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Consortium for
Educational
Communication
Module on
Leaf arrangement,
morphology and structure
By
Dr. EHTISHAM UL HAQ
Lecturer Botany and Research Scholar
CBT, University of Kashmir, Srinagar
1900 06
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Text
Morphology of Leaf
A structurally complete leaf of an angiosperm consists of a petiole
(leaf stalk), a lamina (leaf blade), and stipules (small structures
located on either side of the base of the petiole).
Fig. 4. Morphology of a dicot leaf.
Not every species produces leaves with all of these structural
components. In certain species, paired stipules are not obvious
or are altogether absent. A petiole may be absent, or the blade
may not be laminar (flattened). The petiole mechanically links
the leaf to the plant and provides the route for transfer of water
and sugars to and from the leaf.
In monocots, the leaf is almost always broadly sheathed at the
base, with edges either fused or overlapping (Fig. 5). In taxa
such as grasses (Poaceae) and gingers (Zingiberaceae), there is
an abaxial flap or ligule at the junction of sheath and blade. A leaf
that lacks a petiole is said to be sessile.
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Fig. 5. Monocot leaf
Two basic forms of leaves can
be described considering the way the blade (lamina) is divided. A
simple leaf that has an undivided blade.
Fig. 6. Simple leaves
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A compound leaf
that has a fully sub-divided blade, each leaflet of the blade is
separated and possess the secondary vein. Because each leaflet
can appear to be a simple leaf, it is important to recognize where
the petiole occurs to identify a compound leaf.
Fig. 7. Various types of compound leaves
Compound leaves are a characteristic of some families of higher
plants, such as Fabaceae. (Haupt and Wing, 1953). The middle
vein of a compound leaf or a frond, when it is present, is called a
rachis. Following are some important types of compound leaves:
•
Palmately compound leaves have the leaflets radiating from
the end of the petiole, like fingers of the palm of a hand, e.g.
Cannabis (hemp) and Aesculus (buckeyes).
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Fig. 8. Palmately compound leaf
•
Pinnately compound leaves have the leaflets arranged along
the main or mid-vein. These are of two types:
•
o
Imparipinate (0dd pinnate): with a terminal leaflet, e.g. Fraxinus (ash), Rosa indica etc.
Fig. 9. Odd pinnate compound leaf
Paripinate (even pinnate): lacking a terminal leaflet, e.g.
Swietenia (mahogany), Casia etc.
Fig. 10. Even pinnate compound leaf
Pinnately compound leaves can be:
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•
Bipinnate. Compound leaves are twice divided: the leaflets
are arranged along a secondary vein. Each leaflet is called
a “pinnule”. The pinnules on one secondary vein are called
“pinna”; e.g. Albizia (silk tree), Mimosa pudica etc.
Fig.11. Bipinnately compound leaf
Tripinnate: In which the leaflets are
themselves bipinnate; also called “thrice-pinnate“ as in
Moringa.
Fig.12 Tripinnately compound leaf
Decompound leaf – In which the leaflets are themselves
compound as in Dauccus.
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Fig. 13. Decompound
leaf
•
Trifoliate (or trifoliolate): a pinnate leaf with just
three leaflets, e.g. Trifolium (clover), Laburnum (laburnum).
Fig. 14. Trifoliate leaf
Leaf Petiole
Petiolated leaves have a petiole (leaf stem). Sessile leaves do not
have a petiole, and the blades are directly attached to the stem.
In clasping or decurrent leaves, the blade partially or wholly
surrounds the stem, often giving the impression that the shoot
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grows through the leaf. When this is the case, the leaves are called
“perfoliate”, such as in Claytonia perfoliata. In peltate leaves, the
petiole attaches to the blade inside from the blade margin.
Peltate
Fig. 15. Various forms of leaf bases and their attachments
with petioles
In some Acacia species, such as the Koa
Tree (Acacia koa), the petioles are expanded or broadened and
function like leaf blades; these are called phyllodes. There may or
may not be normal pinnate leaves at the tip of the phyllode.
Fig. 16. The broadened petiole of Acacia koa
Stipule. A stipule, present on the leaves of many dicotyledons,
is an appendage on each side at the base of the petiole resembling
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a small leaf. Stipules may be lasting (a stipulate leaf, such as in
roses and beans), or maybe shed as the leaf expands, leaving a
stipule scar on the twig (an exstipulate leaf) (Haupt and Wing,
1953).
