The presence of variety of plants in the form of different tans (specie, genera) is called diversity of plant. Different plants have different morphology, physiology, cytology and anatomy. It produces diversity among plants. Diversity includes classification and nomenclature of plants.
SCOPE AND CLASSIFICATION OF PLANT KINGDOMS
The study of kinds and diversity of organisms and the evolutionary relationships among them is called systematics or taxonomy. The study of systematics gives the order and relationships among the organism. This order and relationship arise from evolutionary processes. These studies also give description of the new species. It organizes the animals into groups (taxa). This grouping is based on degree of evolutionary relatedness.
A. TAXONOMIC HIERARCHY
(a) Taxonomic hierarchy based on morphology
The modern classification system has been given by Carolus Linnaeus. This system of classification is still used today. Carolus Linnaeus believed that different species could be grouped into same
– Diversity of Plants: Introduction
categories on the basis of similarities between them. The group of animal with similar characteristics forms a taxon. For example, Amaltas (cassia fistula) shows similarities with other cassia specie (cassia senna). Both have compound leaves and produce bean. Therefore, all such plants are placed in the same taxon. Moreover. Cassia also shares characters with Bauhinia verigata (Kachnar). So they are place in the same family.
Carolus Linnaeus recognized five taxa. Modern taxonomists use eight taxa including earlier five taxa. The taxa are arranged hierarchically. It means arrangement of classification from broad to specific. These taxa are: Kingdom, Phylum, Class, Order, Family, Genus, and Species.
(b) Taxonomic hierarchy based on evolution
Carolus Linnaeus did not accept evolution. But still many of his groupings show evolutionary relationships. Morphological similarities between two plants have a genetic basis. It gives rise common evolutionary history. Thus the plants are grouped according to similar characteristics. Carolus Linnaeus grouped them according to their evolutionary relationships. The members of the same taxonomic group are more closely related to each other than the members of different taxa.
KINGDOMS OF LIFE
(a) Kingdom classification system
In 1969, Robert H. Whittaker described a system of classification. This classification forms five kingdoms. This basis of classification of Whittaker is:
(b)Mode of nutrition.
There are following five kingdoms:
- Monera: The members of this kingdom are prokaryotes. The kingdom Monera contains bacteria and the cyanobacteria.
- Protista Members of the kingdom Protista are eukanjotic. They consist cf single cells or colonies of cells This kingdom includes Amoeba. Paramecium, etc.
- Plantae: Members of the kingdom Plantae are eukaryotic, multicellular, and photosynthetic. Plants have walled cells. They are usually non,motile.
- Fungi: Members of the kingdom Fungi are also eukaryotic and multicellular. They also have walled cells and are usually non‑10Master Success Text Book of Botany A
motile. Mode of nutrition distinguishes fungi from plants. Fungi are decomposers. Fungi digest organic matter outside the body and absorb the broken products.
5. Animalia: Members of the kingdom Animalia are eukaryotic and multicellular. They feed by ingesting other organisms or parts of other organisms. Their cells lack walls and they are usually motile.
- Domain classification system
The systematists have concluded on the basis of studies of ribosomal RNA that all life shares a common ancestor. They find that there are three major evolutionary lineages. Each of these lineages is called a domain. The domain is present above the kingdom. There are three domains:
- Arcbaea: These are prokaryotic microbes. They live in extreme
environment such as high temperature rift valleys on ocean floor, or high-salt or acidic environments. All members of the Archaea inhabit anaerobic environments. These environments show the conditions on the earth at the time of origin. The Archaea are the most primitive life form. Ancient Achaeans gave rise to two other domains of organisms.
- Eubacteria: These include true bacteria. These are included prokaryotic microorganisms.
- Eukarya: The Eukarya include all eulcaryotic organisms. The Eukarya diverged more recently than the Eubacteria from the Archaea. Thus, the Eukarya are more closely to Archaea than the Eubacteria. It includes the other four kingdoms of eukaryotes: Protista (Protoctista), fungi, Plantae and Animalia.
Classification of Plants
Plant biologists use the term division for the major plant groups within the plant kingdom. Division is equal to phylum. Divisions are further subdivided into classes, orders, families, and genera.
The classification scheme used in this text recognizes twelve divisions within the kingdom Plantae. These divisions are: Nonvascular nlants
|Division Anthocero ,sida
Vascular Seed less plants
1. Division Psilopsida I Whiskfems
Gymnosperm: Naked seeded plants
Angiosperms: Flowering plants
BASIC CONCEPT OF EVOLUTION IN PLANT DIVERSITY
Nearly all plants reproduce sexually. Most plants are also capable of asexual propagation. Plants produce their gametes within gametangia. Gamentaniga has protective jackets of sterile (non-reproductive) cells. It prevents the delicate gametes from drying during their development. The egg is fertilized within the female organ. The zygote develops into an embryo. Embryo is retained for some time within the jacket of protective cells.
Alternation of generation occurs in plants. The phenomenon in which haploid gametophyte generation and diploid sporophyte generation alternates with each other is called alternation of generation. It occurs in all plants. The sporophyte and gametophyte generations differ in morphology. So they are heteromorphic. Diploid sporophyte is the more noticeable individual in all groups. There are two one main trend in the evolution of plants.
- There is trend towards reduction of haploid generation and dominance of diploid generation.
- There is replacement of flagellated sperm by pollen. It is an adaptation for terrestrial environment.
Major periods of plant evolution
The fossil record shows that there are four major periods at evolution:
1. First period: Origin of plant from aquatic ancestors: The first period of evolution was associated with the origin of plants from aquatic ancestors. Plants evolve from algae during the mid-Silurian period about 425 million years ago. The first terrestrial
12 Master Success Text Book of Botany A
adaptation was formation cuticle and jacketed gametangia. It protects gametes and embryos. Then evolution of vacular tissue takes place. Vascular tissues consist of cells joined into tubes. Vascular tissue transport nutrients throughout the plant. Vascular tissue evolved relatively early in plant history. Most mosses lack vascular tissue. Therefore, they are called nonvascular plants. However, water conducting tubes are present in some mosses. But it is not clear whether tubes of mosses are analogous or homologous to the water-conducting tissue of other plants.
Second period: Diversification of vascular plants: The second major period of plant evolution was diversification of vascular plants during early Devonian period, about 400 million years ago. The earlier vascular plants lacked seeds. This condition is
Diversity of Plants. Introduction
present in ferns and a few other groups of vascular plants.
- Third period: Evolution of Seed: The third major period of plant evolution begun with the origin of the seed. Some Seed protects .embryo from desiccation and other hazards. A seed consists of an embryo packed ovule along with store of food within a protective covering. The first seed producing vascular plants arose about 360 million years ago, near end of Devonian period. These plants have naked seeds. They are not protected by covering. These plants are gymnosperm (conifers). They include pines and other cones. Gymnosperms coexisted with ferns and seedless plants for more than 200 million years.
Fourth period: Evolution of flowering plants: Flowering plant evolved about 130 million years age during early Cretaceous period. The flower is a complex reproductive structure. Their seeds are protected within fruit. The flowering plants are called angiosperms.