Biology is the science of life forms and living processes. The living world

comprises an amazing diversity of living organisms. Early man could

easily perceive the difference between inanimate matter and living

organisms. Early man deified some of the inanimate matter (wind, sea,

fire etc.) and some among the animals and plants. A common feature of

all such forms of inanimate and animate objects was the sense of awe

or fear that they evoked. The description of living organisms including

human beings began much later in human history. Societies which

indulged in anthropocentric view of biology could register limited

progress in biological knowledge. Systematic and monumental

description of life forms brought in, out of necessity, detailed systems

of identification, nomenclature and classification. The biggest spin off

of such studies was the recognition of the sharing of similarities among

living organisms both horizontally and vertically. That all present day

living organisms are related to each other and also to all organisms

that ever lived on this earth, was a revelation which humbled man and

led to cultural movements for conservation of biodiversity. In the

following chapters of this unit, you will get a description, including

classification, of animals and plants from a taxonomist’s perspective.

DIVERSITY IN THE LIVING WORLD

Chapter 1

The Living World

Chapter 2

Biological Classification

Chapter 3

Plant Kingdom

Chapter 4

Animal Kingdom

UNIT 1

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Born on 5 July 1904, in Kempten, Germany, ERNST MAYR, the

Harvard University evolutionary biologist who has been called

‘The Darwin of the 20

century’, was one of the 100 greatest

scientists of all time. Mayr joined Harvard’s Faculty of Arts

and Sciences in 1953 and retired in 1975, assuming the title

Alexander Agassiz Professor of Zoology Emeritus. Throughout

his nearly 80-year career, his research spanned ornithology,

taxonomy, zoogeography, evolution, systematics, and the

history and philosophy of biology. He almost single-handedly

made the origin of species diversity the central question of

evolutionary biology that it is today. He also pioneered the

currently accepted definition of a biological species. Mayr was

awarded the three prizes widely regarded as the triple crown of

biology: the Balzan Prize in 1983, the International Prize for

Biology in 1994, and the Crafoord Prize in 1999. Mayr died at

the age of 100 in the year 2004.

Ernst Mayr

(1904 – 2004)

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How wonderful is the living world ! The wide range of living types is

amazing. The extraordinary habitats in which we find living organisms,

be it cold mountains, deciduous forests, oceans, fresh water lakes, deserts

or hot springs, leave us speechless. The beauty of a galloping horse, of

the migrating birds, the valley of flowers or the attacking shark evokes

awe and a deep sense of wonder. The ecological conflict and cooperation

among members of a population and among populations of a community

or even the molecular traffic inside a cell make us deeply reflect on – what

indeed is life? This question has two implicit questions within it. The first

is a technical one and seeks answer to what living is as opposed to the

non-living, and the second is a philosophical one, and seeks answer to

what the purpose of life is. As scientists, we shall not attempt answering

the second question. We will try to reflect on – what is living?

1.1 WHAT IS ‘LIVING’?

When we try to define ‘living’, we conventionally look for distinctive

characteristics exhibited by living organisms. Growth, reproduction, ability

to sense environment and mount a suitable response come to our mind

immediately as unique features of living organisms. One can add a few

more features like metabolism, ability to self-replicate, self-organise,

interact and emergence to this list. Let us try to understand each of these.

All living organisms grow. Increase in mass and increase in number

of individuals are twin characteristics of growth. A multicellular organism

IVING

ORLD

HAPTER

1.1 What is ‘Living’?

1.2 Diversity in the

Living World

1.3 Taxonomic

Categories

1.4 Taxonomical

Aids

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4 BIOLOGY

grows by cell division. In plants, this growth by cell division occurs

continuously throughout their life span. In animals, this growth is seen

only up to a certain age. However, cell division occurs in certain tissues to

replace lost cells. Unicellular organisms grow by cell division. One can

easily observe this in in vitro cultures by simply counting the number of

cells under the microscope. In majority of higher animals and plants,

growth and reproduction are mutually exclusive events. One must

remember that increase in body mass is considered as growth. Non-living

objects also grow if we take increase in body mass as a criterion for growth.

