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Tropical Leaves


What is living?

Anything which shows characteristics of life is called living.

The fundamental characteristics of life are- growth, reproduction, metabolism, cellular organization and consciousness.



The fundamental characteristics of life are-

  1. Growth

  2. Reproduction

  3. Metabolism

  4. Cellular organization and

  5. Consciousness.


The fundamental characteristics of life are-

1. Growth: Increase in body mass is considered s growth.

Increase in body mass and increase in number of cells are twin characteristics of growth.

But growth is not a defining property of living organisms. Because, some non-living objects like mountains, boulders and sand mounds do grow. This kind of growth exhibited by non-living objects by accumulation of materials on the surface.


2. Reproduction: It’s a biological process in which an organism produces progeny of their own kind.

Lower organisms reproduce by asexual method like budding (hydra), spore formation (fungi), regeneration (planaria), fragmentation (few algae) et.,

Higher organisms reproduce through sexual method by producing gametes.

In unicellular organisms growth and reproduction occur by increasing number of cells. We are not clear about the usage of these two terms.

Reproduction is also cannot be a defining characteristic of living organisms. Because, many organisms which do not reproduce such as mules, sterile worker bees, infertile human couple etc.,


3. Metabolism: The sum total of all the chemical reactions occurring in our body is metabolism.

Small and big chemicals in our body are constantly being made and changed into some other biomolecules. These conversions are called metabolic reactions.

Non-living objects do not exhibit metabolism. So, metabolism is considered as defining character of organisms. However, metabolisms can be demonstrated outside the body in cell-free system.

Anabolism is the process of building up or synthesis of complex substances from simpler ones, e.g., Photo synthesis.

Catabolism is the process of breakdown of complex substances into simpler substances, e.g., Respiration, releasing waste outside.


Metabolism —> Anabolism + Catabolism


4. Cellular Organization: All living organisms made of cells without exception. Cell is the fundamental unit of living organisms where metabolism, growth and reproduction initiate. So, the cellular nature or organization is a defining characteristic of living organisms.


5. Consciousness: An ability of living organisms to sense their surroundings or environment and respond to environmental stimuli is called consciousness.

Plant respond to external factors like light, water, temperature, pollutants etc.,

In both plants and animals’ photoperiod affects reproduction in seasonal breeders.

So, all organisms are ‘aware’ of their surroundings. Human being is the only organism who is aware of himself, i.e., has self-consciousness. Hence consciousness becomes the defining property of living organisms.


There are about 1.7 – 1.8 million species are known and described.

Biodiversity can be defined as ‘the variety and variability of life on Earth.

Taxonomy: The systematic arrangement of organisms that includes characterization, identification, nomenclature and classification of organisms is called taxonomy.

Carl Linnaeus is considered as father of taxonomy.

The term taxonomy was first introduced by A.P de Candolle in1813.



The scientific method of naming an organism by using two words is called binomial nomenclature.

For plants scientific names are based on agreed principles and criteria, which are provided in International Code for Botanical Nomenclature (ICBN).

Animal taxonomists have evolved International Code of Zoological Nomenclature (ICZN).

The scientific names ensure that each organism has only one name. They also ensure that such a name has not been used for any other known organism.


Universal rules of nomenclature as follows:

  1. Biological names are generally in Latin and written in italics. They are Latinized or derived from Latin.

  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 word genus starts with a capital letter while the specific epithet starts with a small letter. E.g. Mangifera indica

  5. 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.


Advantages of Binomial Nomenclature

  1. Binomial names are universally acceptable and recognised.

  2. They remain same in all languages.

  3. The names are small and comprehensive.

  4. There is a mechanism to provide a scientific name to every newly discovered organism.

  5. The names indicate relationship of a species with other species present in the same genus.

  6. A new organism can be easily provided with a new scientific name.



Taxonomy: The systematic arrangement of organisms that includes characterization, identification, nomenclature and classification of organisms is called taxonomy.

External and internal structure, along with the structure of cell, development process and ecological information of organisms are essential and form the basis of modern taxonomic studies.

Taxonomical Hierarchy refers to the taxonomic classification of living organisms in successive levels of complexity. The highest rank in such a classification “Kingdom” while “species” is the lowest.


Taxa: a taxonomic group of any rank, such as a species, family, or class.

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SPECIES: Group of closely related organisms which interbreed themselves to produce fertile offspring is called species.

Based on the distinct morphological differences it is able to distinguish one species from the other closely related species.

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 epithet representing different organisms, but having morphological similarities.


GENUS: Genus is the aggregates of closely related species.

Genus comprises a group of related species which has more characters in common in comparison to species of other genera.

For example, potato, tomato and brinjal are three different species but all 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.


FAMILY: 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.

For examples, among plants 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.


ORDER: Order 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.

For examples, 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 Cancidae.



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.

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.

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.

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Taxonomical aids are techniques, procedures and stored information that are helpful in identification and classification of organisms.

Herbarium, Botanical gardens, Museum, Zoological parks and Key are some examples.



  • Herbarium is a store house of collected plant specimens that are dried, pressed and preserved on sheets.

  • Further, these sheets are arranged according to a universally accepted system of classification.

  • 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.


  • These are specialized gardens have collections of living plants for reference.

  • In botanical garden each plant is labelled with botanical name and its family, which is usful of identification.

  • The famous botanical gardens are at Kew (England), Indian Botanical Garden, Howrah (India) and at National Botanical Research Institute, Lucknow (India).

MUSEUM:  Museum have collections of preserved plant and animal specimens for study and reference. 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.


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.

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.Each statement in the key is called a lead.

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.