Diversity in the Living World | Part 1
One of the cornerstones of science that is virtually unchallenged is that there is order in nature. The order found in living beings is a natural consequence of the shared evolutionary process that influences life. There are about 10 million species of organisms on earth, but only one-third of them have been identified so far.
The diversity is actually the product of the past 3.4 billion years of organic evolution. Also, during the same lengthy period, a lot many more species appeared and were lost (extinct). They are estimated to be at least fifty time more than the existing species.
The requirement is to assort the various varieties under species and then on the basis of similarities and dis-similarities (differences) arrange them into species and then into higher categories (such as genus, family, order, class and phylum).
The classification of organisms, therefore, is nothing but arranging the organisms into groups or sets on the basis of similarities and differences.
The Hierarchy of Classification-Group
Biologists, such as Ernst Haeckel (1894), Robert Whittaker (1959) and Carl Woese (1977) have tried to classify all living organisms into broad categories, called kingdoms. The classification Whittaker proposed has five kingdoms. Monera, Protista, Fungi, Plantae and Animalia, and is widely used. These groups are formed on the basis of their cell structure, mode and source of nutrition and body organization. The modification Woese introduced by dividing the Monera into ARchaebacteria (or Archaea) and Eubacteria (or Bacteria) is also in use.
Further classification is done by naming the sub-groups at various levels as given in the following scheme :
Phylum (for animals) / Division (for plants)
Thus, by separating organisms on the basis of a hierarchy of characteristics into smaller and smaller groups, we arrive at the basic unit of classification, which is a ‘species’. So what organisms can be said to belong to the same species? Broadly, a species includes all organisms that are similar enough to breed and perpetuate.
Plant Kingdom (Kingdom Plantae)
Plant kingdom, as suggested by Eichler (1883) is subdivided into two sub-kingdoms : Cryptogamae and Phanerogamae.
Sub-kingdom Cryptogamae (Crypto-hidden, gamous-marriage) :
These are also known as lower plants, flowerless or seedless plants. These plants do not bear external flowers or seeds and hence are considered to have hidden reproductive organs. It is further divided into three divisions.
(i) Division Thallophyta (Thallus undifferentiated, phyta-plant) :
· The plant body is not differentiated into stem, root and leaves, but is in the form of an undivided thallus.
· There is no vascular system.
· The reproductive organs are single celled and there is no embryo formation after fertilization. This division includes three distinct sub divisions : algae, fungi and lichens.
(ii) Division Bryophyta :
They are the simplest land plants. Plant body is flat and lack true leaves and roots. A true vascular system is absent. Sex organs are multicellular. An embryo is formed upon fertilization. It includes liverworts, horn-worts and mosses. Some common Bryophyta are Riccia, Marchantic, Funeria.
(iii) Division Pteridophyta :
Plant body is made up of stem, leaves and roots. Vascular system is present. Reproductive organs are multicellular, Fertilized egg develops into embryo ; for example, all types of ferns.
These are also called seed plants. Body is differentiated into true stem, leaves and root. Vascular system (xylem and phloem) is well developed. Sex organs are multicellular. An embryo develops from fertilized egg.
On the basis of absence or presence of fruits, there are two sub-divisions :
(a) Gymnospermae and
Both belong to division spermatophyta.
(a) Subdivision Gymnospermae :
Seeds are not enclosed in fruits. Examples : Pinus, Cycas, Cedrus.
(b) Subdivision Angiospermae :
Seeds are enclosed by a fruit. On the basis of the number of cotyledons, the angiosperms are distinguished into two groups : dicotyledons (pea, grams, etc). and monootyledons (wheat, rice, etc).