Trinomial System

Proposed by Huxley and Stricklandt  

1. According to this system name of any plant or species is composed of three names

(i) Generic name

(ii) Specific name

(iii) Subspecifie name (Name of variety)

When members of any species have large variations then trinomial system is used. On the basis of dissimilarities this species is classified into subspecies eg.

  • Brassica oleracea var. botrytis (Cauliflower)
  • Brassica oleracea var. capitata (Cabbage)
  • Brassica oleracea var. caulorapa (Knol-Khol)

Taxonomic Categories

Their are 7 main taxonomic categories. They are obligate categories i.e, they are strictly used at the time of any plant classification. There are some extra categories, like sub division, sub order, sub family, tribe, sub tribe,. etc. They are not regularly used. They are used only when they are needed.

Taxon: Plant groups or animal groups included in categories are called taxon. Adolf Mayer - First proposed the term 'Taxon" - for animals.

H.J. Lan: First proposed the term "Taxon" - for plants. The classification of any plant or animal is written in descending order. Hierarchy - Descending arrangement of taxonomic categories is known as hierarchy.

Species: Smallest taxonomic category: It is basic unit of classification.' Suffix for taxa (Taxon)

Suffix for tax a (tax on )




Sub div



Opsida, phycease, ae







Sub family




Sub tribe


Note: There is no suffix for Genus, Species and Kingdom.


John Ray: Proposed the term and concept of species.

To explain the species, different concepts were proposed, which are as follows:

(A) Biological concept of species:

(1) Mayer proposed the biological concept of species.

(2) Mayer defined the "species" 'in the form of biological concept.

(3) According to Mayer "All the members that can interbreed. among themselves and can produce fertile off springs are the members of same species"

But this definition of Mayer was incomplete because this definition is applicable to sexually reproducing living beings because there are many organisms that have only asexual mode of reproduction ego Bacteria, Mycoplasma, BGA

(4) The main character in determination of any species is interbreeding. But this, character is not used in taxonomy. In taxonomy, the determination of species is based on other characters. ego - Mainly morphological characters

(5) In higher plants, the determination of species is mainly based on the morphology of flower (floral morphology). Because floral (reproductive) characters are more conservative as compared to vegetative (Root/Stem, Leaf) characters i.e. they do not shows any major changes.

(6) When the species is determined on the basis of interbreeding then it is called as biological species. eg. All the 'humans in this world can interbreed among themselves.

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