Taxonomy is the branch of biology dealing with the identification, nomenclature and classification of organisms based on their characteristics. A taxon is a unit or group of organisms of any rank in classification (for example, mammals, insects, dogs). Taxonomic hierarchy is the arrangement of taxonomic categories in a definite descending order, from kingdom down to species.

Binomial nomenclature is the system of naming organisms with two words, given by Carolus Linnaeus, where the first word is the genus and the second is the species (for example, Mangifera indica for mango). Rules: (1) the generic name begins with a capital letter and the specific name with a small letter; (2) the biological name is printed in italics (or underlined separately when handwritten) and is usually in Latin.

The taxonomic categories in descending order are: Kingdom, Phylum (Division in plants), Class, Order, Family, Genus and Species. Species is the lowest category and represents a group of individual organisms with fundamental similarities that can interbreed. As we move up the hierarchy, the number of common characteristics decreases while the number of organisms in the group increases.

R. H. Whittaker (1969) proposed five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. The main criteria used were: cell structure (prokaryotic or eukaryotic), body organisation (unicellular or multicellular), mode of nutrition (autotrophic or heterotrophic), reproduction and phylogenetic relationships.

Kingdom Monera includes bacteria, which are prokaryotic (they lack a true membrane-bound nucleus and membrane-bound organelles). They are microscopic, unicellular organisms with a cell wall (except mycoplasma). Their nutrition may be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic). They are grouped as Archaebacteria (live in extreme habitats) and Eubacteria (true bacteria).

Kingdom Protista includes single-celled eukaryotes. They have a well-defined nucleus and membrane-bound organelles. Some have flagella or cilia for locomotion, and they may be autotrophic or heterotrophic. This kingdom forms a link between plants, animals and fungi. Examples: Chrysophytes (diatoms), dinoflagellates, Euglenoids (Euglena), slime moulds and protozoans (Amoeba, Paramecium).

Fungi are eukaryotic, mostly multicellular (except yeast), heterotrophic organisms with a cell wall made of chitin. Their body is made of thread-like hyphae forming a mycelium. Nutrition is saprophytic (on dead organic matter), parasitic or symbiotic; they store food as glycogen. The four classes are Phycomycetes (e.g. Rhizopus, Mucor), Ascomycetes (e.g. Penicillium, yeast), Basidiomycetes (e.g. mushroom, Puccinia) and Deuteromycetes (imperfect fungi, e.g. Alternaria).

Viruses are non-cellular infectious agents that are inert outside the host but reproduce inside a living host cell. A virus has a protein coat called a capsid enclosing genetic material of either DNA or RNA (never both). Viroids are the smallest infectious agents, made of free naked RNA without a protein coat (e.g. potato spindle tuber disease). Lichens are symbiotic associations between an alga (phycobiont) and a fungus (mycobiont) and are good indicators of air pollution.

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms. They reproduce vegetatively, asexually (by spores) and sexually. The three classes are: Chlorophyceae (green algae, with chlorophyll a and b, e.g. Spirogyra, Chlamydomonas); Phaeophyceae (brown algae, with chlorophyll a, c and fucoxanthin, e.g. Laminaria, Fucus); and Rhodophyceae (red algae, with chlorophyll a, d and phycoerythrin, e.g. Polysiphonia, Gracilaria).

Bryophytes are called the amphibians of the plant kingdom because they can live in soil but need water for sexual reproduction, as the male gametes (antherozoids) require a film of water to swim to the female sex organ. They lack true vascular tissue and true roots (they have rhizoids). Examples: Funaria (moss), Marchantia and Riccia (liverworts).

Pteridophytes are the first terrestrial plants to have true vascular tissues (xylem and phloem). The plant body is a sporophyte differentiated into true root, stem and leaves. They reproduce by spores produced in sporangia, and water is required for fertilisation. Examples: Selaginella, Equisetum and ferns such as Pteris and Dryopteris.

Gymnosperms are plants that bear naked seeds, that is, the seeds are not enclosed inside a fruit because the ovules are not enclosed in an ovary. Characteristics: (1) they are woody perennials (trees or shrubs); (2) the ovules are exposed and develop into naked seeds; (3) they are heterosporous, producing haploid microspores and megaspores, and the pollen grains are usually dispersed by wind. Examples: Cycas, Pinus and Ginkgo.

Animals are classified on the basis of certain fundamental features: (1) levels of organisation (cellular, tissue, organ or organ-system); (2) symmetry (asymmetrical, radial or bilateral); (3) germ layers (diploblastic or triploblastic); (4) presence and nature of a body cavity or coelom (acoelomate, pseudocoelomate or coelomate); (5) segmentation of the body; and (6) presence of a notochord.

Diploblastic animals have cells arranged in two embryonic germ layers, an outer ectoderm and an inner endoderm (e.g. coelenterates). Triploblastic animals have three germ layers: ectoderm, mesoderm and endoderm (e.g. from flatworms upward). A coelomate animal is one that has a true body cavity (coelom) lined by mesoderm, for example annelids, arthropods and chordates.

Porifera (sponges): (1) they are primitive multicellular animals with a cellular level of organisation and body with pores (ostia) and a canal system; (2) the body is supported by spicules and they show intracellular digestion (e.g. Sycon, Spongilla). Coelenterata (Cnidaria): (1) they are diploblastic with tissue-level organisation and radial symmetry; (2) they possess stinging cells called cnidoblasts and show two body forms, polyp and medusa (e.g. Hydra, jellyfish).

Arthropoda is the largest phylum of the animal kingdom. Characteristics: (1) they are triploblastic, bilaterally symmetrical, segmented and coelomate; (2) they have a chitinous exoskeleton and jointed appendages (the name means jointed legs); (3) the body is divided into head, thorax and abdomen, with an open circulatory system and respiration by gills, book gills, book lungs or tracheae. Examples: cockroach, prawn, butterfly, honeybee and spider.

The three fundamental (defining) characteristics of chordates are: (1) presence of a notochord, a flexible rod-like structure on the dorsal side that supports the body; (2) a dorsal, hollow, tubular nerve cord; and (3) paired pharyngeal gill slits at some stage of life. In addition they are triploblastic, coelomate, bilaterally symmetrical and have a post-anal tail. Examples: fishes, amphibians, reptiles, birds and mammals.

A tap root system has one main primary root growing vertically downward from which lateral roots (secondary and tertiary) arise; it develops from the radicle and is characteristic of dicots (e.g. mustard, mango). A fibrous root system has a cluster of thin, thread-like roots of similar size arising from the base of the stem; the primary root is short-lived and it is characteristic of monocots (e.g. wheat, grass).

Modifications of the stem include: (1) underground stems for storage such as the tuber of potato and the rhizome of ginger; (2) stem tendrils for climbing, as in gourds and grapevine; (3) thorns for protection, as in Citrus and Bougainvillea; and (4) runners or stolons for vegetative propagation, as in grass and strawberry. These modifications help in storage, support, protection or propagation.

Placentation is the arrangement of ovules within the ovary of a flower. Types: (1) Marginal - the placenta forms a ridge along the ventral suture and ovules are borne on it, as in pea. (2) Axile - the placenta is axial and ovules are attached to it in a multilocular ovary, as in China rose, tomato and lemon. (3) Parietal - ovules develop on the inner wall of the ovary, as in mustard. (4) Free central - ovules borne on a central axis with no septa, as in Dianthus. (5) Basal - a single ovule is attached at the base, as in sunflower.