the angiosperms show such a large diversity in external structure or morphology, they are all
characterised by presence of roots, stems, leaves, flowers and fruits
possible variations in different parts, found as adaptations of the plants to their environment,
e.g., adaptions to various habitats, for protection, climbing, storage, etc
The underground part of the flowering plant is the root system while the portion above the
ground forms the shoot system
ROOT
In majority of the dicotyledonous plants, the direct elongation of the radicle leads to the
formation of primary root which grows inside the soil.
It bears lateral roots of several orders that are referred to as secondary, tertiary, etc. roots.
The primary roots and its branches constitute the tap root system, as seen in the mustard
plant.
In monocotyledonous plants, the primary root is short lived and is replaced by a large
number of roots. These roots originate from the base of the stem and constitute the fibrous
root system, as seen in the wheat plant.
In some plants, like grass, Monstera and the banyan tree, roots arise from parts of the plant
other than the radicle and are called adventitious roots.
The main functions of the root system are absorption of water and minerals from the soil,
providing a proper anchorage to the plant parts, storing reserve food material and synthesis
of plant growth regulators.
The root is covered at the apex by a thimble-like structure called the root cap. It protects the
tender apex of the root as it makes its way through the soil.
A few millimeters above the root cap is the region of meristematic activity. The cells of this
region are very small, thin-walled and with dense protoplasm. They divide repeatedly.
The cells proximal to this region undergo rapid elongation and enlargement and are
responsible for the growth of the root in length. This region is called the region of
elongation.
The cells of the elongation zone gradually differentiate and mature. Hence, this zone,
proximal to region of elongation, is called the region of maturation.
From this region some of the epidermal cells form very fine and delicate, thread-like
structures called root hairs. These root hairs absorb water and minerals from the soil
Stem
The stem is the ascending part of the axis bearing branches, leaves, flowers and fruits. It
develops from the plumule of the embryo of a germinating seed.
The stem bears nodes and internodes. The region of the stem where leaves are born are called
nodes while internodes are the portions between two nodes.
The stem bears buds, which may be terminal or axillary. Stem is generally green when young
and later often become woody and dark brown.
The main function of the stem is spreading out branches bearing leaves, flowers and fruits. It
conducts water, minerals and photosynthates. Some stems perform the function of storage of
food, support, protection and of vegetative propagation
Leaf
The leaf is a lateral, generally flattened structure borne on the stem. It develops at the node
and bears a bud in its axil. The axillary bud later develops into a branch.
Leaves originate from shoot apical meristems and are arranged in an acropetal order. They
are the most important vegetative organs for photosynthesis
A typical leaf consists of three main parts: leaf base, petiole and lamina .The leaf is attached
to the stem by the leaf base and may bear two lateral small leaf like structures called
stipules.
In monocotyledons, the leaf base expands into a sheath covering the stem partially or
wholly. In some leguminous plants the leafbase may become swollen, which is called the
pulvinus.
The petiole help hold the blade to light. Long thin flexible petioles allow leaf blades to flutter
in wind, thereby cooling the leaf and bringing fresh air to leaf surface.
The lamina or the leaf blade is the green expanded part of the leaf with veins and veinlets.
There is, usually, a middle prominent vein, which is known as the midrib.
Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals
and food materials. The shape, margin, apex, surface and extent of incision of lamina varies
in different leaves.
The arrangement of veins and the veinlets in the lamina of leaf is termed as venation. When the
veinlets form a network, the venation is termed as reticulate. When the veins run parallel to
each other within a lamina, the venation is termed as parallel.
Leaves of dicotyledonous plants generally possess reticulate venation, while parallel venation is
the characteristic of most monocotyledons
A leaf is said to be simple, when its lamina is entire or when incised, the incisions do not touch
the midrib.
When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets,
the leaf is called compound. 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.
The compound leaves may be of two types. In a pinnately compound leaf a number of leaflets
are present on a common axis, the rachis, which represents the midrib of the leaf as in neem.
In palmately compound leaves, the leaflets are attached at a common point, i.e., at the tip of
petiole, as in silk cotton.
Phyllotaxy is the pattern of arrangement of leaves on the stem or branch. This is usually of three
types – alternate, opposite and whorled.
In alternate type of phyllotaxy, a single leaf arises at each node in alternate manner, as in china
rose, mustard and sun flower plants.
In opposite type, a pair of leaves arise at each node and lie opposite to each other as in
Calotropis and guava plants. If more than two leaves arise at a node and form a whorl, it is
called whorled, as in Alstonia.
Inflorescence
A flower is a modified shoot wherein the shoot apical meristem changes to floral meristem.
Internodes do not elongate and the axis gets condensed.
The apex produces different kinds of floral appendages laterally at successive nodes instead
of leaves. When a shoot tip transforms into a flower, it is always solitary.
The arrangement of flowers on the floral axis is termed as inflorescence. Depending on
whether the apex gets developed into a flower or continues to grow, two major types of
inflorescences are defined – racemose and cymose.
In racemose type of inflorescences the main axis continues to grow, the flowers are borne
laterally in an acropetal succession. In cymose type of inflorescence the main axis
terminates in a flower, hence is limited in growth. The flowers are borne in a basipetal order
