Living organisms can be easily identified and distinguished from non-living objects. A virus shows both the characters of living and non-living.
A living organism shows 1) Growth
                                           2) Reproduction
                                           3) Metabolism
                                           4) Irritability

GROWTH: It is the fundamental property. An irreversible, Permanent increase in size, shape, dry weight due to cell division is called growth. Unicellular organisms show growth just by increase in size due to swelling. Cell division in them results in reproduction. Multicellular organisms show growth by cell division. The growth in plants is continuous through out their life span where as in animals it occurs only upto a certain stage.
              Non-living objects also show growth and it is from outside and not permanent. The growth is permanent as already said in living and it is from inside.
So growth is not a defining property of living organisms.

REPRODUCTION:
        Life begets life. Living organisms, whether unicellular or multicellular show sexual reproduction or asexual reproduction or both.
Algae: Filamentous algae like Spirogyra show asexual reproduction by spores
              & fragmentation. Algae also show sexual reproduction.
Fungi: Fragmentation is common. Yeast shows budding like Hydra (animal).
Bryophyta: Asexual reproduction is common by producing gemma tubers. In
                        Mosses Protonema shows fragmentation.
          Thus there are several examples for various kinds of reproduction in rest of the plant groups. Mules, sterile, worker bees, infertile human couples, Gall flowers, Neuter flowers do not show reproduction though they are very much living. Hence, reproduction is not a defining property of living organisms.

METABOLISM
          All chemical reactions which occur inside the living organisms are collectively called as Metabolism. It is 2 types. (1) Anabolism, (2) Catabolism.
Anabolism: It includes constructive processes. The most important one
                         is Photosynthesis. Protein syntheseis, lipid synthesis, DNA synthesis, 
                         storage, repair, reproduction, growth etc., are also anabolic. These are
                         all energy requiring reactions.
Catabolism: It includes destructive processes Respiration is the most important catabolic process. Other examples include digestion. These are energy evolving reactions.
There are no living organisms without showing metabolism. So we can say metabolism
                                        is a defining property of living organisms.

IRRITABILITY
            The power of living organisms to show response to the stimulus is called irritability. The organism is said to possess consciousness. The source of stimulus may be light water, air, temperature, sun, pollutants, chemicals and man. Gravitational force is also a source of stimulus. Plants, animals, micro-organisms whether unicellular or multicellular show irritability.

Thus we can say irritability or consciousness is the defining property of living organisms.
Exceptionally human beings show self-consciousness. But not all. Patients in coma do not show self-consciousness. Thus we can say self consciousness is not a defining property of human beings.

INTERACTIONS
              A unique phenomenon that is responsible for the living organisms to express life. The interactions result in emergent properties at a higher level like in
         1) Molecular interactions between glucose molecules in macromolecules like starch.
         2) Interactions between different organelles of a cell.
         3) Interactions between different cells of a tissue.
         4) Interactions between different continent tissues of a organ.
         5) Interactions between different organs of an organism.

But a property shown at higher level of organisation is not seen in its constiuents.
        All the living organisms, irrespective of time they live or lived have a common ancestral genetic stock.
The defining properties of living organism are metabolism, irritability and interactions
Conclusion: The living organisms are self replicating, evolving and self
                          regulating interactive systems capable of responding to the external stimuli.

 BIODIVERSITY

            It refers to the number of species of plants and animals microscopic or macro scopic living on the earth. In plants they include Algae Fungi Bryophytes Pteridophytes
Gymnosperms, Angiosperm, Bacteria, Lichen etc. The so far estimated identified number of species ranges between 1.7 to 1.8 million. This number may increase if the taxonomists explore new and even old areas. The number may even decrease due to extinction of some species. It occurs as a result of hunting, deforestation and pollution. Biodiversity is the richness of the universe.

TAXONOMY 

This term was coined by A.P. de Candolle (French Botanist). This word is derived from Greek (Taxis = arrangement, Nomos = law Taxonomy may be defined as the branch of Biology that deals with identification, nomenclature and (Botany) classification.
Linnaeus (Sweedish Naturalist) called it as "Systematics" which deals with the study of different kinds of organisms. Their derivatives and also the relationship among them. The term 'Systema' is derived from Latin, which means the systematic arrangement of organisms. Systematics also takes into account of evolutionary relationships between organisms. Later Sir Julian Huxley (1940) called it is new systematics.

The main objectives of Taxonomy are: 1) Identification
                                                                    2) Nomenclature and
                                                                    3) Classification

IDENTIFICATION
         It is the first step in Taxonomy. It means to determine whether the collected plant is entirely new or already known. B.S.I. (Botanical Survey of India) located at IBG (Indian Botanical Garden), Howrah helps in identification. It also has branches in other parts of India.

Identification can be done by
       1) Direct Method - by comparing with already existing herbarium specimens.
       2) Indirect Method - by taking the help of keys in floras.
       3) Modern Method - by computer punch card

NOMENCLATURE
       It is the next step after identification. Every plant is called by a local name in different parts of the state, country and world. It leads to confusion. To avoid this, one plant should be known by one name.
The systematic naming of an organism is called nomenclature. So every plant is given a scientific name based on principles and criteria proposed by ICBN (International Code of Botanical Nomenclature- Cambridge). It helps in proper communication and identification of a plant.
       According to it the scientific name of a plant has 2 parts It is called Binomial Nomenclature.
       It was first used by Gaspard Bauhin. He also used trinomial nomenclature. Linnaeus followed binomial nomenclature & popularised it through his "Species Plantarum". Later it is followed by all the biologists.

