ADVERTISEMENTS:
A taxon is a biological group of any rank. Taxonomy is, therefore, the study of grouping or classification. It provides evidences by use of which a system of classification is worked out.
Systematics is a wider term comprising identification of an individual organism, assigning a proper rank to the same and giving it a name. However, taxonomy and systematics are considered by many to denote the same thing and the terms are treated as synonymous.
The aim of taxonomy, therefore, is:
ADVERTISEMENTS:
(a) To classify the plant Kingdom and
(b) To identify and name a plant and fix its rank in a recognised system of classification.
This is done by using almost entirely the external morphological characters, i.e., gross morphology. Such taxonomy is called “classical” or “orthodox” or “alpha” taxonomy to distinguish it from “experimental” taxonomy where cytological or embryological evidences are also taken into consideration.
The latter is also known as “omega” taxonomy or “biosystematics” and concerns chromosome morphology, chromosome number, breeding behaviour and barriers.
ADVERTISEMENTS:
Recent trend in taxonomy is to recognise character combination for taxonomical grouping. Here mathematical techniques are applied to assess similarity and disparity through diagrams and using mathematical coefficients. This is called “numerical” taxonomy.
To make use of the results of chemical analysis or tests of plant-parts and products in study of taxonomy is called Chemotaxonomy. This involves a study of the starch grains and rap-hides, study of plant-products, e.g. alkaloids, resins, etc. and serological tests.
Chemical characters, however, should not be regarded as more important than other characters. They are often affected by environmental factors such as temperature, mineral deficiencies, etc.
It is possible that chemical and morphological features may have followed different evolutionary pathways. Comparative chemistry and serology are aids to establishing consanguinity, not phylogeny proper, which involves the tracing of phyletic lines back into the past.
“Many different types of information can be and should be woven into classificatory scheme and there is no obvious advantage in attaching overriding importance to any one type.” Basic unit of classification is the species. Each species has a combination of characters peculiar to its own.
Therefore species is a group of plants having a combination of characters common to each individual in that group. Such characters are morphological, cytological, embryological, etc. Further the plants belonging to a particular species are capable of free interbreeding, and carry and express one or more gene pools.
A genus is a natural group of closely related species. When a species is more or less isolated the genus may often be monotypic, i.e., consisting of a single species only. Similarly a family is a natural group of closely related genera.
Closely related families are grouped into an order, orders into a- class and classes into a division, while the divisions are grouped under a kingdom. Intermediate subgroups are also recognised and Intra specific categories are also known.
The International Code of Botanical Nomenclature provides a list of the groups of different ranks in the following sequence:
In modern systems of classification of plants the above sequence is maintained. As mentioned before, species is the basic unit of classification. Therefore, a plant must belong to a species which must be under a genus.
A genus again must belong to a family, a family to an order, an order, to a class and a class to a division of the plant- kingdom. The intermediate groups may or may not exist. For instance, the family Ceratophyllaceae is not subdivided into subfamilies, tribes or sub-tribes as Ceratophyllum is the only genus in that family.