Fig. 17. Leaves of Rosa indica with
stipules
The situation, arrangement, and structure of the stipules is
called the “stipulation”. It may be:
o
free
o
adnate : fused to the petiole base
o
ochreate : provided with ochrea, or sheath-formed stipules,
e.g. rhubarb,
o
encircling the petiole base
o
interpetiolar : between the petioles of two opposite leaves.
o
intrapetiolar : between the petiole and the subtending stem
Venation
The pattern or arrangement of veins in the leaf blade is known
as leaf venation. If there is only one prominent vein in a leaf, it
is called mid-vein or primary vein; branches from this vein are
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called secondary veins. Tertiary veins usually link the secondaries,
forming a ladder like (scalariform) or netlike (reticulate) pattern.
Most frequent types of venations are:
•
Unicostate pinnate venation–
The veins arise pinnately from a single mid-vein and subdivide
into veinlets. These, in turn, form a complicated network. This
type of venation is typical of dicotyledons.
Fig. 18. Leaf with reticulate venation
Multicostate or Palmate venation- Here more than one
equally strong vein entering the leaf blade. It is of following two
types:
•
Three main veins branch at the base
of the lamina and run essentially parallel subsequently, as in
Plantago major. A similar pattern (with 3-7 veins) is especially
conspicuous in Melastomataceae.
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Fig. 19. The leaf of Plantago major with three
main veins
•
Several main veins diverge from near the
leaf base where the petiole attaches, and radiate toward the
edge of the leaf, e.g. Acer (maples).
Fig. 20. Acer leaf with palmate
netted venation
•
•
Parallel-veined– veins run parallel for the
length of the leaf, from the base to the apex. Commissural
veins (small veins) connect the major parallel veins. Typical for
most monocotyledons, such as grasses.
Fig. 21. Monocot leaf with parallel venation
Dichotomous – There are no dominant bundles, with the
veins forking regularly by pairs; found in Ginkgo and some
pteridophytes.
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Fig. 22.
Ginkgo leaf with dichotomous venation
Fig. 23. Different features of leaves
Leaf Arrangement (Phyllotaxy)
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The arrangement of leaves on the stem is called phyllotaxy. The
leaves may be arranged in one the following patterns in a plant:
•
Alternate – Here the leaves are borne singly
and are usually arranged in spiral pattern along the stem.
Alternate leaves are sometimes placed along the two sides
of the stem (2-ranked or distichous), or on three sides of the
stem (3-ranked or tristichous).
Fig. 24. Alternate arrangement of
leaves
•
Opposite – Two leaves are borne on
opposite sides at each node on the stem. Opposite leaves may
be spiraled, as in red Mangroves (Rhizophora); 2-ranked as in
many Zygophyllaceae; or decussate (the leaves of adjacent
nodes rotated at 90°. The decussate condition is the most
common arrangement among vascular plant species.
Fig. 25. Opposite arrangement
of leaves
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•
•
Whorled – Three or more leaves are attached at
each node on the stem. As with opposite leaves, successive
whorls may or may not be decussate. Opposite leaves may
appear whorled near the tip of the stem.
Fig. 26. Whorled arrangement of
leaves
Rosulate – leaves form a rosette
Fig. 27. Rosette arrangement of
leaves
As a stem grows, leaves tend to appear arranged around the
stem in a way that optimizes yield of light. In essence, leaves
form a helix pattern centered around the stem, either clockwise
or counterclockwise, with (depending upon the species) the same
angle of divergence. There is a regularity in these angles and they
follow the numbers in a Fibonacci sequence: 1/2, 2/3, 3/5, 5/8,
8/13, 13/21, 21/34, 34/55, 55/89. This series tends to a limit
close to 360° x 34/89 = 137.52 or 137° 30’, an angle known in
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mathematics as the golden angle. In the series, the numerator
indicates the number of complete turns or “gyres” until a leaf
arrives at the initial position. The denominator indicates the
number of leaves in the arrangement (Mauseth and James, 2008).
. This can be demonstrated by the following:
•
alternate leaves have an angle of 180° (or 1/2)
•
120° (or 1/3) : three leaves in one circle
•
144° (or 2/5) : five leaves in two gyres
•
135° (or 3/8) : eight leaves in three gyres.
