In this article we will discuss about:- 1. Principle of Classification of Plants 2. Units of Classification of Plants 3. Nomenclature.
Principle of Classification of Plants:
Diverse forms of plants, ranging from the smallest bacteria to the gigantic forest trees, exist on the earth. They are not only numerous but varied in nature. It has been mentioned than 340,000 plants have been identified, described and named by the botanists.
The number is increasing, as newer ones are being discovered. Quite a large number of them possibly still remain unknown. The main object of systematic botany is to bring about an orderly system out of chaos by classifying the plants into groups, on the basis of their relationship.
Even the early Greek naturalists devised classifications. They used as their bases arbitrary characters like utility, habit, etc. So their systems were artificial. The present system of classification began with Carolus Linnaeus, the famous Swedish botanist of the 18th century, who was referred to as the ‘father of classification’.
He classified plants into groups on the basis of reproductive or sexual characters, particularly on the characters of the stamens.
This classification was also artificial, as it was based on one character. It can be compared to the arranging of books in a library in alphabetic order of the names of authors, disregarding altogether the subject matters.
In the nineteenth century George Bentham and Joseph Hooker introduced a natural system based on a number of characters and attempted to trace the ‘blood relationship’ amongst the plants through common descent. Though other systems have since then been suggested, Bentham and Hooker’s classification is widely accepted in our country and is followed in the herbaria.
According to this system, the plant kingdom is divided broadly into two sub-kingdoms: Cryptogamia or flowerless plants and Phanerogamia or Spermatophyta, or the flowering plants.
Flowerless plants reproduce by simple fission or by minute, usually unicellular spores; and the flowering ones reproduce by seeds containing multicellular embryo. Cryptogamia, again, are divided into three classes, viz. thallophyta, bryophyte and pteridophyta. Thallophyta include the lowest plants. The plant body of a thallophytes an undifferentiated mass of cells called thallus.
The green aquatic thallophytes like pond-scum, sea-weeds are called algae; and the non-green ones like moulds and mushrooms are fungi. Bryophytes like mosses may have small stems and simple leaves but not true roots.
Hairy bodies called rhizoids, carry on the functions of roots. They always lack in vascular tissues, and usually live in moist situations. Some bryophytes have thalloid bodies with rhizoids to fix them on the substrata. Pteridophytes are the highest cryptogams.
They are primarily land plants with the plant bodies well-differentiated into roots, stems, and leaves which flourish well in moist and shady places. They possess distinct vascular bundles. Ferns belong to this group. Phanerogams or Spermatophytes are the highest plants which bear flowers and reproduce by seeds. They possess complex vegetative organs and very well-developed vascular tissues.
Spermatophyte are divided into Gymnospermia or naked-seeded plants {e.g. pine, cycad, etc.) and Angtospermia or covered-seeded plants. In gymnosperms ovules remain exposed and are directly fertilised; whereas in angiosperms ovules remain within the ovary and which, as we know, with the stimulus of fertilisation develops into the fruit containing seeds.
Anglo- sperms are dicotyledonous (e.g. pea, mango) and monocotyledonous (e.g. maize, rice), according to the number of cotyledons in the embryo. They are, again, divided into a number of sub-classes.
Darwin’s doctrine of evolution, which is certainly the most epoch-making event in the history of biological studies, maintains that the higher forms have been derived from the simple ones that lived in the past.
That doctrine prepared the way to natural classification based on relationship through common descent. The above system^ shows, beyond doubt, a gradual ascent in the complexity of structures from the lowest to the highest plants indicating, at the same time, gradual adaptation from aquatic to terrestrial or land habit.
Units of Classification of Plants:
The species is the unit of classification. All individual plants, which resemble one another in important vegetative and reproductive characters and differ visibly from other plants, constitute a species. All the plants of a species are thus supposed to have descended from the same ancestor. Let us take an example.
All banyan trees have similar characters, no matter where they grow. So they form a species, say banyan species. Similarly pipul forms a species; fig forms a third species. Plants belonging to the same species may show variations in minor characters due to environmental conditions and hybridisation. They are but varieties.
Banyan, pipul and fig plants bear the same type of reproductive organs. They form a genus (pl. genera). Thus a genus is a collection of species having similar reproductive characters. Allied genera constitute a family or natural order; natural orders are again grouped into cohort, cohorts into series; and so on, till plant kingdom is reached.
Nomenclature of Classification of Plants:
Plants identified and described must be named. Though almost all the plants have common names but they do not serve our purpose. For proper co-ordination amongst workers names should be internationally acceptable. Linnaeus introduced ‘binomial system’ of nomenclature which has been followed all over the world. Every name has two parts, the generic part and the specific part.
The generic part of the name is written in capital and the specific part in small initial letter. Both the names as a rule are indicated in italics and the name of the author in Roman script. Banyan is Ficus benghalensis, pipul is Fie us religiosa and fig is Fious hispida. So all the three belong to the same genus Ficus but form different species.
The abbreviated name of the author or the person who first published the name of the plant with a suitable description, is put after the specific name. Pea is Pisum sativum Linn. Here Linn indicates that Linnaeus was the author.