Classification of Plants and Related Organisms, Biology tutorial

Classification of Organisms:

Categorization of living and non-living objects around us provides us sense of order of world around us. People all over world have been categorizing plants and animals some time immemorial. Generally, such categorizations are utilitarian. Therefore plants have been categorized as those yielding medicines, grains, fruits, tubers, fibres and several other products. Plants have categorized as shrubs, herbs and trees. One of the objectives of categorizations is to recognize the particular organism. Characteristic size, shape, color and smell of leaves verifies identity of curry leaf plant. Biologists also categorize and name organisms. This is science of TAXONOMY. Not like common people who deal with limited number of organisms around them, biologists ought to name and categorize millions of organisms. More than 1.7 million organisms have been named, explained and categorized by biologists. Novel organisms are explained and named every day. It is evaluated that there may be approx 30 million organisms yet to be recognized. For taxonomists each species is evenly significant and as worth studying as any other species. Regarding vast number of organisms to be categorized it is not surprising that taxonomy is extremely specialized and challenging field of biology. As all organisms are associated to each other by common ancestors modern taxonomists are also interested in evolutionary history of organisms they study. The comprehensive study of diversity between organisms and setting up relationship among the organisms and meaning and categorizing them is called as systematic. Taxonomy then turns into section of systematic, managing only with naming and categorizing organisms. Though, such distinction is hardly ever made in common practice and most biologists use 2 terms, systematic and taxonomy, interchangeably.

Taxonomists recognize every species by providing the unique name to it. Name of species specific terms are in Latin. The common black pepper is recognized by scientific name, piper nigrum, name Linnaeus himself gave to the plant. Betel plant is associated but distinct species and is known by binomial piper betel. Close relationship between black pepper and betel leaf plants is pointed by common generic name, piper. By convention, generic and specific names of plants are written in italics or underlined.

Two species of Piper and other species of the genus, and other associated genera are clustered together in family Piperaceae. Associated families in turn can be grouped under the order, and associated orders under class. Classes in turn are clustered under division and at last kingdom. Black pepper and betel leaf plants are members of class Dicotyledonae of division Anthonphyta (flowering plants) of kingdom plantae. International Code of Botanical Nomenclature (ICBN) gives taxonomists with rules and suggestions to guide them in selection and application of names to TAXON. Taxon (plural, taxa) is any taxonomic category like species genus, family etc. Each plant which has correctly described, recognized and named is represented by the dried herbarium specimen in one of several reputed herbaria around world. This special specimen is called as type specimen and is final reference point in disputes concerning identity of species.

Earliest systems of categorization were artificial. In artificial system characters like flower color, habit of plant or shape of leaves may be utilized for categorization. While such categorization help in accurate identification of plant they don't explicitly bring out evolutionary relations between plants. When Linnaeus grouped plants known to him into 24 classes, he followed the artificial system of classification. He categorized plants on basis of number, union and length of stamens of the flowers. Most modern systems of categorization are natural. The natural system of categorization is also called as phylogenetic system. The phyloenetic system of categorization groups organisms according to evolutionary affinities. The variety of characters is utilized to arrive at such categorization. These may be biochemical, anatomical, morphological, and molecular. In recent years variations which exist at molecular level in organization of DNA, ribosomal RNA and proteins have been used to find evolutionary relationships.

Five Kingdoms:

Once upon a time, every living thing was lumped together in 2 kingdoms, that is plants and animals.  Animals incorporated every living thing which moved, ate, and grew to some size and stopped growing. Plants incorporated every living thing which didn't move or eat and which continued to grow all through life. It turned into extremely hard to cluster some living things in one or other, so two kingdoms were extended into 5 kingdoms: Protista (single-celled eukaryotes); Monera (the prokaryotes); Fungi (fungus and related organisms); Animalia (animals); Plantae (plants);. Several biologists identify six different kingdoms, separating Monera into Eubacteria and Archeobacteria.

Before the discovery of the microscope it was easy to assign all green and photosynthetic organisms to the plant kingdom and the heterotrophic to the animal kingdom. Fungi such as the mushroom and puffballs were considered to be plants that have lost their chlorophyll and thus were treated as plants. The microscope revealed the existence of thousands of unicellular organisms including bacteria that could not be readily included in the plant or animal kingdom. More than a hundred years ago a third kingdom, PROTISTA, was proposed to include all organisms that remained unicellular throughout their life. Yet, biologists continued to assign even these unicellular organisms either to the plant or animal kingdom thus creating somewhat artificial assemblages. One reason for this situation was that biologists were wither botanists or zoologists and it was necessary to bring all organisms under either one of their purviews for a scientific study of these organisms.

