THE LEAF
The leaf is a lateral, generally flattened structure on
the stem. It develops at the node and bears a bud in its
axil. The bud later develops into a branch. Leaves
originate from shoot apical meristems and are arran
an acropetal order. They are the most important
vegetative organs for photosynthesis.
Parts of a Leaf
A typical leaf has three main parts
i. Leaf Base ( Hypopodium)
The leaf is attached to the stem by the leaf base. In
monocots, the leaf base is said to be sheathing as it
expands and partially and wholly surrounds the stem. In
dicots, the leaf base bears two lateral outgrowths called
stipules.
In some leguminous plants, the leaf base may become
swollen which is called the pulvinus. Leaves with stipules
are called stipulate and those without them are termed as
exstipulate.
ii. Petiole (Mesopodium)
It is the stalk of a leaf. Petiole helps to hold the leaf blade
towards light. Petiole raises the lamina high to the level of
stem so as to provide maximum required exposure to light
and air.
iii. Lamina (Epipodium)
The lamina or leaf blade is the green, expanded part of the
leaf with veins and veinlets. It has a prominent median
vein called the midrib. It produces thinner lateral veins
in turn branch to form veinlets.
The lamina is the seat of photosynthesis, gaseous exchange,
transpiration and other metabolic activities. The shape,
margin, apex, surface and extent of incision of lamina vane
in different leaves.
Venation
The arrangement of veins and veinlets in the lamina of leaf
called venation. The midrib, veins and veinlets contain
vascular tissues, i.e., xylem and phloem for conduction
water, mineral salts and food.
Leaves have mainly two types of venation
i. Reticulate Venation
When the veinlets form a network, the venation is called
reticulate. It is found in dicot leaves. However, some
monocot leaves like Smilax, Dioscorea and Alocasia alq) show
reticulate venation.
ii. Parallel Venation
When the veins run parallel to each other within a lamina,
the venation is termed as parallel, e.g., Calophyllum,
Zingiber officinale, etc. It can be of the following types as
shown in the figure below.
Types of Leaves
Leaves can be of following types
i. Simple Leaves
A leaf having a single or undivided lamina is
called simple leaf. The lamina of a simple leaf
may be incised, but the incisions do not touch
the midrib. An axillary bud is always present in
the axil of leaf petiole and stem.
The lamina can have various types of incisions
which may reach upto half (fid), more than half
(partite) or near the base or midrib (sect).
ii. Compound Leaves
A leaf is called compound when the incision of
the leaf blade goes down to the midrib (rachis)
or to the petiole so that the leaf is broken up
into a number of segments called leaflets.
A bud is present in the axil of petiole in both
simple and compound leaves, but not in the
axil of leaflets of the compound leaf.
A compound leaf can be of following two types
(a) Pinnately Compound Leaves In these
leaves, the incision of lamina is directed
towards the midrib, which is known as
rachis. Leaflets are arranged on both side
on the rachis, e.g., neem, rose, etc.
(b) Palmate Compound Leaves The leaflets are attached at a
common point, i.e., at the tip of petiole as in silk cotton.
Phyllotaxy
The pattern of arrangement of leaves on the stem or branch is called
phyllotaxy. It helps to avoid overcrowding and provides every leaf with
optimum sunshine.
Phyllotaxy is usually of three types
i. Alternate (Spiral) Phyllotaxy
A single leaf arises at each node in alternate manner, e.g.. China rose,
mustard and sunflower plants.
ii. Opposite Phyllotaxy
A pair of leaves arises at each node and opposite to each other, e.g.,
Calotropis and Psidium guajava (guava plants).
iii- Whorled (Verticillate) Phyllotaxy
If more than two leaves arise at a node and form a whorl. it is called
whorled. The leaves of one whorl generally alternate with those of the
adjacent whorls in order to provide maximum exposure, e.g., Nerium
(kaner), Alstonia.
Modifications of Leaves
Leaves of plants are modified to perform different additional functions
in addition to their main function, i.e., photosynthesis.
i. Leaf Tendrils
These are thread-like sensitive structures, which can coil
around a support to help the plant in climbing, e.g., wild pea
(Lathyrus aphaca), Pisum sativum (sweet pea) and Gloriosa
superba (glory lily).
ii. Phyllode
It is a green, short-lived and flattened petiole or rachis of a
leaf, which performs the function of photosynthesis,
e.g., Australian Acacia. Phyllodes develop usually vertically
and possess fewer stomata hence, reduce transpiration.
iii. Bladder
The segments of the leaf modify into bladder-like structures,
which trap small insects present in the water, e.g.,
bladderwort ( Utricularia).
iv. Pitcher
It is a petiole modified into a tendril to hold the pitcher
upright. The leaf base is expanded to carry out
photosynthesis. The leaf apex is modified into a lid,
e.g., Nepenthes, Dischidia and Sarracenia.
v. Leaf Spines
The entire leaf or a part of a leaf may be modified into a
pointed structure called a spine, as in Opuntia.
vi. Scale Leaves
These are thin, membranous leaves found at the nodal
region. Each scale leaf contains an axillary bud in its axil,
e.g., Zingiber officinale (ginger).