by microbiology Doctor dr (doctor-dr)(doctor_dr) plant diversity
An Outline of Classification - Microorganisms and Plants
The following is a summary of the classification of living organisms:
Viruses
Viruses cannot be fitted into a classification of living things because they are not cellular. Viruses consist of a strand of nucleic acid and a protein coat. They are classified separately according to their chemical and physical properties. The viruses exhibit following characteristics:
i. They are ultramicroscopic, ranging in size from 20 to 300 nanometers, and are too tiny to be seen with a light microscope.
ii. Because they can pass through bacteria-retention filters, they're known as filterable toxins.
iii. Obligate parasites are disease-causing organisms that can only replicate themselves inside particular living cells. Infected cells eventually degrade and die.
iv. Viruses also target and kill bacteria. Bacteriophages are the name for these viruses.
v. Animal virus illnesses include smallpox, influenza, rabies, and poliomyelitis, whereas plant virus diseases include tobacco mosaic virus, turnip mosaic virus, yellow mosaic virus, and others.
Super Kingdoms - Prokaryota and Eukaryota
All cellular creatures investigated so far are classified into one of two groups: prokaryota and eukaryota.
Prokaryotes are a group of creatures that first arose around 3500 million years ago and are commonly referred to as bacteria. Prokarytoes (pro=before + karyon=nucleus) have no genuine nuclei in their cells. To put it another way, their genetic material (DNA) is not encased by nuclear membranes and is free to move about in the cytoplasm.
Protista, fungus, green plants, and animals are all eukaryotes. They originally emerged some 2000 million years ago, in the late pre-Cambrian epoch, and are thought to have developed from prokaryotes. Eukaryotic cells (en=true) are far more sophisticated, with a genuine nucleus, in which genetic material is surrounded by a nuclear envelope to create a distinct, readily identifiable structure.
Super Kingdom: Prokaryota
Kingdom: Monera
The kingdom exhibits following general characteristics.
i. The genetic material is circular DNA, occurring naked in the cytoplam and normally attached to the cell membrane.
ii. The cytoplasm contains few organelle, and none are surrounded by a two membrane envelopes Ribosomes are present, but are small as compared with eukaryotes.
iii. The cell wall is a rigid structure made of polysaccharides with amino residues combined in it.
iv. Some organisms have the ability to fix atmospheric nitrogen into organic compounds (amino acids).
The kingdom contains all bacteria and cyanobacteria and can be broadly classified into two phyla: the cyanobacteria and bacteria.
i. Phylum: Cyanobacteria
The phylum includes spherical or rod-shaped unicellular organisms (Chroococcus), or simple or branched filamentous prokaryotes (Oscillatoria, Nostoc, Anabaena). They exhibit following general characteristics:
i. They are photosynthetic with photosynthetic pigments held in plasma membranes around the periphery of the cells.
ii. The photosynthetic pigments are chlorophyll (green), phycoerythrin (red) and phycocyanin (blue). The blue pigment is dominant.
iii. Certain species of cyanobacteria live mutualistically with fungi forming lichens.
iv. Many species contain gas vacuoles, and the cells float at the surface of the water in which they live.
At one time the cyanobacteria are classified with algae as blue-green algae.
ii. Phylum: Bacteria
The phylum includes bacteria, the smallest cellular organisms that occupy virtually all habitats. The study of bacteria is bacteriology and those who study bacteria are known as bacteriologists.
Bacteria exhibit following general characteristics.
i. They are unicellular prokaryotes but the cells often remain attached after division, forming clumps, chains or simple filaments. They are about 1 um in diameter and range between 0.1-10 um in length.
ii. The nutrition is varied: many bacteria are heterotrophs and absorb externally digested food, others are autotrophs and get nutrition either by chemosynthesis or photosynthesis.
iii. Many bacteria cause decay and, with fungi, facilitate recycling of nutrients.
iv. Certain bacteria are parasites on other organisms and cause specific diseases. such as food poisoning and thyphoid in animals, and soft rots of various vegetable plants.
v. Bacteriologists divide the bacteria into several phyla on the basis of differences in metabolism and cell structure.
