UNIT 2
The description of the diverse forms of life on earth was made only by
observation – through naked eyes or later through magnifying lenses
and microscopes. This description is mainly of gross structural features,
both external and internal. In addition, observable and perceivable
living phenomena were also recorded as part of this description. Before
experimental biology or more specifically, physiology, was established
as a part of biology, naturalists described only biology. Hence, biology
remained as a natural history for a long time. The description, by itself,
was amazing in terms of detail. While the initial reaction of a student
could be boredom, one should keep in mind that the detailed description,
was utilised in the later day reductionist biology where living processes
drew more attention from scientists than the description of life forms
and their structure. Hence, this description became meaningful and
helpful in framing research questions in physiology or evolutionary
biology. In the following chapters of this unit, the structural organisation
of plants and animals, including the structural basis of physiologial or
behavioural phenomena, is described. For convenience, this description
of morphological and anatomical features is presented separately for
plants and animals.
STRUCTURAL ORGANISATION
IN PLANTS AND ANIMALS
Chapter 5
Morphology of
Flowering Plants
Chapter 6
Anatomy of Flowering
Plants
Chapter 7
Structural Organisation in
Animals
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KATHERINE ESAU was born in Ukraine in 1898. She studied
agriculture in Russia and Germany and received her doctorate
in 1931 in United States. She reported in her early publications
that the curly top virus spreads through a plant via the food-
conducting or phloem tissue. Dr Esau’s Plant Anatomy published
in 1954 took a dynamic, developmental approach designed to
enhance one’s understanding of plant structure and an
enormous impact worldwide, literally bringing about a revival
of the discipline. The Anatomy of Seed Plants by Katherine Esau
was published in 1960. It was referred to as Webster’s of plant
biology – it is encyclopediac. In 1957 she was elected to the
National Academy of Sciences, becoming the sixth woman to
receive that honour. In addition to this prestigious award, she
received the National Medal of Science from President George
Bush in 1989.
When Katherine Esau died in the year 1997, Peter Raven,
director of Anatomy and Morphology, Missouri Botanical
Garden, remembered that she ‘absolutely dominated’ the field
of plant biology even at the age of 99.
Katherine Esau
(1898 – 1997)
2020-21
The wide range in the structure of higher plants will never fail to fascinate
us. Even though 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.
In chapters 2 and 3, we talked about classification of plants based
on morphological and other characteristics. For any successful attempt
at classification and at understanding any higher plant (or for that
matter any living organism) we need to know standard technical terms
and standard definitions. We also need to know about the 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.
If you pull out any weed you will see that all of them have roots, stems
and leaves. They may be bearing flowers and fruits. The underground
part of the flowering plant is the root system while the portion above the
ground forms the shoot system (Figure 5.1).
5.1 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,
tertiary, etc. roots. The primary roots and its branches constitute the
M
ORPHOLOGY OF
F
LOWERING
P
LANTS
C
HAPTER
5
5.1 The Root
5.2 The Stem
5.3 The Leaf
5.4 The Inflorescence
5.5 The Flower
5.6 The Fruit
5.7 The Seed
5.8 Semi-technical
Description of a
Typical
Flowering Plant
5.9 Description of
Some Important
Families
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66 BIOLOGY
Flower
Shoot
system
Root
system
Fruit
Bud
Stem
Leaf
Node
Internode
Primary
root
Secondary
root
{
Figure 5.2 Different types of roots : (a) Tap (b) Fibrous (c) Adventitious
(c)(b)
Figure 5.1 Parts of a flowering plant
Fibrous roots
Adventitious roots
Laterals
(a)
Main root
tap root system, as seen in the mustard
plant (Figure 5.2a). 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 (Figure
5.2b). 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
(Figure 5.2c). 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.
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MORPHOLOGY OF FLOWERING PLANTS
67
5.1.1 Regions of the Root
The root is covered at the apex by a thimble-like
structure called the
root cap (Figure 5.3).
It
protects the tender apex of the root as it makes
its way through the soil. A few millimetres 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.
5.1.2 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 (Figure 5.4 and 5.5). 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 that
support a banyan tree are? 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 (Figure 5.5b).
