- Leaves 1
Leaves are best known as the photosynthetic organs of plants, and much of the
leaf "architecture" reflects this function. Leaves are part of the plant's shoot
system, attached to stems at nodes. The regions along the stem between leaves
are internodes. Leaves exhibit far more variation in shape (morphology) than do
stems and roots. Leaf shape, size, venation pattern, margins, tips and bases are
all used in identification of plant species (with appropriate vocabulary).
Most leaves, however, have two common features: the blade (or lamina), the
flattened portion of the leaf, and the p etiole, or leaf stalk, which attaches the
leaf to the stem. Leaves that do not have a petiole are s essile, and often sheath
the stem at the base of the leaf. S tipules , small leaf-like growths near the base
of the petiole, may or may not be present. Buds are located in the axil of a leaf
with the stem.
Leaf morphology varies in monocots and eudicots, too, with monocots generally
having linear leaves that sheath the stem at the base and eudicots having almost
any shape, but typically with a petiole.
Eudicot Leaf
Monocot Leaf
Leaves have a vascular connection to the stem through the petiole. Vascular
tissue in leaves comprises the veins. In early development, a procambium strand
from the shoot meristem branches out into each leaf primordium. This is the l eaf
trace . Similar strands of procambium branch out into buds, the b ud traces.
They leave a procambium gap in the stem tissue called the leaf trace gap and bud
trace gap. The traces and gaps can often be seen in the shoot meristem.
Leaf venation patterns are also an important distinction between monocots and
eudicots. Monocots usually have parallel veins; eudicots have n etted veins,
generally with a significant midvein. Eudicot leaves may have p innate venation
(with veins branching regularly from the midvein) or palmate venation (with major
veins radiating from the leaf base). The Ginkgo tree, which is a gymnosperm, has
leaves with d ichotomous venation. It is unique.
Netted veins
Parallel veins
Ginkgo leaf
Leaves - 2
Leaf blades may be s imple , d issected or c ompound with leaflets along the
petiole (or rachis) along one plane. A compound leaf can be distinguished from a
simple leaf by the location of buds. There are no buds in the axils of leaflets.
Compound leaves may be pinnately compound or palmately compound. Compound
leaves with three leaflets, such as clover, are said to be t ernate .
The phyllotaxy, or arrangement of leaves on the stem may be helical (sometimes
called alternate with one leaf per node), opposite (two leaves per node) or whorled
(three or more leaves per node). The number of leaves along a stem before
overlapping is called ranking. In some cases, this leads to a very ordered
arrangement, such as the ranking of leaves on most plants in the mint family or in
many grasses.
A lternate
Opposite
Whorled
The many variations in leaves provide for better survival in specific habitats. For
example, thickened epidermis and epidermal hairs protect and minimize water loss
for leaves of dry areas as well as reflect intense sunlight. Large air spaces make
floating leaves buoyant. Dissected leaves in water offer less resistance to water
force. There are also differences in leaves found in sun and in shade, and in some
juvenile and adult leaf forms.
Leaves - 3
Leaf Functions
P hotosynthesis
A primary function of leaves is photosynthesis. As you observe the
structure of leaves in lab, note the design of the leaf relative to what it
needs for photosynthesis: light, water, CO2 and chlorophyll.
T ranspiration
Transpiration is the evaporation of water through leaf surfaces, which
creates a tension (or negative pressure) that draws water upward through
the xylem tissue from roots and stems throughout the plant.
Gas exchange
o CO2 in for the process of photosynthesis
o O2 in or out, depending on diffusion gradients. Oxygen, a by-product of
photosynthesis, is required for cell respiration in all living plant cells.
Leaf Structure
Epidermis
The leaf epidermis comprises the outer layer of leaf cells. Since most leaves
are flattened there is an upper and a lower epidermis. For vertical leaves
the upper epidermis is the inner side of the leaf and the lower epidermis, the
outer side
The epidermis layer is generally one cell thick; some leaves may have a multilayered hypodermis
The epidermis is covered with a waxy c uticle (thickness varies).
E pidermis Functions
Protection
Gas exchange
S pecial cells in leaf epidermis
Guard cells t hat form stomata
Guard cells are concentrated on lower epidermis in most leaves.
