
leaf is called simple. The margins of simple leaves may be entire and smooth or
they may be lobed in various ways. The coarse teeth of dentate margins project
at right angles, while those of serrate margins point toward the leaf apex.
Crenulate margins have rounded teeth or scalloped margins. Leaf margins of
simple leaves may be lobed in one of two patterns, pinnate or palmate. In
pinnately lobed margins the leaf blade (lamina) is indented equally deep along
each side of the midrib (as in the white oak, Quercus alba), and in palmately
lobed margins the lamina is indented along several major veins (as in the red
maple, Acer rubrum). A great variety of base and apex shapes also are found. The
leaf may also be reduced to a spine or scale.


pain bush
Pain bush, or African poison ivy (Smodingium argutum). The species is poisonous
because of the sap it emits.
JMK

leaf types
Common leaf morphologies.
Encyclopædia Britannica, Inc.

Veins, which support the lamina and transport materials to and from the leaf
tissues, radiate through the lamina from the petiole. The types of venation are
characteristic of different kinds of plants: for example, dicotyledons such as
poplars and lettuce have netlike venation and usually free vein endings;
monocotyledons like lilies and bamboo have parallel venation and rarely free
vein endings.


leaf: veins
Close-up of veins in a leaf.
© Corbis



LEAF MODIFICATIONS

Whole leaves or parts of leaves are often modified for special functions, such
as for climbing and substrate attachment, storage, protection against predation
or climatic conditions, or trapping and digesting insect prey. In temperate
trees leaves are simply protective bud scales; in the spring when shoot growth
is resumed, they often exhibit a complete growth series from bud scales to fully
developed leaves.



Spines are also modified leaves. In cacti, spines are wholly transformed leaves
that protect the plant from herbivores, radiate heat from the stem during the
day, and collect and drip condensed water vapour during the cooler night. In the
many species of the spurge family (Euphorbiaceae), the stipules are modified
into paired stipular spines and the blade develops fully.
In ocotillo (Fouquieria splendens), the blade falls off and the petiole remains
as a spine.


fishhook cactus
Fishhook cactus (Mammillaria).
Gary M. Stolz/U.S. Fish and Wildlife Service

Many desert plants, such as Lithops and aloe, develop succulent leaves for water
storage. The most common form of storage leaves are the succulent leaf bases of
underground bulbs (e.g., tulip and Crocus) that serve as either water- or
food-storage organs or both. Many nonparasitic plants that grow on the surfaces
of other plants (epiphytes), such as some of the bromeliads, absorb water
through specialized hairs on the surfaces of their leaves. In the water
hyacinth (Eichhornia crassipes), swollen petioles keep the plant afloat.


aloe
Gelatinous interior of the leaves of aloe (Aloe vera), a succulent plant.
Raul654

Leaves or leaf parts may be modified to provide support. Tendrils and hooks are
the most common of these modifications. In the flame lily (Gloriosa superba),
the leaf tip of the blade elongates into a tendril and twines around other
plants for support. In the garden pea (Pisum sativum), the terminal leaflet of
the compound leaf develops as a tendril. In nasturtium (Tropaeolum majus)
and Clematis, the petioles coil around other plants for support. In catbrier
(Smilax), the stipules function as tendrils. Many monocotyledons have sheathing
leaf bases that are concentrically arranged and form a pseudotrunk, as
in banana (Musa). In many epiphytic bromeliads, the pseudotrunk also functions
as a water reservoir.



tendrils
Tendrils of catbrier (Smilax rotundifolia). The stipules elongate and coil
around other plants for support.
Runk/Schoenberger—Grant Heilman Photography, Inc.

Carnivorous plants use their highly modified leaves to attract and trap insects.
Glands in the leaves secrete enzymes that digest the captured insects, and the
leaves then absorb the nitrogenous compounds (amino acids) and other products of
digestion. Plants that use insects as a nitrogen source tend to grow in
nitrogen-deficient soils.


slender pitcher plant
Pitcher-shaped leaves of the carnivorous slender pitcher plant (Nepenthes
gracilis).
© So happy/Fotolia



SENESCENCE

Leaves are essentially short-lived structures. Even when they persist for two or
three years, as in coniferous and broad-leaved evergreens, they make little
contribution to the plant after the first year. The fall of leaves, whether in
the first autumn in most deciduous trees or after several years in evergreens,
results from the formation of a weak zone, the abscission layer, at the base of
the petiole. Abscission layers may also form when leaves are seriously damaged
by insects, disease, or drought. As a result, a zone of cells across the petiole
becomes softened until the leaf falls. A healing layer then forms on the stem
and closes the wound, leaving the leaf scar, a prominent feature in many winter
twigs and an aid in identification.



fall foliage
Pigments other than chlorophyll give this maple leaf its autumn colours.
© Corbis
Understand why leaves change its color in autumn
Learn why leaves of deciduous trees change colour in autumn.
Contunico © ZDF Enterprises GmbH, MainzSee all videos for this article

In perennial plants, leaf fall is usually associated with approaching winter
dormancy. In many trees leaf senescence is brought about by declining day length
and falling temperature toward the end of the growing season. Chlorophyll
production in deciduous plants slows as the days get shorter and cooler, and
eventually the pigment is broken down completely. Yellow and orange pigments
called carotenoids become more conspicuous, and, in some
species, anthocyanin pigments accumulate. Tannins give oak leaves and certain
other plants their dull brown colour. These changes in leaf pigments are
responsible for the autumn colours of leaves. There are some indications that
day length may control leaf senescence in deciduous trees through its effect on
hormone metabolism; both gibberellins and auxins have been shown to retard leaf
fall and to preserve the greenness of leaves under the short-day conditions of
autumn.

The Editors of Encyclopaedia Britannica
This article was most recently revised and updated by Melissa Petruzzello.



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External Websites
 * Palomer College - WAYNE'S WORD - Leaf Terminology
 * The Spruce - Types of Leaf Arrangements

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2022.
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External Websites
 * Palomer College - WAYNE'S WORD - Leaf Terminology
 * The Spruce - Types of Leaf Arrangements

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 * leaf - Children's Encyclopedia (Ages 8-11)
 * leaf - Student Encyclopedia (Ages 11 and up)



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