Escherichia coli, a rod-shaped bacterium, serve as typical bacterium for detailed studies.
Super Kingdom: Eukaryota
Kingdom: Protoctista
Protoctista is the most controversial group because it is probably an unnatural group. It contains eukaryotes that are generally regarded as identical and similar to the ancestors of modern plants, animals and fungi. The Protoctista include two sub-kingdoms: the Protozoa and Algae. Both sub-kingdoms formerly had taxonomic status. The algae was classified alongwith fungi in the division: Thallophyta and protozoans as phylum: Protozoa in invertebrates. These two diverse groups are placed together on the basis of a relatively simple level of organization.
i. Sub-Kingdom: Protozoa
The protozoa are a diverse group of microscopic eukaryotic creatures. The majority of the creatures are single-celled animal-like organisms. (Nutrition is primarily heterotrophic, although depending on the food available, the mode of nutrition may alter.
The most commonly occurring phyla of the sub-kingdom are:
a) Phylum: Rhizopoda (rhizopods)
Protozoans having pseudopodia which are used for locomotion and feeding movements are included in this phylum, for example Amoeba.
b) Phylum: Zoomastigina (flagellates)
The protozoans having at least one flagellum for locomotion and exhibiting heterotrophic mode of nutrition are placed in this phylum, for example Trypanosoma.
c) Phylum: Apicomplexa (sporozoans)
This phylum comprises of protozoans that are mostly parasites with complex. life cycles involving more than two hosts species, and reproduce asexually by multiple fission. The common example is malarial parasite, the Plasmodium.
d) Phylum: Ciliophora (ciliates)
The protozoans covered by rows of short, flexible flagella called cilia are included in this phylum.
e) Phylum: Euglenophyta (euglenoid flagellates)
The phylum includes a group of organisms intermediate between the protozoa and algae. Some of these are photosynthetic while the others non-photosynthetic. All species have one or two long, conspicuous flagella used in locomotion. All are without cell wall and the body is covered by a tough protein layer, the pellicle. The chloroplast in photosynthetic species contain chlorophyll a and b, carotene and xanthophyll. Food reserved are stored as carbohydrate-called paramylon. The common example is Euglena.
ii. Sub-Kingdom: Algae
The sub-kingdom includes oxygen-producing photosynthetic eukaryotic organisms with a cell structure similar to that of green plants, Algae show great diversity in structure, ranging from unicellular (Chlamydomonas) and simple filamentous (Spirogyra) to huge seaweeds (Fucus). Algae occur in water or on very damp surfaces. Their bodies lack true stems, roots and leaves, i.e, a thallus. Algae classified into distinct phyla on the basis of their photosynthetic pigments.
a. Phylum: Chlorophyta (green algae)
The green algae is very similar to green plants in cell structure and biochemistry. Chloroplasts contain the pigments chlorophylls a and b, carotene and xanthophyll. The dominant pigment is chlorophyll. Starch is the stored food. The cell walls are made of cellulose. Plant body is single-celled (Chlamydomonas), or a filament (Spirogyra), or a colonial form (Volvox), or shaped as flattened thallus, i. e., thalloid (Ulva). Most green algae are found in freshwater.
b. Phylum: Bacillariophyta diatoms
Diatoms have unusual wall made from celluloses and impregnated with silica. The walls consist of two halves called valves, which fit together like the halves of a petri dish. Chloroplasts contain pigments chlorophylls a and c, carotene and the xanthophyll fucoxanthin Oil is stored as food reserve. The example are Navicula and Pinnularia.
c. Phylum: Xanthophyta (yellow-green algae)
The members of this division are characterized by their yellow-green colour due to dominance of xanthophyll diadinoxanthin. The cell wall consists of two overlapping halves. The reserve food is oil and chrysolaminarin, a leucosin. The most commonly occurring genus is Vaucheria.
