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)

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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

ORPHOLOGY OF

LOWERING

LANTS

HAPTER

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

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).

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

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

(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

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

Fleshy

leaves

Leaf

tendril

(a) Pea

(b) Cactus

Leaves

modified

into spines

Figure 5.9 Different types of phyllotaxy :

(a) Alternate (b) Opposite

(a) China rose

(b) Guava

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