Classification of plant kingdom can be dealt with as follows—
Systematics : This deals with the study of diversity of organisms at all levels of organisation. It deals from cellular to population level.
Taxon : Taxon is the unit of classification. The taxon includes species, genus family, order, series, class and division.
Species : It is a group of individuals which resemble closely in their anatomy. One species has the same number of chromosomes. While writing species’ name the first letter should be small letter.
Genus : The genus occupies the next higher number. It is the assemblage of similar and related species. While writing genus the first letter should be capital.
Family : Family is next to genus and includes one or more related genera.
Order : It is the next higher taxon to family. It includes the number of related families.
Class : It is the next higher level which includes the organisms of related orders.
Phylum : It is next higher taxon which includes different classes of organisms. Phylum is a big group having living organisms with the same pattern of organisation.
Kingdom : It is the highest level in the classification. It includes different phyla.
Clade : It tells about the common ancestor and all the species descended from the common ancestor.
Common ancestor : A common ancestor is one which has at least two lines of descent.
Convergent Evolution : It tells about the similarity in structure in somewhat related groups due to the adaptation to the environment.
Homology : Similarity in structure given by common ancestor.
Phylogenetic tree : It is a diagram which indicates common ancestors and lines of descent.
Phylogeny : This exhibits evolutionary history of a group of organisms.
Five Systems of Classification : Classification of plant kingdom has been two kingdom, three kingdom and five kingdom classification. Now a days five kingdom classification is most prevalent. This was given by R.H. Whittaker (1969). According to this classification the five kingdoms are—
1. Monera
2. Protista
3. Fungi
4. Plantae and
5. Animalia.
Characterstic features of the main groups of plant kingdom are as follows.
Kingdom Monera (Unicellular Prokaryotes)
(i) Monera has unicellular organisms.
(ii) Most of them are microscopic but some can be seen with the naked eyes also.
(iii) They have prokaryotic cell structure.
(iv) Plasma membrane usually produced inwards as mesosomes.
(v) Some possess cell walls made of polysaccharides (complex sugars) and amino acids.
(vi) There are no chloroplasts. The photosynthetic pigment is generally associated with membranous vesicles.
(vii) Asexual reproduction occurs either by simple fission or by budding.
Archaebacteria
1. This includes the most simple and ancient bacteria.
2. The cell walls are made up of polysaccharides and proteins.
3. These are the oldest living fossils.
4. The cell membrane of this bacteria is resistant to extreme heat and pH which is due to the presence of branched chain lipids in it (cell membrane).
Examples : Archaeoglobus, Methanobacterium, etc.
Bacteria (Eubacteria)
1. The Schizophyta includes true bacteria.
2. Bacteria are unicellular and microscopic organisms.
3. They are omnipresent, i.e. distributed in almost all ecological niches.
4. They are minute in their size—2 to 10 m in length and 0.2 to 0.5 m in width.
5. They change their shape according to environmental conditions and nutrient availability, e.g. Acetobacter.
6. They have a rigid cell wall made up of peptidoglycan.
7. Motile bacteria has flagella.
Cyanobacteria (Blue-green-algae)
(i) They have prokaryotic type of cells.
(ii) Cells do not have any organised nucleus.
(iii) Flagella are absent in the whole life cycle.
(iv) Apart from chlorophyll ‘a’ they contain b carotenes, myxoxanthim myxoxanthophyll and biliproteins.
(v) Some members like Nostoc have filamentous plant body.
(vi) Sexual reproduction is totally absent.
(vii) Thallus remains surrounded by mucilage.
(viii) Cell wall is composed of a gelatinous sheath.
(ix) Cells contain organelles like cyanophycean granules, gas vacuoles, polyhederal bodies, ribosomes, polyglucoside bodies, polyphosphate bodies, etc.
(x) Cell division is of amitotic type.
Useful activities of Cyanobacteria
1. Many cyanobacteria are capable of fixing atmospheric nitrogen and converting it into ammonia which increases the fertility of the soil.
2. They are of great help in reclaiming barren soils and make them more productive.
3. They are cultivated in tanks as a source of protein rich food for fish and other animals.
4. By the addition of this bacteria to rice fields, the yield of rice crop can be increased considerably.
Harmful activities of Cyanobacteria
1. Cyanobacteria contaminate water by giving it colour, fishy taste and foul odour.
2. When present in large number, they deplete the oxygen supply of water thus causing death to fishes.
3. Some cyanobacteria produce animal killing toxins.
Kingdom Protista
Characteristics
1. The kingdom protista includes aquatic unicellular micro-organisms.
2. Their cells are eukaryotic.
3. They possess membrane bound organelles like nucleus, mitochondria, chloroplasts, Golgi bodies and different types of microbodies.
4. They may be photosynthetic or heterotrophic. The heterotrophic forms may be parasitic or saprophytic.
5. They may have cilia or flagella for locomotion.
6. Reproduction is both asexual and sexual. Asexual reproduction takes place by binary fission and cyst formation.
7. Many protists live in haploid state. e.g. Diatoms, dinoflagellates, Amoeba, Paramoecium, Euglena, etc.
Diatoms : Diatoms are unicellular or filamentous photosynthetic eukaryotic protists.
Dinoflagellates : These are aquatic unicellular algae. They are responsible for marine food chain. They include more than 1000 species–almost all of them being marine.
Alga (pl. (Algae) : Aquatic, plant like organism carrying out photosynthesis and belonging to the kingdom protista.