Guard cells are most often pairs of "bean-shaped" cells with thick,
rigid inner walls. They contain chloroplasts. The opening between
guard cells forms s tomata for gas exchange
Stomata open and close with changes in turgor caused by solute
differences between the guard cells and the surrounding epidermal
cells. The thinner walls of the guard cell stretch as it swells with
water whereas the thicker, inner walls do not. This differential
stretching creates the stoma. Chemical signals, photosynthesis
and osmotic changes are necessary for most stomatal function.
We will discuss this mechanism later.
Leaves - 4
Epidermal hairs (trichomes)
Silky, woolly, prickly, felt-like, scaly, glandular, etc.
L eaf Mesophyll
Mesophyll includes all of the internal cells of the leaf outside of the vascular
bundles or veins
Mesophyll cells are mostly parenchyma cells along with some fibers. Fibers
are most commonly associated with veins.
P hotosynthesis occurs in the mesophyll layers
Eudicot Leaf Mesophyll
Palisade mesophyll
Located near upper epidermis
Cells elongated to surface
Contain many chloroplasts
Spongy mesophyll
Located near lower epidermis
Many air spaces
Isodiametric shape
Important in transpiration and CO2 movement
Eudicot leaf, xs, midvein region
Eudicot leaf, xs, blade region
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Monocot Leaf Mesophyll
Monocots usually do not have a distinctive palisade and spongy
mesophyll.
Monocots have parallel veins with a general mesophyll of loosely packed
parenchyma cells on both sides of the veins extending to the epidermis
layers.
Monocot leaf, xs
Vascular Tissue in Leaves (Veins)
Vascular tissue provides s upport for the leaf. Veins branch extensively
throughout the mesophyll of leaves. This is most evident in decomposing
leaves, with their leaf skeleton of veins.
Vascular tissue c onducts water and solutes and maintains vascular
connections with stem and root.
Leaf vascular tissue includes:
Xylem (Toward upper epidermis) composed of vessels and fibers
Phloem (Toward lower epidermis) composed of sieve tubes, companion
cells and transfer cells
B undle sheath of fibers or parenchyma
B order parenchyma at tips of "veinlets".
Veins may extend to leaf tip cells called h ydathodes . Hydathodes
function in g uttation. Guttation is described in the section on water
and transport in plants.
The leaf petiole, which is the vascular connection the from stem to
leaf blade.
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Vein Patterns
As discussed, in e udicots veins form a network throughout the mesophyll,
branching from the midvein. In a leaf cross section, the large midvein is
conspicuous, with b undle sheath extensions that often go from the upper
epidermis to the lower epidermis providing additional support. There may be some
secondary growth in the midvein. Branching veins may be seen either in cross
section or in longitudinal or even oblique sections of leaves. Branching veins may
also have bundle sheath extensions.
In a m onocot leaf cross section, the parallel veins are seen in cross section and
may be uniform in size and distribution across the leaf's interior.
Some grasses may have large thin-walled cells along both sides of the midvein in
the upper epidermis. These cells, called bulliform cells, help the leaf to fold or
roll inward during water deficit periods. Folding of the leaf minimizes evaporation.
In addition, many monocots have enlarged bundle sheath cells, surrounding the
veins, which have a function in C-4 photosynthesis for some plants.
Bundle Sheath Cells in Sugar Cane and in Corn leaves
Leaves - 7
Some Leaf Variations
Hydromorphic Leaves
Floating Leaves
Floating leaves will have stomata on the upper epidermis with air
channels into the palisade parenchyma.
The spongy mesophyll will be filled with huge air spaces (aerenchyma)
Vascular tissue may be reduced, especially in the amount of x ylem in
the branching veins.
Submerged Leaves
Plants that grow completely underwater will typically have dissected
leaves, or narrow linear leaves. These shapes minimize resistance to
water currents, and probably prevent damage to the leaf.
Some aquatic plants produce dissected leaves where submerged, and
less dissected shapes when the leaves are produced above the surface
of the water. They are a good example of environmental control of
gene expression.
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Xeromorphic Leaves
Plants that live in arid environments are subject to drought, and often,
intense sunlight. Such plants are called xerophytes. Water loss is a serious
problem and xerophytes have modifications to minimize water loss.
Thick c uticle
The upper e pidermis may be several layers thick.