Mountains, boulders and sand mounds do grow. However, this kind of

growth exhibited by non-living objects is by accumulation of material on

the surface. In living organisms, growth is from inside. Growth, therefore,

cannot be taken as a defining property of living organisms. Conditions

under which it can be observed in all living organisms have to be explained

and then we understand that it is a characteristic of living systems. A

dead organism does not grow.

Reproduction, likewise, is a characteristic of living organisms.

In multicellular organisms, reproduction refers to the production of

progeny possessing features more or less similar to those of parents.

Invariably and implicitly we refer to sexual reproduction. Organisms

reproduce by asexual means also. Fungi multiply and spread easily due

to the millions of asexual spores they produce. In lower organisms like

yeast and hydra, we observe budding. In Planaria (flat worms), we observe

true regeneration, i.e., a fragmented organism regenerates the lost part of

its body and becomes, a new organism. The fungi, the filamentous algae,

the protonema of mosses, all easily multiply by fragmentation. When it

comes to unicellular organisms like bacteria, unicellular algae or Amoeba,

reproduction is synonymous with growth, i.e., increase in number of cells.

We have already defined growth as equivalent to increase in cell number

or mass. Hence, we notice that in single-celled organisms, we are not very

clear about the usage of these two terms – growth and reproduction.

Further, there are many organisms which do not reproduce (mules, sterile

worker bees, infertile human couples, etc). Hence, reproduction also cannot

be an all-inclusive defining characteristic of living organisms. Of course,

no non-living object is capable of reproducing or replicating by itself.

Another characteristic of life is metabolism. All living organisms

are made of chemicals. These chemicals, small and big, belonging to

various classes, sizes, functions, etc., are constantly being made and

changed into some other biomolecules. These conversions are chemical

reactions or metabolic reactions. There are thousands of metabolic

reactions occurring simultaneously inside all living organisms, be they

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unicellular or multicellular. All plants, animals, fungi and microbes exhibit

metabolism. The sum total of all the chemical reactions occurring in our

body is metabolism. No non-living object exhibits metabolism. Metabolic

reactions can be demonstrated outside the body in cell-free systems. An

isolated metabolic reaction(s) outside the body of an organism, performed

in a test tube is neither living nor non-living. Hence, while metabolism is

a defining feature of all living organisms without exception, isolated

metabolic reactions in vitro are not living things but surely living reactions.

Hence, cellular organisation of the body is the defining feature of

life forms.

Perhaps, the most obvious and technically complicated feature of all

living organisms is this ability to sense their surroundings or environment

and respond to these environmental stimuli which could be physical,

chemical or biological. We sense our environment through our sense

organs. Plants respond to external factors like light, water, temperature,

other organisms, pollutants, etc. All organisms, from the prokaryotes to

the most complex eukaryotes can sense and respond to environmental

cues. Photoperiod affects reproduction in seasonal breeders, both plants

and animals. All organisms handle chemicals entering their bodies. All

organisms therefore, are ‘aware’ of their surroundings. Human being is

the only organism who is aware of himself, i.e., has self-consciousness.

Consciousness therefore, becomes the defining property of living

organisms.

When it comes to human beings, it is all the more difficult to define

the living state. We observe patients lying in coma in hospitals virtually

supported by machines which replace heart and lungs. The patient is

otherwise brain-dead. The patient has no self-consciousness. Are such

patients who never come back to normal life, living or non-living?

In higher classes, you will come to know that all living phenomena

are due to underlying interactions. Properties of tissues are not present

in the constituent cells but arise as a result of interactions among the

constituent cells. Similarly, properties of cellular organelles are not present

in the molecular constituents of the organelle but arise as a result of

interactions among the molecular components comprising the organelle.

These interactions result in emergent properties at a higher level of

organisation. This phenomenon is true in the hierarchy of organisational

complexity at all levels. Therefore, we can say that living organisms are

self-replicating, evolving and self-regulating interactive systems capable

of responding to external stimuli. Biology is the story of life on earth.

Biology is the story of evolution of living organisms on earth. All living

organisms – present, past and future, are linked to one another by the

sharing of the common genetic material, but to varying degrees.