d. Phylum: Pheaophyta (brown algae)
Nearly all brown algae is marine found in intertidal zone of a rocky shore. There are only three freshwater genera. The plant body is a multicellular thallus with a degree of tissue differentiation, however, water-conducting cells have not evolved. Chloroplasts contain the pigments chlorophylls a and c with carotene and fucoxanthin a brown pigment that dominates the others. The food reserves include a carbohydrate, laminarin. Reproduction is sexual, and many species have haploid and diploid alternation of generations. They are commonly called seaweeds, for example Fucus.
e. Phylum: Rhodophyta (red algae)
The red algae are marine plants of the lower part of the intertidal zone. Chloroplasts contain chlorophyll a, carotene and xanthophyll pigments. But the dominant pigment is red phycobilin, which absorbs wavelengths of light penetrating deeper seawater. The plant body is composed of branched filaments or aggregations of filaments. The life cycles of red algae are complex, usually with alternation of generations. The examples are Batrachospermum and Polysiphonia.
Phylum: Oomycota
This phylum was originally classified with fungi but now included in Protoctista. The plant body consists of hyphae which have not cross-walls, i. e., non septate. The cell walls contain cellulose. Asexual reproduction is by means of biflagellate zoospores produced in sporangia. The sexual reproduction is by oogamy, involving fusion of an oosphere (female gamete) with a male gamete to produce an oospore. The phylum includes disease causing agents, for example Phytophthora infestans that causes potato blight disease and Pythium - a parasite that causes damping off of seedlings.
2. Kingdom: Fungi
The fungi are a large and successful group of organisms of about 80,000 Classifi named species. They range in size from the unicellular yeasts to the large toadstools, puffballs and stinkhorns, and occupy a very wide range of habitats. Fungi are eukaryotic organisms with protective walls composed of chitin. The plant body is a mycelium consisting of thread-like hyphae. The hyphae may be divided by Septa (cross walls) into short, multinucleate section. The fungi lack chlorophyll and their nutrition is non-photosynthetic. Some feed as parasites, others are mutualistic, but most are saprotrophic absorptive, secreting enzymes on to food and absorbing the products of external digestion. The reserve food, if present, is glycogen. Most fungi are known to reproduce sexually and asexually. The gametes and spores produced are without flagella.
The kingdom Fungi is further classified into:
i. Phylum: Zygomycota (zygomycetes - moulds)
The phylum includes saprotrophic fungi widespread in soils and dung, and common in decaying food. The mycelium is branched and composed of non-septate hyphae. Asexual reproduction is by conidia or sporangia containing spores. Sexual reproduction is by conjugation, involving fusion of two gametangia to produce a resistant zygospore. The examples are Rhizopus stolonifer, commonly called bread mould; and Mucor, a common saprotrophic mould.
ii. Phylum: Ascomycota (ascomycetes the sac fungi)
The yeasts (Saccharomyces), cup-fungi (Peziza), Penicillium, and powdery mildews (Phyllactinia) are included in this phylum. The plant body or mycelium is composed of septate, multinucleate hyphae. The septum has a central pore so that the cytoplasm is continuous throughout the mycelium. Asexual reproduction is by conidia. Sexual reproduction involves production of specialized sexual spores, the ascospores produced inside a special structure, the ascus. In most species there is a cup-shaped or flask-shaped fruiting body in which the lasc are formed.
iii. Phylum Basidiomycota (basidiomycete club fungi)
The phylum include mushrooms (Agaricus), rusts (Puccinia), smuts (Albugo), toadstools, puffballs and bracket fungi. The organisms are composed of septate hyphae. Asexual reproduction is by formation of spores but not very common. Sexual reproduction involves production of special sexual spores, the basidiospores, occurring on the club-shaped tip of a special hypha, the basidium.
iv. Phylum Deuteromycota (fungi imperfecti)
The phylum include fungi in which sexual reproduction has never been observed, and whose classification is uncertain, for example Alternaria.
v. Phylum: Mycophycophyta (lichens)
The phylum contains lichens, Lichens are difficult to classify within the existing scheme. Lichens are dual organisms, formed by an association between an alga (green alga or cyanobacterium) and a fungus (usually an ascomycete). The lichen thallus consists of compact fungal hyphae surrounding algal cells and taking the form of an encrusting (crustose), a leaf-like thallus (foliose) or a tiny shrub (fruticose). Each component is dependent upon the other; the algal cell manufacture sugar, and the fungi retain ions as and when they become available.