Figure 5.3 The regions of the root-tip
Figure 5.4 Modification of root for support:
Banyan tree
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68 BIOLOGY
5.2 THE STEM
What are the features that distinguish a stem from a root? 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.
5.2.1 Modifications of Stem
The stem may not always be typically like what they are expected to be.
They are modified to perform different functions (Figure 5.6). Underground
stems of potato, ginger, turmeric, zaminkand, Colocasia are modified to
store food in them. They also act as organs of perennation to tide over
conditions unfavourable for growth. Stem tendrils which develop from
axillary buds, are slender and spirally coiled and help plants to climb
such as in gourds (cucumber, pumpkins, watermelon) and grapevines.
Axillary buds of stems may also get modified into woody, straight and
pointed thorns. Thorns are found in many plants such as Citrus,
Bougainvillea. They protect plants from browsing animals. Some plants
of arid regions modify their stems into flattened (Opuntia), or fleshy
cylindrical (Euphorbia) structures. They contain chlorophyll and carry
Figure 5.5 Modification of root for : (a) storage (b) respiration: pneumatophore in
Rhizophora
(b)
(a)
Carrot
Turnip
Sweet
potato
Asparagus
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MORPHOLOGY OF FLOWERING PLANTS
69
Figure 5.6 Modifications of stem for : (a) storage (b) support (c) protection
(d) spread and vegetative propagation
out photosynthesis. Underground stems of some plants such as grass
and strawberry, etc., spread to new niches and when older parts die new
plants are formed. In plants like mint and jasmine a slender lateral branch
arises from the base of the main axis and after growing aerially for some
time arch downwards to touch the ground. A lateral branch with short
internodes and each node bearing a rosette of leaves and a tuft of roots is
found in aquatic plants like Pistia and Eichhornia. In banana, pineapple
and Chrysanthemum, the lateral branches originate from the basal and
underground portion of the main stem, grow horizontally beneath the
soil and then come out obliquely upward giving rise to leafy shoots.
5.3 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.
A typical leaf consists of three main parts: leaf base, petiole and lamina
(Figure 5.7 a). The leaf
is attached to the stem by the leaf base and may
(a)
(b)
(c)
(d)
Axillary bud
modified
into tendril
Roots arising
from nodes
Stem modified
into spine
Ginger
Zaminkand
Potato
Bougainvillea sp.
Oxalis sp.
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70 BIOLOGY
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.
5.3.1 Venation
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 (Figure 5.7 b). When the veins run
parallel to each other within a lamina, the venation
is termed as parallel (Figure 5.7 c). Leaves of
dicotyledonous plants generally possess reticulate
venation, while parallel venation is the characteristic
of most monocotyledons.
5.3.2 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
(Figure 5.8). 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.
Figure 5.7 Structure of a leaf :
(a) Parts of a leaf
(b) Reticulate venation
(c) Parallel venation
(b) (c)
(b) Silk Cotton
(a)
Lamina
Petiole
Stipule
Leaf
base
Axillary
bud
(a) Neem
Figure 5.8 Compound leaves :
(a) pinnately compound leaf
(b) palmately compound leaf
Rachis
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MORPHOLOGY OF FLOWERING PLANTS
71
In palmately compound leaves, the
leaflets are attached at a common point, i.e.,
at the tip of petiole, as in silk cotton.
5.3.3 Phyllotaxy
Phyllotaxy is the pattern of arrangement of
leaves on the stem or branch. This is usually
of three types – alternate, opposite and
whorled (Figure 5.9). 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.
5.3.4 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
(Figure 5.10 a, b). The fleshy leaves of onion
and garlic store food (Figure 5.10c). 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.
5.4 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
Figure 5.10 Modifications of leaf for :
(a) support: tendril (b) protection:
spines (c) storage: fleshy leaves
(c) Onion
Fleshy
leaves
Leaf
tendril
(a) Pea
(b) Cactus
Leaves
modified
into spines
Figure 5.9 Different types of phyllotaxy :
(a) Alternate (b) Opposite
(c) Whorled
(c) Alstonia
(a) China rose
(b) Guava
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