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1.2 DIVERSITY IN THE LIVING WORLD

If you look around you will see a large variety of living organisms, be it

potted plants, insects, birds, your pets or other animals and plants. There

are also several organisms that you cannot see with your naked eye but

they are all around you. If you were to increase the area that you make

observations in, the range and variety of organisms that you see would

increase. Obviously, if you were to visit a dense forest, you would probably

see a much greater number and kinds of living organisms in it. Each

different kind of plant, animal or organism that you see, represents a

species. The number of species that are known and described range

between 1.7-1.8 million. This refers to biodiversity or the number and

types of organisms present on earth. We should remember here that as

we explore new areas, and even old ones, new organisms are continuously

being identified.

As stated earlier, there are millions of plants and animals in the world;

we know the plants and animals in our own area by their local names.

These local names would vary from place to place, even within a country.

Probably you would recognise the confusion that would be created if we

did not find ways and means to talk to each other, to refer to organisms

we are talking about.

Hence, there is a need to standardise the naming of living organisms

such that a particular organism is known by the same name all over the

world. This process is called nomenclature. Obviously, nomenclature or

naming is only possible when the organism is described correctly and we

know to what organism the name is attached to. This is identification.

In order to facilitate the study, number of scientists have established

procedures to assign a scientific name to each known organism. This is

acceptable to biologists all over the world. For plants, scientific names are

based on agreed principles and criteria, which are provided in International

Code for Botanical Nomenclature (ICBN). You may ask, how are animals

named? Animal taxonomists have evolved International Code of Zoological

Nomenclature (ICZN). The scientific names ensure that each organism

has only one name. Description of any organism should enable the people

(in any part of the world) to arrive at the same name. They also ensure

that such a name has not been used for any other known organism.

Biologists follow universally accepted principles to provide scientific

names to known organisms. Each name has two components – the

Generic name and the specific epithet. This system of providing a name

with two components is called Binomial nomenclature. This naming

system given by Carolus Linnaeus is being practised by biologists all

over the world. This naming system using a two word format was found

convenient. Let us take the example of mango to understand the way of

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providing scientific names better. The scientific name of mango is written

as Mangifera indica. Let us see how it is a binomial name. In this name

Mangifera represents the genus while indica, is a particular species, or a

specific epithet. Other universal rules of nomenclature are as follows:

1. Biological names are generally in Latin and written in italics.

They are Latinised or derived from Latin irrespective of their

origin.

2. The first word in a biological name represents the genus while

the second component denotes the specific epithet.

3. Both the words in a biological name, when handwritten, are

separately underlined, or printed in italics to indicate their Latin

origin.

4. The first word denoting the genus starts with a capital letter

while the specific epithet starts with a small letter. It can be

illustrated with the example of Mangifera indica.

Name of the author appears after the specific epithet, i.e., at the end of

the biological name and is written in an abbreviated form, e.g., Mangifera

indica Linn. It indicates that this species was first described by Linnaeus.

Since it is nearly impossible to study all the living organisms, it is

necessary to devise some means to make this possible. This process is

classification. Classification is the process by which anything is grouped

into convenient categories based on some easily observable characters.

For example, we easily recognise groups such as plants or animals or

dogs, cats or insects. The moment we use any of these terms, we associate

certain characters with the organism in that group. What image do you

see when you think of a dog ? Obviously, each one of us will see ‘dogs’

and not ‘cats’. Now, if we were to think of ‘Alsatians’ we know what we are

talking about. Similarly, suppose we were to say ‘mammals’, you would,

of course, think of animals with external ears and body hair. Likewise, in

plants, if we try to talk of ‘Wheat’, the picture in each of our minds will be

of wheat plants, not of rice or any other plant. Hence, all these - ‘Dogs’,

‘Cats’, ‘Mammals’, ‘Wheat’, ‘Rice’, ‘Plants’, ‘Animals’, etc., are convenient

categories we use to study organisms. The scientific term for these

categories is taxa. Here you must recognise that taxa can indicate

categories at very different levels. ‘Plants’ – also form a taxa. ‘Wheat’ is

also a taxa. Similarly, ‘animals’, ‘mammals’, ‘dogs’ are all taxa – but you

know that a dog is a mammal and mammals are animals. Therefore,

‘animals’, ‘mammals’ and ‘dogs’ represent taxa at different levels.