Lichens are first colonizers of bare rocks, and they have the capacity to survive extremely adverse conditions. They are susceptible to air-borne pollution, however. The examples are Physcia and Cladonia.
3. Kingdom: Plantae (green plants)
The green plants are multicellular eukaryotes with cell walls containing cellulose and other polysaccharides. They are autotrophic and the majority are photosynthetic. Photosynthesis occurs in chloroplasts which contain the pigments chlorophylls a and b, xanthophyll and carotene. The life histories of green plants involve two generations which alternate with each other. First, a diploid generation occurs, known as sporophyte, during which spores are formed. This is followed by a haploid generation, called gameotphyte, during which sex cells or gametes are produced.
The International Code of Botanical Nomenclature recommends that each major group of the plant kingdom should be called a division.
i. Division: Bryophyta
Bryophytes are the first plants to colonize the land and are generally thought to have evolved from green algae. It is the simplest group of land plants evolved about 420 million years ago. They exhibit alternation of generations during which the gametophyte is the conspicuous and dominant generation. The sporophyte is attached to and dependent upon the gametophyte for its function. Bryophytes are largely restricted to moist environments. They lack true roots and the plants are anchored by thin filamentous outgrowths, the rhizoids. The water-conducting (xylem) and elaborated food-conducting (phloem) tissue are absent. Bryophytes have evolved antheridia (male reproductive organs) and archegonia (female reproductive organs) but they require water for transport of male gamete to female gamete. Asexual reproduction is by spore formation. The spores are produced in specialized spore producing structures, the capsule.
Bryophyta includes two main classes, the Hepaticeae (liverworts) and the Musci (mosses). Liverworts and mosses have sometimes been described as amphibians of the plant kingdom, because they require water at least once during their life cycle, therefore found in moist habitats always.
a. Class: Hepaticeae (liverworts)
Liverworts either have a flat thallus (Marchantia, Riccia, Pellia) or a stem (Lophocolea) with three ranks of leaves. These are anchored by unicellular rhizoids. Spores are produced in capsule. The wall of the capsule splits into four valves for spore dispersal. The elators aid in dispersal of spores.
b. Class: Musci (mosses)
Mosses are small, leafy structures with spirally arranged leaves, and anchored by multicellular rhizoids. The capsule of the sporophyte has an elaborate mechanism of spore-dispersal, dependent on dry conditions and involving teeth or pores. The elators are absent. Common examples of mosses are Funaria, Mnuium, and Sphagnum a bog-moss.
Tracheophytes
The remaining groups of kingdom plantae are collectively known as tracheophytes. Tracheophytes have a conspicuous and dominant sporophyte generation in the form of a plant differentiated into stem, leaf and root. Vascular tissue (xylem and phloem) is present. It is composed of lignified cells mostly. The leaves are elaborate structures and their surfaces are covered with cuticle. All the vascular plants were formerly included in one division, the Tracheophyta with club mosses, horsetails and ferns, and the more advanced groups, the seed-bearing plants (gymnosperms and angiosperms), being classified as sub-divisions.
ii. Division: Lycopodophyta (club mosses)
The division includes plants with small, spirally arranged leaves. The leaves are microphyllous, i. e., have a single mid-vein. The sporangia are usually produced in cones or strobili. The sporangia are produced on the dorsal surfaces of leaves called sporophylls. Some club mosses are homosporous (Lycopodium) while others are heterosporous (Selaginella, Marselia). Heterospory is regarded as first step towards seed formation. The gametophyte is independent but smaller and inconspicuous usually called a prothallus.