Kingdom Fungi
These are a group of eukaryotic, heterotrophic organisms. They are of diverse froms, sizes, physiology and modes of reproduction.
Mycology : Mycology is the study of fungi. The study began with mushrooms, the largest among fungi. P.A. Micheli (1729) is called the Founder and Father of Mycology.
Characteristics
(i) The fungi are without chlorophyll, and are hence heterotrophic.
(ii) They are either saprophytic, absorbing nourishment from dead organic substances, or are parasitic, living on tissues of the host.
(iii) The body of a fungus (with the exception of yeast) is called the mycellium; it is made up of filaments called hyphae.
(iv) The reserve food is in the form of glycogen (not starch) and oil droplets.
(v) The cell wall is composed of fungal cellulose, often called chitin; cell structure is eukaryotic.
(vi) Nearly all fungi produce motile or non-motile asexual spores.
Kingdom Plantae
Characteristics
(i) The members of this kingdom are all multicellular.
(ii) The cell structure is eukaryotic. The cell possesses a cell wall composed chiefly of cellulose or of cellulose and pectin.
(iii) They are autotrophic.
(iv) They possess differentiated tissues and organs.
(v) They either do not possess vascular tissues for conduction (non-vascular e.g., bryophytes) or possess them (vascular; e.g. tracheophytes).
(vi) The chief mode of reproduction is sexual.
(vii) The life cycle consists of two successive generations called gametophytic (producing gametes) and sporophytic (producing spores) respectively. The two generations alternate.
Classification :
Plantae
Higher Algae Embryophyta
1. Chlorophyta
2. Phaeophyta Bryophyta Tracheophyta
3. Rhodophyta Pteriodophyta Gamnospermae Angiospermae Monocotyledoneae Dicotyledoneae
Algae : Algae are simple without any vascular tissue, and body consists of thallus. Algae are classified on the basis of their photosynthetic pigments, reserve food, composition of the cell wall, the presence or absence of flagella, morphology and methods of reproduction.
Chlorophyta (Green Algae) : Chlorophyta, also called green algae, are chiefly fresh water forms. These plants are of different shapes and sizes. Some green algae are epiphytic or epizoic while others have symbiotic relationship with fungi e.g. Lichens.
Phaeophyta (Brown Algae) : These are called brown algae. They are mostly marine, may grow up to 40—60 m. Larger forms such as Laminaria have lamina which is photosynthetic, stem-like stipe and holdfast which anchors them to rocks.
Rhodophyta (Red Algae) : Rhodophyta, or Red algae, comprises of a distinct, primitive marine group having unicellular to multicellular plant or coral-like. Phycobilin pigments provide red colour to this group.
Bryophyta
(i) The bryophytes are chloroplast containing green plants. They are ‘amphibious’ and grow generally in moist shady places, mostly in tropics. The group includes liverworts, hornworts, and mosses. The main plant body in which gametophyte is dominant phase.
(ii) The plant body may be flattened. The leaves are green.
(iii) They do not possess specialized vascular tissues, and absorb moisture directly from air through their ‘leaves’.
(iv) Free water is necessary for the completion of the sexual cycle.
Classification Hepaticopsida (Liverworts) : The gametophytes are dorsoventrally differentiated. Sporophytes simple or differentiated, e.g. Riccia, Marchantia. Anthoceratopsida (Hornworts): Gametophyte of a simple structure; Sporophyte differentiated into capsule and foot, e.g. Anthoceros, Notothylas.
Pteridophyta
(i) These are mostly terrestrial plants. They have the vascular or conducting tissues (xylem and phloem).
(ii) The plant body, representing the sporophyte, is differentiated into distinct roots, stem and leaves. They do not produce seeds.
(iii) The sporophyte reproduces asexually by producing spores.
(iv) The gametophyte is small but an independent structure called the prothallus, and it produces both antheridia and archegonia.
Spermatophyta
1. These bear flowers and seeds.
2. The flower consists of stamens and carpels with or without non-essential floral parts (perianth).
3. All are heterosporous.
4. The sporophyte is the independent and dominant generation.
5. Pollen grain grows into a pollen tube which carries the male gametes to the vicinity of egg cell.
6. Young sporophyte (embryo) develops at the expense of stored food material in the parent sporophyte.
7. Plants show well developed vascular tissues.
8. Fertilization does not need water.
Gymnosperms
(i) They are ever green plants and have naked seed.
(ii) It is sporophyte. May be branched or unbranched. Plants have two kind of leaves.
(iii) Tap roots are well developed.
(iv) These plant possess the vascular tissues, xylem and phloem.
(v) In most gymnosperms the reproductive organs are grouped together.
(vi) Wind pollination present.
(vii) Gametophytic generation is reduced and the plants are heterosporous, i.e., there are two kinds : male and female plants.
Angiosperms (Flowering Plants)
(i) The adult plant is the diploid (2n) sporophyte. It is differentiated into an underground root system and an aerial shoot system.
(ii) The plants possess well developed conducting tissues. The xylem has vessels and the phloem has companion cells.
(iii) The plants produce flowers.
(iv) A flower has sepals, petals, stamens and carpels.
(v) The carpel encloses the ovules within the ovary. The stamen bears anthers or microsponrangia which produce the microspores or pollen grains.
(vi) Fertilization involves two nuclear fusions.
(vii) The ovules form the seeds, and the ovary forms the fruit.
Angiosperms are divided into 1. Monocot plants and 2. Dicot plants.
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