Hence, based on characteristics, all living organisms can be classified

into different taxa. This process of classification is taxonomy. External

and internal structure, along with the structure of cell, development

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process and ecological information of organisms are essential and form

the basis of modern taxonomic studies.

Hence, characterisation, identification, classification and nomenclature

are the processes that are basic to taxonomy.

Taxonomy is not something new. Human beings have always been

interested in knowing more and more about the various kinds of

organisms, particularly with reference to their own use. In early days,

human beings needed to find sources for their basic needs of food, clothing

and shelter. Hence, the earliest classifications were based on the ‘uses’ of

various organisms.

Human beings were, since long, not only interested in knowing more

about different kinds of organisms and their diversities, but also the

relationships among them. This branch of study was referred to as

systematics. The word systematics is derived from the Latin word

‘systema’ which means systematic arrangement of organisms. Linnaeus

used Systema Naturae as the title of his publication. The scope of

systematics was later enlarged to include identification, nomenclature

and classification. Systematics takes into account evolutionary

relationships between organisms.

1.3 TAXONOMIC CATEGORIES

Classification is not a single step process but involves hierarchy of steps

in which each step represents a rank or category. Since the category is a

part of overall taxonomic arrangement, it is called the taxonomic category

and all categories together constitute the taxonomic hierarchy. Each

category, referred to as a unit of classification, in fact, represents a rank

and is commonly termed as taxon (pl.: taxa).

Taxonomic categories and hierarchy can be illustrated by an example.

Insects represent a group of organisms sharing common features like

three pairs of jointed legs. It means insects are recognisable concrete

objects which can be classified, and thus were given a rank or category.

Can you name other such groups of organisms? Remember, groups

represent category. Category further denotes rank. Each rank or taxon,

in fact, represents a unit of classification. These taxonomic groups/

categories are distinct biological entities and not merely morphological

aggregates.

Taxonomical studies of all known organisms have led to the

development of common categories such as kingdom, phylum or division

(for plants), class, order, family, genus and species. All organisms,

including those in the plant and animal kingdoms have species as the

lowest category. Now the question you may ask is, how to place an

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organism in various categories? The basic requirement is the knowledge

of characters of an individual or group of organisms. This helps in

identifying similarities and dissimilarities among the individuals of the

same kind of organisms

as well as of other kinds of organisms.

1.3.1 Species

Taxonomic studies consider a group of individual organisms with

fundamental similarities as a species. One should be able to distinguish

one species from the other closely related species based on the distinct

morphological differences. Let us consider Mangifera indica, Solanum

tuberosum (potato) and Panthera leo (lion). All the three names, indica,

tuberosum and leo, represent the specific epithets, while the first words

Mangifera, Solanum and Panthera are genera and represents another

higher level of taxon or category. Each genus may have one or more than

one specific epithets representing different organisms, but having

morphological similarities. For example, Panthera has another specific

epithet called tigris and Solanum includes species like nigrum and

melongena. Human beings belong to the species sapiens which is

grouped in the genus Homo. The scientific name thus, for human being,

is written as Homo sapiens.

1.3.2 Genus

Genus comprises a group of related species which has more characters

in common in comparison to species of other genera. We can say that

genera are aggregates of closely related species. For example, potato and

brinjal are two different species but both belong to the genus Solanum.

Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with several

common features, are all species of the genus

Panthera. This genus differs

from another genus Felis which includes cats.

1.3.3 Family

The next category, Family, has a group of related genera with still less

number of similarities as compared to genus and species. Families are

characterised on the basis of both vegetative and reproductive features of

plant species. Among plants for example, three different genera Solanum,

Petunia and Datura are placed in the family Solanaceae. Among animals

for example, genus Panthera, comprising lion, tiger, leopard is put along

with genus, Felis (cats) in the family Felidae. Similarly, if you observe the

features of a cat and a dog, you will find some similarities and some

differences as well. They are separated into two different families – Felidae

and Canidae, respectively.

