The Plant Kingdom
All the plants of this universe constitute the highest taxonomic rank, known as the Plant Kingdom or Plant World. Our understanding of the plant kingdom has changed over time. The members of kingdoms Fungi, Monera and Protista having cell walls have now been excluded from kingdom Plantae (metaphyta) though earlier classification put them in the same kingdom. So, the cyanobacteria, also called blue-green algae are not algae anymore. Now kingdom Plantae includes Algae, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.
♦Plant classification is an arrangement of plants into groups and subgroups on the basis of their affinities.
♦ For the classification of plants, several systems were proposed from time to time by various botanists.
C. Linnaeus (1735) using the number and position of stamens, their fusion and other sexual characters divided Plant kingdom into 24 classes, 23 of flowering plants and one of the non-flowering plants. He included all non-flowering plants such as algae, fungi, lichens, mosses and ferns in a separate 24th class called Cryptogamia. His system was based on a few characters. He himself stated that his system of classification did not consider relationships. Moreover, his system places widely unrelated families of monocotyledons and dicotyledons in one class. However, his system of classification was the best artificial system. Since his classification was based on the reproductive structures, it is called Sexual System of Classification. Further, he based his classification mainly on the number of stamens; it is also called Numerical System of Classification.
♦ Subsequently, several systems of classification of plants were proposed by taxonomists. Out of these, the common system was proposed by Schimper (1879), Eichler (1883), etc.
♦ The system proposed by Eichler, who was famous for his work entitled Bluthendiagramme (1875-78) was actually the first phylogenetic system of plant classification. He divided the plant kingdom into two sub-kingdoms, named as Cryptogamae and Phanerogamae, on the basis of the absence or presence of flowers and seeds. The Cryptogame (Gk. Kryptos = concealed; games = marriage) include all non-flowering plants such as algae, fungi, lichens, mosses and ferns, whereas all flowering plants which bear seeds are included in Phanerogamae (Gk. Phaneros = visible; gamos = marriage). Cryptogams are further classified into three divisions namely Thallophyta, Bryophyta and Pteridophyta on the basis of simplicity and complexity of the plant body. Thallophyta (Gk. Thallose = undifferentiated body; phyton = plant) includes plants whose body is not differentiated among root, stem and leaf. Bacteria and viruses were considered parts of fungi. Bryophyta (Gk. Bryon = moss; phyton = plant) are amphibians of plant kingdom owing to the amphibious habitat of the plants. They are characterised by the presence of conspicuous, green, well developed, nutritionally independent gametophytes to which are always attached physically and nutritionally dependent sporophytes. Bryophytes were divided into three classes: Hepaticae (Liverworts), Anthocerotae (Hornworts) and Musci (mosses). The Pteridophyta (Gk. Pteron = feather; phyton = plant) are the spore-bearing most primitive vascular cryptogams. They are an assemblage of flowerless, seedless and spore-bearing plants that have successfully invaded the land. Pteridophytes were classified into four classes: Psilopsida, Lycopsida, Sphenopsida and Pteropsida. The phanerogams are also known as spermatophytes (Gk. Sperma = seeds; phyton = plant) since they produce seeds. These seed-bearing plants were further grouped into two division Gymnospermae and Angiospermae. The Gymnosperms (Gk. Gymnos = naked; sperm = seeds) are naked seeded plants with their ovules freely exposed on open megasporophyll. Goebel has rightly described them “Phanerogams without ovary”.
The gymnosperms were divided into Cycads, Conifers, Cedrus, etc. The angiosperms included all the flowering plants which produce seeds and have ovules enclosed in an ovary or fruit. They were classified into Monocotyledonae with one seed leaf or cotyledon, e.g., grasses, lilies, orchids, etc.) and Dicotyledonae (with two seed leaves or cotyledons, e.g., legumes, crucifers, cucurbits, composites, etc.).
♦In 1942, Oswald Tippo proposed a modern classification of the plant kingdom. He divided entire plant Kingdom into two major groups, Thallophyta and Embryophyta on the basis of absence and presence of an embryo. Embryophyta was divided into two groups, Bryophyta and Tracheophyta on the basis of absence and presence of vascular tissue. Tracheophyta was divided into Pteridophyta and Spermatophyta on the basis of absence or presence of seeds. Spermatophyta was divided into Gymnosperms and Angiosperms on the basis of nature of seeds. Finally, Angiosperms were divided into Dicotyledons and Monocotyledons based on the number of cotyledons.
♦ Algae and fungi (in Five Kingdom System, Fungi have their own Kingdom) are considered together in Thallophyta (having undifferentiated plant body), though there is a basic difference in the mode of nutrition (i.e., autotrophic in algae and heterotrophic in fungi).
♦ The term ‘alga’ (L. alga – seaweed) was first introduced by Linnaeus (1755) but the present day algae were delimited by de Jussieu (1789).
♦ Fritsch (1935) included algae under all holophytic organisms (as well as their numerous colourless derivatives) that fail to reach the level of differentiation, characteristic of archegoniate plants. The study of algae is called phycology or algology.
Characteristics of Algae
♦ Algae usually occur in a variety of habitats such as water, land as well as on the other plants and even animals. Some grow in marine water called seaweeds.
♦ P, The plant body is acellular, colonial, filamentous, parenchymatous or pseudo parenchymatous.
♦ The whole body is covered by mucilage. The mucilage protects the algae from desiccation, epiphytic growth, toxic chemicals and decaying effect of water.
♦ Vascular tissues are absent.
♦ A mechanical tissue is absent.
♦Nutrition is autotrophic.
♦ Every algal group has a specific reserve food.
♦ A variety of pigments in algae provide different colours.
♦ Vegetative and asexual modes of reproduction are abundant.
♦ Sexual reproduction involves isogamy, anisogamy and oogamy.
♦ Sex organs are unicellular and non-jacketed.
♦ An embryo stage is absent.
♦ The life cycle is various-haplontic, diplontic or diplohaplontic.
CLASSIFICATION OF ALGAE
♦ Algae are usually differentiated on the basis of their pigments and storage products. Algae included under kingdom Plantae by Whittaker (1969) are of three types: red algae, brown algae and green algae.