Notes on Plant Taxonomy:- 1. Objectives of Plant Taxonomy 2. Principles of Plant Taxonomy 3. Classification 4. Identification.
Objectives of Plant Taxonomy:
The first object of plant taxonomy is to identify all the kinds of plants on earth with their names, distinctions, distribution, habit, characteristics and affinities. It also tries to correlate the studies with scientific data contributed by various researches in the field of botanical science. It gives an accumulated information and scientific knowledge of the world’s plant resources.
The second objective is to arrange the kinds of plants into a scheme of classification or an orderly arrangement. There are some species that are closely related to each other than others. Such species are placed in a higher group; similarly the closely related higher groups to still higher groups and so on.
The third objective is to study the factors of evolution to find out the origin of species and their interrelationships. Hence a taxonomist not only studies the species existing today but also reveals the changes that they have undergone through the past.
The fourth objective of plant taxonomy is the correct naming of plants according to the international code of nomenclature. The naming of the plant is guided and regulated by international rules of botanical nomenclature.
In this system, to every plant a binomial name is given e.g., Cicer arietinum L.; the first name refers to its genus, the second to its species, L. for Linnaeus – the name of the person or author who first observed and reported the plant.
The fifth is the documentation which includes the preservation of living or fossil flora in a herbarium.
Manson H.L. (1950) in his paper “Taxonomy, Systematic botany and Biosystematics” thought that the science of taxonomy may be a synthesis of four inter-related fields viz.,
It is the fact finding field of taxonomy which includes genetically and cytological studies as well as other techniques applicable to the problem.
The taxonomic system:
It is based on the facts found by systematic botany and includes:
(a) Taxonomic concepts of plant groups or taxa.
(b) Concepts of the evolutionary sequence of characters.
(c) Classification and arrangement of taxa.
(d) Description of taxa and phytography.
It is a method of naming plants based on international rules which permit only a single valid name for each kind of plant.
It includes the preservation of living or fossil type specimens and illustrations in a museum or herbarium.
By all these fields of plant taxonomy light has already been thrown on plant or plant population, their units, interrelationship, distribution and evolutionary tendencies.
Principles of Plant Taxonomy:
Taxonomy is a functional science. The direction and character of its functions are governed by principles. The principles developed with the increase in knowledge of plants themselves.
The principles are as follows:
It developed in the nineteenth century. This was mainly concerned with the observation of similarities, and differences in the gross morphological characters of plants known at that time. This began with the works of Tournefort, de Jussieu and Linnaeus. In this principle the plants were described and classified on the basis of morphological characters.
This principle was introduced in the 20th century. Primary importance was given to morphological distinctness and affinity, but it was influenced appreciably by the findings of cytologist, geneticist, anatomist, physiologist and embryologist.
Modern taxonomists of the 20th century use phylogeny as the main principle of plant taxonomy. Phylogeny is the evolutionary history of a taxon. By this principle attempt is made to account for the origin and development of species. To determine the origin of a species a taxonomist has to depend on the science of palaeobotany which includes all taxa of extinct plant groups.
Classification of Plant Taxonomy:
Classification is the ordering of organisms into groups (or sets) on the basis of their relationships i.e. phylogenetic relationship.
Types of Botanical Classification:
i. Empirical classification:
In this classification botanists have arranged the plants in alphabetical (ABC …… ) order. The system may be compared with the alphabetical arrangement of words in dictionary. There was no character into consideration.
ii. Reasonable classification:
In this classification the plants were placed together on the basis of some natural characters which may link them.
This type of classification may further be divided into following three types:
(a) Practical classification:
This classification is mainly based on the properties of plants particularly to their value or use to human race.
(b) Artificial classification:
This is more or less arbitrary as the plants are classified on the basis of one or at the most few characters, which, however, do not throw any light on the affinities or relationship of the plants with one another.
According to recent understanding, Artificial classification (key classification) is a classification structured for convenience, using easily observed phenotypic characters and not necessarily indicating phylogenetic relationships.
Natural classification (phylogenetic classification) is a hierarchical classification based on hypothetical phylogenetic relationships such that the members of each category in the classification share a single common ancestor.
(c) Natural system of classification:
This system is based not only on the characters of reproductive organs and structural relationship but all the other important characters are also taken into consideration and the plants are classified according to their related character. It helps us not only to ascertain the name of a plant but also its relationship and affinities with other plants. All the modern systems of classification are natural.
(d) Phylogenetic system of classification:
This type of system classifies plants according to their evolutionary and genetic relationships. It enables us to find out the ancestors or derivatives of any taxon. Our present day knowledge is insufficient to construct a perfect phylogenetic classification and all present phylogenetic systems are formed by the combination of natural and phylogenetic evidences.
Identification of Plant Taxonomy:
This is the allocation or assignment of additional unidentified objects to the correct class once a classification has been established.
Phylogeny is the origin and evolution of taxa.
It stresses the ancestral relationship of taxa to one another.
It emphasises belonging to particular line of descent (phyletic line).