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1.3.4 Order

You have seen earlier that categories like species, genus and

families are based on a number of similar characters. Generally,

order and other higher taxonomic categories are identified based

on the aggregates of characters. Order being a higher category,

is the assemblage of families which exhibit a few similar

characters. The similar characters are less in number as

compared to different genera included in a family. Plant families

like Convolvulaceae, Solanaceae are included in the order

Polymoniales mainly based on the floral characters. The animal

order, Carnivora, includes families like Felidae and Canidae.

1.3.5 Class

This category includes related orders. For example, order Primata

comprising monkey, gorilla and gibbon is placed in class

Mammalia along with order Carnivora that includes animals like

tiger, cat and dog. Class Mammalia has other orders also.

1.3.6 Phylum

Classes comprising animals like fishes, amphibians, reptiles, birds

along with mammals constitute the next higher category called

Phylum. All these, based on the common features like presence

of notochord and dorsal hollow neural system, are included in

phylum Chordata. In case of plants, classes with a few similar

characters are assigned to a higher category called Division.

1.3.7 Kingdom

All animals belonging to various phyla are assigned to the

highest category called Kingdom Animalia in the classification

system of animals. The

Kingdom Plantae, on the other hand, is

distinct, and comprises all plants from various divisions.

Henceforth,

we will refer to these two groups as animal and

plant kingdoms.

The taxonomic categories from species to kingdom have been

shown in ascending order starting with species in Figure 1.1.

These are broad categories. However, taxonomists have also

developed sub-categories in this hierarchy to facilitate more

sound and scientific placement of various taxa.

Look at the hierarchy in Figure 1.1. Can you recall the basis

of arrangement? Say, for example, as we go higher from species

to kingdom, the number of common characteristics goes on

Figure 1.1 Taxonomic

categories

showing

hierarchial

arrangement

in ascending

order

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1.4 TAXONOMICAL AIDS

Taxonomic studies of various species of plants, animals and other

organisms are useful in agriculture, forestry, industry and in general in

knowing our bio-resources and their diversity. These studies would

require correct classification and identification of organisms. Identification

of organisms requires intensive laboratory and field studies. The collection

of actual specimens of plant and animal species is essential and is the

prime source of taxonomic studies. These are also fundamental to studies

and essential for training in systematics. It is used for classification of an

organism, and the information gathered is also stored along with the

specimens. In some cases the specimen is preserved for future studies.

Biologists have established certain procedures and techniques to store

and preserve the information as well as the specimens. Some of these are

explained to help you understand the usage of these aids.

1.4.1 Herbarium

Herbarium is a store house of collected plant specimens that are dried,

pressed and preserved on sheets. Further, these sheets are arranged

decreasing. Lower the taxa, more are the characteristics that the members

within the taxon share. Higher the category, greater is the difficulty of

determining the relationship to other taxa at the same level. Hence, the

problem of classification becomes more complex.

Table 1.1 indicates the taxonomic categories to which some common

organisms like housefly, man, mango and wheat belong.

Common Biological Genus Family Order Class Phylum/

Name Name Division

Man Homo sapiens Homo Hominidae Primata Mammalia Chordata

Housefly Musca Musca Muscidae Diptera Insecta Arthropoda

domestica

Mango Mangifera Mangifera Anacardiaceae Sapindales Dicotyledonae Angiospermae

indica

Wheat Triticum Triticum Poaceae Poales Monocotyledonae Angiospermae

aestivum

TABLE 1.1 Organisms with their Taxonomic Categories

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according to a universally accepted system of classification. These

specimens, along with their descriptions on herbarium sheets, become a

store house or repository for future use (Figure 1.2). The herbarium sheets

also carry a label providing information about date and place of collection,

English, local and botanical names, family, collector’s name, etc. Herbaria

also serve as quick referral systems in taxonomical studies.

1.4.2 Botanical Gardens

These specialised gardens have collections of living plants for reference.

Plant species in these gardens are grown for identification purposes and

each plant is labelled indicating its botanical/scientific name and its family.

The famous botanical gardens are at Kew (England), Indian Botanical

Garden, Howrah (India) and at National Botanical Research Institute,

Lucknow (India).