Internal Structure of a leaf (Anatomy)
A leaf is
a plant organ which is made up of a collection of tissues in a
regular organisation. The major tissue systems are:
Fig. 28. Anatomical features
of a leaf
1.The epidermis that covers the upper and lower surfaces
2.The mesophyll inside the leaf that is rich in chloroplasts
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(also called chlorenchyma)
3.The arrangement of veins (the vascular tissue)
Fig. 29. Detailed
anatomy of leaf
These three tissue systems typically form a regular organization
at the cellular scale.
Epidermis
The epidermis is the outer layer of cells covering the leaf. It forms
the boundary separating the plant’s inner cells from the external
world. The epidermis serves several functions: protection against
water loss by way of transpiration, regulation of gas exchange,
secretion of metabolic compounds, and (in some species)
absorption of water. Most leaves show dorsiventral anatomy:
The upper (adaxial) and lower (abaxial) surfaces have somewhat
different construction and may serve different functions.
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The epidermis is usually transparent (epidermal cells lack
chloroplasts) and coated on the outer side with a waxy cuticle
that prevents water loss. The cuticle is in some cases thinner on
the lower epidermis than on the upper epidermis, and is generally
thicker on leaves from dry climates as compared with those from
wet climates (Mauseth and James, 2008).
The epidermis tissue includes several differentiated cell types
such as epidermal cells, epidermal hair cells (trichomes), stomata
complex, guard cells and subsidiary cells. The epidermal cells are
typically more elongated in the leaves of monocots than in those
of dicots.
The epidermis is studded with pores called stomata (fig.28) part
of a stoma complex consisting of a pore surrounded on each side
by chloroplast-containing guard cells, and two to four subsidiary
cells that lack chloroplasts. Opening and closing of the stoma
complex regulates the exchange of gases and water vapor
between the outside air and the interior of the leaf and plays an
important role in allowing photosynthesis without letting the leaf
dry out. In a typical leaf, the stomata are more numerous on the
abaxial (lower) epidermis than on the adaxial (upper) epidermis,
and more numerous in plants from cooler climates (Mauseth and
James, 2008).
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Fig. 30. Epidermis with stomata
Mesophyll
Most of the interior of the leaf between the upper and lower layers
of epidermis is a parenchyma (ground tissue) or chlorenchyma
tissue called the mesophyll (Greek for “middle leaf”). This
assembled tissue is the primary location where photosynthesis
occurs in the plant.
In ferns and most flowering plants, the mesophyll is divided into
two layers:
•
•
An upper palisade layer of tightly packed, vertically elongated
cells, one to two cells thick, directly beneath the adaxial
epidermis. Its cells contain many more chloroplasts than the
spongy layer. These long cylindrical cells are regularly arranged
in one to five rows.
Beneath the palisade layer is the spongy layer. The cells of
the spongy layer are more rounded and not so tightly packed.
There are large intercellular air spaces. These cells contain
fewer chloroplasts than those of the palisade layer. The pores
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or stomata of the epidermis open into sub-stomatal chambers,
which are connected to the air spaces between the spongy layer
cells.
These two different layers of the mesophyll are absent in
many aquatic and marsh plants. Instead, use a homogeneous
aerenchyma (thin-walled cells separated by large gas-filled
spaces) for their gaseous exchanges. Leaves are normally
green in color, which comes from chlorophyll found in plastids
in the chlorenchyma cells. Plants that lack chlorophyll can not
photosynthesize. In some plants variously coloured leaves are
present and their colour is determined by chromo-plasts.
Veins
The veins are the vascular tissue of the leaf and are located in
the spongy layer of the mesophyll. They are typical examples of
pattern formation through ramification. The pattern of the veins
is called venation.
The veins are made up of:
•
•
Xylem: vascular component that bring water and minerals from
the roots into the leaf.
Phloem: vascular component that usually move sap, with
dissolved sucrose, produced by photosynthesis in the leaf, out
of the leaf.
The xylem typically lies on the adaxial side of the vascular
bundle and the phloem typically lies on the abaxial side. Both
are embedded in a dense parenchyma tissue, called the pith
or sheath, which usually includes some structural collenchyma
tissue.