The p alisade parenchyma may also be thickened.
Veins may have bundle sheath extensions
The lower epidermis may have several layers and a thickened cuticle.
Deep invaginations of the lower epidermis layer, called s tomatal
crypts , are common. Crypts often have many hairs, which in the
depression of the crypt create a microenvironment that is more
humid. Stomata are located in the crypts.
Some xeromorphic plants may have succulent leaves, and may have
stomata that open at night rather than in the daylight to minimize
water loss.
Nerium oleander, xs
Pine leaf, xs
Conifer Leaves (Needles)
The needles of conifers have many of the same adaptations of xeromorphic
leaves. Conifers predominate in areas that have long, cold winters and dryer
summers. To survive, conifers are adapted to conditions of moisture
stress. A typical conifer needle will have s unken stomata, often in
channels on the lower surface, a t hick cuticle, h ypodermis layers, and, in
addition, an e ndodermis surrounding the vein (or rarely, veins). As with
other conifer organs, the leaves will have resin canals, lined with
parenchyma.
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Other Leaf Variations
Phase changes
As discussed in our section on genetic controls of development, the leaf shapes in
some plants change from the juvenile regions of the plant compared to those in
areas that are more mature, changes that are controlled by gene signals in the
meristem. Because plants have modular growth, the originating meristem exerts
genetic control over meristems derived from it, so plants may have juvenile and
mature leaves at the same time. All meristems derived from an originating bud,
for example, will have the genetic phase shape of that original bud.
Phase changes are also responsible for submerged and above the water
differences in leaf shape.
Other phase changes occur when a shoot transitions from a vegetative to a
reproductive bud. Reproductive buds are terminal in the sense that no apical
meristem is preserved in a shoot bud for new bud primordia.
Shade/Sun Leaves
Leaves in the shade have thinner blades and a larger surface area compared to
those exposed to full sun, which have thicker mesophyll and epidermis, and greater
vascular tissue development. Shade leaves have a lower photosynthetic rate in full
sun, but in low light levels, because they have more surface area, shade leaves
have similar rates of photosynthesis to sun leaves. These leaf developmental
differences are good examples of the influence of environmental conditions on
developmental processes.
Leaf in full sun, xs
Leaf in shade, xs
Leaves - 10
Leaf Abscission (Loss of leaves)
The loss and replacement of leaves in plants is a normal process. A perennial
which loses all of its leaves at one time in response to seasonal or climate
differences, is termed d eciduous . All leaves, however, have a finite life span, and
are lost from the plant, to be replaced by newer leaves produced in the shoot
meristems. The process of leaf loss is called a bscission , and is controlled by
hormones.
The abscission zone is located at the base of the petiole in a region of
undifferentiated, small parenchyma cells. Their walls contain no lignin, and the
vascular cells in the abscission zone are also reduced in size.
The process of abscission is initiated and proceeds as follows:
The parenchyma cells start dividing rapidly.
They secrete a layer of suberin in the walls nearest the stem
The middle lamella, cell walls and cells of the abscission zone dissolve
(enzymatic degradation activated by ethylene)
Leaf abscises
The hormones (which will be discussed later) involved in leaf abscission are:
Auxin
Ethylene
Associated with abscission, and occurring prior to the loss of the leaf, is a process
that involves degrading and moving a number of solutes, minerals and other
substances from the leaf tissue into the stem and roots. The degradation of
chlorophyll and change in leaf color which results, is often referred to as "fall
color". As chlorophyll degrades the natural carotenoids become prominent. In
addition, some leaves may accumulate anthocyanins in their vacuoles, adding reds
or bluish reds to the fall color. Photoperiod is important in triggering the fall
change in pigmentation. Summer moisture also affects fall color. There are a
number of web sites that discuss fall color.
Leaves - 11
Economic uses of Leaves
Leaves are used extensively in medicine and nutrition, as well as other human
economical endeavors. Some examples include:
Food The leafy "greens" are among the most nutritious of foods
Herbs The mint family is a popular herb family
Beverages
Teas
Drug uses
Tobacco
Marijuana
Cocaine
Insecticides
Rotenone
Citronella
Waxes
Carnauba
Aromatic Oils
Phytochemicals Herbal supplements, some with potential health benefits
Medical uses