Kingdoms are separated into groups known as phyla, every phylum is separated in classes, every class in orders, every order in families, every family into genera, and genus in species. The species signifies one kind of organism, kind dog, tiger shark, Ameoba proteus (common amoeba), Homo sapiens (us), or Acer palmatum (Japanese maple).

Monera (comprises Eubacteria and Archaebacteria):

Individuals are single-celled, may or may not move, include cell wall, contain no chloroplasts or other organelles, and contain no nucleus. Monera are generally extremely tiny, though one type that is blue-green bacteria, look similar to algae. They are filamentous and fairly green, long, but include no visible structure inside cells. No visible feeding mechanism. They suck up nutrients through cell wall or create their own by photosynthesis.


Protists are single-celled and generally move by flagella, cilia, or by amoeboid mechanisms. There is generally no cell wall, though few forms may have cell wall. They contain organelles having a nucleus and may contain chloroplasts, so few will be green and others will not be. They are tiny, though several are big adequate to be identified in dissecting microscope or even with magnifying glass. Nutrients are obtained by photosynthesis, ingestion of other organisms, or both.


Fungi are multicellular with the cell wall, organelles comprising nucleus, but no chloroplasts. They contain no mechanisms for locomotion. Fungi range in size from microscopic to extremely large ( like mushrooms). Nutrients are obtained by absorption. For most part, fungi obtain nutrients from decaying material.


Plants are multicellular and many do not move, though gametes of few plants move utilizing cilia or flagella. Organelles comprising nucleus, chloroplasts are present, and cell walls are there. Nutrients are obtained by photosynthesis.


Animals are multicellular, and move with help of cilia, flagella, or muscular organs based on contractile proteins. They contain organelles comprising nucleus, but do not have chloroplasts or cell walls. Animals obtain nutrients by ingestion.

The cyanobacteria are prokaryotes and therefore, true members of kingdom Monera. Fungi too are members if fungal and not plant kingdom. The organisms are comprised in study on plant diversity for many reasons. Cyanobacteria, also called as blue-green algae, are usually studied by phycologists (=algalogists) who study other algal groups. Though they are prokaryotes, cyanobacteria have chlororphyll a and develop oxygen in photosynthesis similar to eukaryotic algae and plants do. Cyanobacteria also dwell in fresh and salt water and terrestrial habitats like other algal groups. Endosymbiont theory recommends that different eukaryotic algal groups have developed through symbiotic acquisition of ancestral cyanobacteria cells as plastids. Two members associated to cyanobacteria have both chlorophylls a and b, pigments discovered in all green algae and plans. Fungal thallus resembles algal thallus in usual construction. Fungi reproduce by spores as several algae do. At least few fungi might have developed from algal ancestor after secondarily loosing photosynthetic pigments. As pathogens several fungi are intimatelu related with plants. Approx 80% of vascular plants have fungal association in the roots. This mycorihizal association assists higher plans get nutrients from soil. Fungi play the significant role in recycling dead plant material. Therefore, knowledge of cyanobacteria and fungi assists us in understanding the relationship to plants and role they play in life of true plants. Some unicellular algae are possibly so distantly associated to plants that they must not be placed in plant kingdom. In contrast, tree algae are closely associated to land plants. It is now thought that land plants developed from the advanced green algal group. Therefore, any circumscription of plant kingdom is probable to make probles. Every algae groups comprising green algae are comprised in kingdom protista that also comprises unicellular protests. Consensus now developing among botanists is to use term, plants to refer only to multicellular land plants. Land plants comprise bryophytes, pteridophytes gymnosperms and angiosperms.

Environmental Degradation and Plant Diversity:

The word biodiversity has assumed great importance in recent years as people of every nations are attempting to document biological wealth of the countries and evolving methods to protect the biodiversity. Over 170 countries are signatories of the significant international agreement which was reached in June 1992 at Convention on Biological Diversity held in Brazil. Different forms of environmental deprivation are now threatening survival of number of organisms. Human being cleared forest, hunted down several animals, polluted soil, water and air, and have harshly modified balance which maintained world's ecosystems over millions of years. Few scientists evaluate that as 100 species may be becoming destroyed every day. This high rate of destruction never happened on earth before. Clearly human beings are completely liable for the unprecedented danger to life on earth. Loss of habitat is one of main reasons of threat to organisms. Moist tropical forest covers only approx 70% of land area of world.

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