Biology

Morphology
of
flowering
plants

THE 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,

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.

REGIONS OF THE ROOT
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.

MODIFICATIONS OF ROOT
Roots in some plants change their shape and
structure and become modified to perform
functions other than absorption and conduction
of water and minerals. They are modified for
support storage of food and respiration. Tap roots
of carrot, turnip and adventitious roots of sweet
potato, get swollen and store food. Can you give
some more such examples? Have you ever
wondered what those hanging structures are that
support a banyan tree? These are called prop
roots. Similarly, the stems of maize and
sugarcane have supporting roots coming out of
the lower nodes of the stem. These are called stilt
roots. In some plants such as Rhizophora growing
in swampy areas, many roots come out of the
ground and grow vertically upwards. Such roots,
called pneumatophores, help to get oxygen for
respiration.

A stem:
Is generally considered to be the
central axis of the plant.
supports the leaves and flowers of a
plant.
has nodes from which new shoots
and sometimes new roots can arise.
is usually found above-ground, but
can be modified and found belowground as well.

Stem Modifications
Tubers:

Potato Tuber

A tuber is an underground stem
that
stores
food.
We know a potato is a tuber
because it has nodes (eyes)
which produce new shoots.

Bulbs: Bulbs come in 2 basic types:

1. Tunicate bulbs; onions and
tulips are
examples.

Onion Bulb

Tunicate bulbs have:
A papery outer covering
called a tunic.
A compressed, modified stem
with nodes, called a basal
plate.
Fleshy scales made of
modified leaf tissue.
Onion Bulb shown in cross section.

Scaly Bulbs:

The other type of true bulb is a
scaly bulb. Scaly bulbs:
Lack a papery covering (tunic).
Have a basal plate.
Have fleshy scales made of leaf
tissue.
Do not form “onion rings” when
cut in cross-section.

Lilium in a scaly bulb

Rhizomes:

Rhizomes are:
underground stems.
horizontally-growing.
produce shoots and adventitious
roots.

The pachymorph rhizome of
Ginger

Stolons:

Stolons are:
Aboveground stems.
Grow horizontally.
Produce shoots and
adventitious roots.
Strawberry and Spider Plant are
examples of plants that
produce stolons.

Wild strawberry stolons.

THE 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.

VENATION: The arrangement of veins and the

Parallel and Reticulate
Venations

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.

TYPES OF LEAVES:

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

pinnately compound leaf and
palmately compound leaves

PHYLLOTAXY
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.

MODIFICATIONS OF LEAVES
Leaves are often modified to perform functions other
than photosynthesis. They are converted into tendrils
for climbing as in peas or into spines for defence as in
cacti. The fleshy leaves of onion and garlic store food.
In some plants such as Australian acacia, the leaves
are small and short-lived. The petioles in these plants
expand, become green and synthesise food. Leaves of
certain insectivorous plants such as pitcher plant,
venus-fly trap are also modified leaves.

THE 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
converted 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.

Cymose Inflorescence

Racemose Inflorescence

THE FLOWER
The flower is the reproductive unit in the angiosperms. It is
meant for sexual reproduction. A typical flower has four
different kinds of whorls arranged successively on the
swollen end of the stalk or pedicel, called thalamus or
receptacle. These are calyx, corolla, androecium and
gynoecium. Calyx and corolla are accessory organs, while
androecium and gynoecium are reproductive organs. In
some flowers like lily, the calyx and corolla are not distinct
and are termed as perianth. When a flower has both
androecium and gynoecium, it is bisexual. A flower having
either only stamens or only carpels is unisexual.

In symmetry, the flower may be actinomorphic (radial
symmetry) or zygomorphic (bilateral symmetry).
When a flower can be divided into two equal radial
halves in any radial plane passing through the centre,
it is said to be actinomorphic, e.g., mustard, datura,
chilly. When it can be divided into two similar halves
only in one particular vertical plane, it is zygomorphic,
e.g., pea, gulmohur, bean, Cassia. A flower is
asymmetric (irregular) if it cannot be divided into two
similar halves by any vertical plane passing through
the centre, as in canna.
A flower may be trimerous, tetramerous or
pentamerous when the floral appendages are in
multiple of 3, 4 or 5, respectively. Flowers with bracts –
reduced leaf found at the base of the pedicel – are
called bracteate and those without bracts, ebracteate

Position of floral parts on
Thalamus : (a) Hypogynous (b) and
(c) Perigynous (d) Epigynous

Parts of Flower.
Each flower normally has four floral whorls, viz., calyx, corolla,
androecium and gynoecium.
CALYX
The calyx is the outermost whorl of the flower and the members are
called sepals. Generally, sepals are green, leaf like and protect the flower
in the bud stage. The calyx may be gamosepalous (sepals united) or
polysepalous (sepals free).
COROLLA
Corolla is composed of petals. Petals are usually brightly coloured to
attract insects for pollination. Like calyx, corolla may be also united
gamopetalous (petals united) or polypetalous (petals free). The shape
and color of corolla vary greatly in plants. Corolla may be tubular, bellshaped, funnel-shaped or wheel-shaped.

Aestivation


The mode of arrangement of sepals or petals in floral bud
with respect to the other members of the same whorl is
known as aestivation. The main types of aestivation are
valvate, twisted, imbricate and vexillary . When sepals or
petals in a whorl just touch one another at the margin,
without overlapping, as in Calotropis, it is said to be
valvate. If one margin of the appendage overlaps that of
the next one and so on as in china rose, lady’s finger and
cotton, it is called twisted. If the margins of sepals or
petals overlap one another but not in any particular
direction as in Cassia and gulmohur, the aestivation is
called imbricate. In pea and bean flowers, there are five
petals, the largest (standard) overlaps the two lateral
petals (wings) which in turn overlap the two smallest
anterior petals (keel); this type of aestivation is known as
vexillary or papilionaceous.

Types of aestivation in corolla : (a)
Valvate (b) Twisted (c) Imbricate
(d) Vexillary

Source: NCERT Textbook,
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