1.4.3 Museum

Biological museums are generally set up in educational institutes such

as schools and colleges. Museums have collections of preserved plant

and animal specimens for study and reference. Specimens are preserved

in the containers or jars in preservative solutions. Plant and animal

specimens may also be preserved as dry specimens. Insects are preserved

in insect boxes after collecting, killing and pinning. Larger animals like

birds and mammals are usually stuffed and preserved. Museums often

have collections of skeletons of animals too.

Figure 1.2 Herbarium showing stored specimens

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1.4.4 Zoological Parks

These are the places where wild animals are kept in protected environments

under human care and which enable us to learn about their food habits

and behaviour. All animals in a zoo ar

e provided, as far as possible, the

conditions similar to their natural habitats. Children love visiting these

parks, commonly called Zoos (Figure 1.3).

Figure 1.3 Pictures showing animals in different zoological parks of India

1.4.5 Key

Key is another taxonomical aid used for identification of plants and animals

based on the similarities and dissimilarities. The keys are based on the

contrasting characters generally in a pair called couplet. It represents

the choice made between two opposite options. This results in acceptance

of only one and rejection of the other. Each statement in the key is called

a lead. Separate taxonomic keys are required for each taxonomic category

such as family, genus and species for identification purposes. Keys are

generally analytical in nature.

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SUMMARY

The living world is rich in variety. Millions of plants and animals have been

identified and described but a large number still remains unknown. The very

range of organisms in terms of size, colour, habitat, physiological and

morphological features make us seek the defining characteristics of living

organisms. In order to facilitate the study of kinds and diversity of organisms,

biologists have evolved certain rules and principles for identification, nomenclature

and classification of organisms. The branch of knowledge dealing with these aspects

is referred to as taxonomy. The taxonomic studies of various species of plants

and animals are useful in agriculture, forestry, industry and in general for knowing

our bio-resources and their diversity. The basics of taxonomy like identification,

naming and classification of organisms are universally evolved under international

codes. Based on the resemblances and distinct differences, each organism is

identified and assigned a correct scientific/biological name comprising two words

as per the binomial system of nomenclature. An organism represents/occupies a

place or position in the system of classification. There are many categories/ranks

and are generally referred to as taxonomic categories or taxa. All the categories

constitute a taxonomic hierarchy.

Taxonomists have developed a variety of taxonomic aids to facilitate

identification, naming and classification of organisms. These studies are carried

out from the actual specimens which are collected from the field and preserved as

referrals in the form of herbaria, museums and in botanical gardens and zoological

parks. It requires special techniques for collection and preservation of specimens

in herbaria and museums. Live specimens, on the other hand, of plants and

animals, are found in botanical gardens or in zoological parks. Taxonomists also

prepare and disseminate information through manuals and monographs for

further taxonomic studies. Taxonomic keys are tools that help in identification

based on characteristics.

Flora, manuals, monographs and catalogues are some other means

of recording descriptions. They also help in correct identification. Flora

contains the actual account of habitat and distribution of plants of a

given area. These provide the index to the plant species found in a

particular area. Manuals are useful in providing information for

identification of names of species found in an area. Monographs contain

information on any one taxon.

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EXERCISES

1. Why are living organisms classified?

2. Why are the classification systems changing every now and then?

3. What different criteria would you choose to classify people that you meet often?

4. What do we learn from identification of individuals and populations?

5. Given below is the scientific name of Mango. Identify the correctly written name.

Mangifera Indica

Mangifera indica

6. Define a taxon. Give some examples of taxa at different hierarchical levels.

7. Can you identify the correct sequence of taxonomical categories?

(a) Species Order Phylum Kingdom

(b) Genus Species Order Kingdom

8. Try to collect all the currently accepted meanings for the word ‘species’. Discuss

with your teacher the meaning of species in case of higher plants and animals

on one hand, and bacteria on the other hand.

9. Define and understand the following terms:

(i) Phylum (ii) Class (iii) Family (iv) Order (v) Genus

10. How is a key helpful in the identification and classification of an organism?

11. Illustrate the taxonomical hierarchy with suitable examples of a plant and an

animal.

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