Leaves are the most active and conspicuous organs of plants. The most
important of their function is absorbing sunlight to be used for
photosynthesis. In order to do this, they expose large amounts of surface
area to the environment. On a global basis, leaves produce more than 200
billion tons of sugars per year. Those sugars sustain most life on this
How leaves form:
Leaves are the most diverse
of all plant organs-because they can be tubular, feathery, needlelike,
cupped, smooth, sticky fragrant, or waxy. They range in size from
pinhead-sized leaves to 20-meter across, and they range in number from
millions to only one or two in the whole lifetime of the plant. Leaves are
formed by the coordinated efforts of several meristems, each of which is
named for its position. The earliest stage of leaf development is a small
bulge at the shoot apex called a leaf buttress or primordium, which
consists of 100 to 300 cells. It is formed by cellular divisions one to
three cell layers below the overlying protoderm. More cellular divisions
and expansions produce a radially symmetrical cone called an apical peg,
which has an apical meristem and a procambial strand that forms the leaf's
midrib. The adaxial surface of the apical peg elongates slower than the
abaxial surface, and arches the leaf primordium over the shoot apical
The leaf then forms an
adaxial meristem that thickens the leaf. It forms an upper leaf zone and a
lower leaf zone. The upper zone contains marginal meristems that form the
flattened blade and stalklike petiole that attaches the leaf to the stem.
The lower leaf zone forms the leaf base.
growth of a leaf involves cellular expansion and division. These divisions
continue until the leaf is one-half to three-fourths grown. Cellular
expansion forms most of the intercellular spaces in a leaf. Stomata
differentiate soon after intercellular spaces form. Except for vascular
tissues which differentiates from the base into the tip of the leaf, other
tissues in leaves differentiate from the tip toward the base. Until it is
30%-40% of its final size, a growing leaf depends on the rest of the plant
for its nutrition.
Structure of Leaves:
There are four basic
kinds of leaves: simple, compound, peltate, and perfoliate:
- Simple leaves-have
a flat, undivided blade that is supported by a stalk called a
petiole. The petiole is typically supported by collenchyma and
- Compound leaves-have
blades divided into leaflets that form in one plane and lack axillary
buds. Each compound leaf has a single bud at the base of its petiole.
There are two kinds of compound leaves: pinnately compound and
palmately compound leaves. Pinnately form in pairs along a central
stalklike rachis, and palmately attach at the same point.
leaves-have petioles that attach to the middle of the blade.
leaves-are sessile leaves that surround and are pierced by the stems.
Leaves consists of
epidermal, ground, and vascular tissues.
- Epidermis-it is compact,
transparent, and usually not photosynthetic. It also contains many
stomata. In horizontally oriented leaves, there are usually more
stomata on the protected lower side that the exposed upper side.
Vertical leaves usually have similar numbers of stomata on both sides.
Although stomata occupy 1% of the leaf surface, they lose huge amounts
of water in to the atmosphere.
- Vascular-Xylem and
phloem in leaves form in strands called veins. Xylem forms on teh
upper side of a vein, and phloem forms on the lower side. Veins are
supported by fibers and are usually surrounded by a layer of
parenchyma cells called the bundle sheath, which extends to the
epidermis of the leaf. These help support the veins and may conduct
water to the epidermal cells. Most dicots and some nonflowering plants
have netted venation, meaning they have one or a few prominent
midveins, from which smaller minor veins branch into a meshed network.
The leaves of monocots have parallel venation, meaning several
prominent and parallel veins interconnect with smaller, inconspicuous
veins. Several factors influence the formation of veins, including the
presence of other veins.
- Ground-The ground tissue
of leaves is called mesophyll. It contains many types of cells,
including sclerenchyma, storage parenchyma, and chlorenchyma. In
general, the arrangement of chlorenchyma is determined genetically,
and is influenced by whether the leaf is oriented horizontally or
vertically when it forms.
Like other organs,
leaves are often modified for functions other than photosynthesis. Below
are a few examples:
- Tendrils-of plants are
leaves modified for support. In some plants the entire leaf is a
tendril; photosynthesis in these plants is delegated to leaflike
structures called stipules at the base of each leaf. Tendrils, of many
plants may be up to 30 cm long, which makes them well suited for
seeking support in the plant's nearby environment.
- Stipules-are small,
leaflike structures at the base of petioles, and have a variety of
functions. Some are photosynthetic, while others form protective
- Spines-leaves modified
- Bud Scales-are tough,
overlapping, waterproof leaves that protect buds from frost,
desiccation, and pathogens. Bud scales form before teh onset of
unfavorable growing seasons ie winter.
- Window leaves-are common
in many desert plants, are shaped like tin ice-cream cones and grow
mostly underground, with only a small transparent "window"
tip protruding above the soil level.
- Bracts are floral leaves
that form at the base of a flower or flower stalk. They are usually
small and scalelike, and protect developing flowers.
- Storage leaves-flowerpot
leaves are typically of rosette plants, and such epiphytes. They are
packed tightly into a flowerpotlike structure that catches falling
water and debris.
leaves-in carnivorous plants, leaves modified for attracting,
trapping, and digesting animals. These adaptations range from sticky
flypaper surfaces to vatlike leaves.
- Leaves modified for
reproduction-form tiny plants at the edges of their leaves. These
plants become new individuals when they are shed from parent leaves.
embryonic leaves. Monocots, usually have one cotyledon, while dicots
- Prophylls-are the
first leaves to form on axillary buds. Monocots have one usually,
where dicots have two, suggesting that tiny leaves may be analogous to
cotyledons. Prophylls protect axillary buds.