Acc. to ICBN
¤   Every plant should have only one correct scientific name.
¤   It must be in Latin or Latinised English.
¤   Binomial nomenclature must be followed.
¤   The first part of the name is Generic name
¤   The second part of the name is specific epither.
¤   Genus is a noun. Species name is adjective.
¤   Genus and species name should be written with capital and small letters respectively.
¤   When hand written, the names of Genus and species must be underlined separately.
e.g.: Mangifera Indica
¤   When printed, in Italics, they need not be underlined.
¤   The species name is followed by authors name in abbreviation.
e.g.: Mangifera indica Linn. (Linn = Linnaeus)
It indicates that the species was discovered by Linnaeus.
¤   If genus and species names are similar, they are called Tautonyms.
e.g.: Malus malus (Apple)
Tautonyms are not legally acceptable. So it is changed to Pyrus malus.
Cajanus cajan is not a tautonym.

CLASSIFICATION
          It is the ultimate objective of Taxonomy - Arrangement of plants into specific groups basing on their similarities and differences is called classification.
Each specific group is called Taxon. It is a unit of classification or Taxonomy or Taxonomic arrangement. All the taxons constitute taxonomic hierarchy. Various taxonomic units in hierarchy are as follows. (with one example)
           Plant kingdom             - plantae
           Division                        - Spermatophyta
           Sub division                 - Angiospermae
           Class                             - Dicotyledonae
           Sub - class                    -
           Series                            -
           Order                            - Sapindales 
           Family                           - Anacardiaceae
           Genus                            - Mangifera
           Species                          - Mangifera indica

Kingdom: It is the highest taxon. It consists sub-categories called Divisions.
Division: It is next to Kingdom consisting sub categories called Sub divisions. In
                   animals it is called Phylum.
Sub-division: For angiosperms it is the highest taxon. It includes 2 classes.
                             1) Dicotyledonae    2) Monocotyledonae
Class: Generally a teacher starts classification of an angiospermic plant from class on wards.
Subclass: Poly petalae, Gamo petalae etc., constitute sub classes.
Series: Sub class includes related series.
                e.g.: Thalamiflorae, Inferae, Coronariae
Order: It includes a group of families with some similarities.
               e.g.: Malvales, Rosales, Polymoniales etc.
Family: It is an important taxon. It includes closely related genera.
Genus: It is an aggregate of closely related species
Species: It is the basic unit of taxonomy consisting a group of plants with all
                  similar characters.
      A species belongs to one genus but a genus may have many species.
e.g.: Hibiscus rosasinensis
        Hibiscus micranthus
        Hibiscus cannabinus
Solanum melongena
        Solanum tuberosum
     As we move from kingdom to species the similarities increase. In reverse, as we move from species to kingdom, common characters decrease.

TAXONOMICAL  AIDS 

      These are useful mainly in identification field studies and other taxonomic studies. The prime source of taxonomical studies is the collection of actual specimens.
      Any institution, book or any unit that helps in identification, field studies which in turn are useful in agriculture, forestry, industry and in general, in knowing biodiversity is called Taxonomical aid. These are as follows:
Herbarium: It is a store house or repository of collected plant specimens. The twigs are collected, dried, pressed in between the old news papers. They are fixed on a sheet with gum tape. It is called herbarium sheet. It carries a label showing the following information.

       Date of collection
       Place of Collection
       Local name of the specimen
       English common name

  Botanical name
       Family
       Order
       Name of the Collector
       Name of the Institution
       Description
       Herbarium of India is preserved at BSI (Botanical Survey of India) located in Indian Botanical Gardens(Howrah). World's largest herbarium is preserved at Royal Botanic Gardens, Kew (London).
Botanic Gardens: The specialised gardens have collection of living plant specimens. These are labelled with 1) Common name 2) Botanical name 3) Family. These are useful for identification.
       The most famous, oldest and largest(in South East Asia) Botanic gardens is Indian Botanic (or Botanical) gardens. It is 225 years old. Now it is called Acharya Jagadish Chandra Bose Indian Botanic Gardens. It is located at Howrah (Kolkata). In India an other famous garden is located at National Botanic Research Institute(NBRI) Lucknow.
       There are only 2 more equally old and contemporary gardens.
¤   Royal Botanic Gardens, Kew (London)
      It is world's largest one.
¤   Botanic Gardens of Adelaide (Australia)
Museums (Bilogical): It is a house storing, preserving the dried, dead labelled, plant and stuffed animal specimens. They are preserved in Jars and Containers in preservative solutions like formalin. These are set up in Educational Institutions like Medical Colleges and Post Graduate Institutions.
Floras: All the plants of various species growing naturally at a particular area is called Flora. It gives the index of plant species with habitat and distribution. The first complete flora of British India was provided by J. D. Hooker in 7 volumes.
Manuals: These are books providing the information of names of various plant species found in an area.
Monographs: It is a book or paper written by an author who worked extensively on a single particular taxon. It gives maximum information about that taxon.
Catalogues: It provides the index of various plant species with scientific names of an area.
Keys: Important characters of plants and animals are prepared in pairs. Each pair
            of contrasting characters is called Couplet. Each character in the pair is called
            Lead. Each lead decides a taxon, and rejects the other - Separate keys
            are prepared for separate taxons.
            e.g.: 1) Reticulate Venation - lead
                    2) Parallel Venation - lead
      This pair is called couplet. First lead accepts dicotyledonae(class) and rejects Monocotyledonae the second lead accepts Monocotyledonae and rejects Dicotyledonae.

Another example: 1) Petals free
                                 2) Petals united
       In this couplet, first lead accepts polypetalae(sub-class) and second lead accepts Gamopetalae(another sub-class).
DIGITAL HERBARIUM: It is the most modern on line Internet facility that provides pictures of the herbarium specimens with related information. Now it is the easiest method and helps in the identification of specimens.