iii. Division: Sphenophyta (horsetails)
The horsetails possess microphyllous leaves arranged in whorls around the stem. The sporangia are produced in cones on distinctive sporangiophores. The plants are homosporous. The only surviving genus is Equisetum.
iv. Division: Filicinophyta (ferns)
The leaves in ferns are macrophyllous, i. e., large and have branching veins, and called fronds. These are tightly coiled and uncoil in early growth and development. The sporangia develop on lower surface of the leaves in clusters called sori. Each sorus contains many stalked sporangia. The ferns are homosporous. The examples are Dryopteris, Pteridium, Adiantum and Marselia.
Seed Plants
The gymnosperms (conifers) and angiosperms (flowering plants) are characterized by formation of seeds, therefore called seed plants. The sporophyte is the dominant generation; gametophyte is very much reduced. The vascular tissue found in root, stem and leaves is more complex. The sporangia develop in cones in gymnosperms, and modified leaves. the stamens and carpels of flowers, in angiosperms. The plants are heterosporous, i. e., produce two types of spores, the microspores and megaspores. The megaspore is retained in the megasporangium (ovule), and a fertilized ovule is a seed. Both groups do not need water for fertilization, they convey sperms to the ovum by a pollen tube.
v. Division :. Cycadophyta
The division includes both extinct and extant gymnospermous seed plants with fern-like leaves. The cycads are also called living fossils because of their resemblances to extinct seed ferns. The male cones of cycads are largest among the gymnosperms. The sporangia are produced in sori. The common examples are Cycas, Zamia, Microcycas, Dioon, etc.
vi. Division: Coniferophyta (conifers)
The division includes cone-producing plants in which the seeds are naked, i. e., not enclosed in a fruit. Leaves are usually waxy and needle-shaped. The sporangia and spores develop in cones. Male and female cones are separate but develop on the same plant. The vessels are absent in xylem tissue and phloem contains albuminous cells similar in function to companion cells, but different in origin. The common examples are Pinus, Cedrus.
vii. Division: Gnetopsida
The members of the division are xerophytic small shrubby trees. The male and female cone are compound structures and usually referred to as flowers. The ovules are surrounded by many envelopes regarded as extra integuments. The micropylar tube is extremely elongated and forms a bristle-like tube. The secondary wood contains vessels. The common genera are Ephedra, Gnetum and Welwitschia.
viii. Division: Angiospermatophyta (flowering plants)
The angiosperms are dominant group of land plants. They are seed-bearing plants, with the seed enclosed in fruits formed from the ovary. Xylem contains vessels and phloem contains companion cells. The flowers are unique to angiosperms. The sporangia and spores develop in the flowers. The development of flowers has been associated with the evolution of complex mechanisms for pollen transfer (pollination) and seed dispersal, sometimes involving insects, birds and mammals, and the agency of wind and water.
Angiospermatophyta includes two classes.
a. Class: Monocotyledoneae (monocots)
The leaves of monocots mostly have parallel veins. The embryo plant in the seed has a single cotyledon (seed-leaf). The vascular bundles are scattered. Vascular cambium is usually absent and there is no secondary growth. Floral parts are usually in threes or multiples of threes. Perianth is not distinguished into calyx and corolla. Usually wind pollinated. The examples are Poa (an annual meadow grass), Triticum (wheat), iris, lilies, orchids.
b. Class: Dicotyledoneae (dicots)
The leaves in dicots have net veins. The embryo plant in the seed has two cotyledons (seed-leaves). The vascular bundles are arranged in a ring. Vascular cambium is usually present giving rise to secondary growth. Floral parts are in fours or fives or multiples of fours and fives. Usually insect pollinated. The examples are Ranunculus (buttercup), Quercus (oak tree), Rosa (rose) and Lathyrus (pea).
