NEET Biology Notes Plant Kingdom
Plant is the kingdom of multicellular and photoautotrophic organisms. Their cell wall is cellulosic, growth indefinitepnd reserve food is mainly starch. Kingdom-Plantae has been described under Algae, Bryophytes, Pteridophytes, Gymnosperms and Angiosperms.
Eichler (1883), divided plant kingdom into two sub-kingdoms, mainly on the basis of the presence or the absence of seeds.
1. Cryptogamae (Gr. Cryptos—hidden; gamous—marriage) Lower plants in which sex organs are hidden and seeds and flowers are absent. It includes thallophytes, bryophytes and pteridophytes.
2. Phanerogamae (Gr. Phaneros—visible/evident; gamous—marriage) Higher plants in which sex organs are evident, seeds present. It includes gymnosperms and angiosperms
These are chlorophyll bearing, simple thalloid autotrophic and largely aquatic (both freshwater and marine) organisms. Algae occur in a variety of habitats like moist stones, soils and wood. Some of them also occur in association with fungi (lichens) and animals (e.g. on sloth bear). In algae, an embryo stage is absent. Sexual reproduction is absent in blue-green algae. Life cycle is of various types in algae, i.e. haplontic, diplontic and diplohaplontic.
The size of algae ranges from the microscopic unicellular forms like Chlamydomonas to colonial forms like Volvox and to the filamentous forms like Ulothrix and Spirogyra and kelps form massive plant bodies.
- Reproduction in algae occur by vegetative, sexual and asexual methods.
- Ectocarpus is a brown alga in which male gametes are flagellated. Spirogyra shows isogamy with non-flagellated gametes.
- Algae are of paramount importance as these are primary producers of energy-rich compounds, which form the basis of the food cycles of all aquatic animals.
- It is a member of class-Chlorophyceae. It occurs in freshwater bodies but Ulothrix flacca is marine and U. implexa is lithophytic. The thallus is unbranched filament consisting of basal elongated and colourless cell called holdfast, the uppermost apical cell and row of rectangular cells (broader than longer). Each cell contains single nucleus, band-like or collar-shaped chloroplast with one or more pyrenoids, a large central vacuole and cell wall of cellulose imprignated by pectin. Vegetative reproduction takes place by fragmentation. Asexual reproduction takes place by zoospores under favourable conditions.
- The nucleus of each cell except holdfast divides mitotically into 2-64 nuclei followed by accumulation of protoplasm around these nuclei resulted in development of bi- or quadriflagellate zoospores. During unfavourable conditions, the
- Ulothrix reproduces asexually by hypnospores, akinetes and palmella stage.
- Sexual reproduction is isogamous type involving fusion of two similar biflagellate gametes to produce zygospore. The zygospore after a resting period undergoes meiosis to produce haploid meiozoospores, which under favourable conditions germinate to produce new filament.
- In Ulothrix, life cycle is of haplontic type.
Batrachospermum (frog spawn alga) is a freshwater filamentous rhodophycean alga. The filament gives a branched beaded appearance. The beads occur in the region of nodes, where whorls of short branches or glomerules occur. Long branches occur at places. Internode are made up of single long cells. A loose cortex is found around them. Cells of short laterals are small and elliptical The alga can multiply vegetatively (by fragmentation and gemmae) and asexually (by monospores). Male sex organs are called spermatangia, while female sex organs are called carpogonia. Meiosis occurs immediately after fertilisation. A haploid carposporophyte or cystocarp is formed. It produces carpospores. A carpospore forms a highly branched filamentous chantransia stage. It is juvenile stage, which’ can multiply by monospores. The adult alga grows over the chantransia stage.
Useful Activities of Algae
Carbohydrates, inorganic compounds and vitamins are found in algae hence, they are used as food, e.g. Porphyra (containing 30-35% protein, 40-45% carbohydrate, vitamin-A and C), Ulva, Alaria, Chlorella (containing 30% carbohydrate, 30% proteins, 15% lipid), Chondrus, Codium, etc.
Many seaweeds such as Fucus, Laminaria, Ascophyllum and Sargassum are used as fodder.
Chlorella (green alga) and Spirulina (blue-green alga) are rich in proteins. They can be used as food supplements even by space travellers.
- Agar-agar (polysaccharide consists of agarose and agaropectin) is obtained commercially from species of Gelidium, Gracilaria and Gigartina. It is used in manufacture of processed cheese, pudding, creams, jellies and culture media.
- Carrageenin (polysaccharide colloid) is obtained from cell wall of Chondrus crispus and Gigartina stellata and used in stabilisation of emulsions in paints and cosmetics and in alcohol and sugar industry.
- Alginate (salts of alginic acid) is extracted from Laminaria, Ascophyllum, Fucus, Macrocystis and useful as thickner, emulsifier and gelling agent.
- Diatomite (deposits of diatoms) is insoluble and chemically inert, therefore used as filter, insulator in boilers and blast furnaces and an absorbant of nitroglycerine.
- Laminaria and Fucus are source of iodine and bromine. Blue-green algae like Nostoc, Anabaena, Aulosira, Oscillatoria, etc., act as nitrogen fixing agent. The antibiotic chlorellin is obtained from Chlorella. Chlorella, Chlamydomonas, Euglena, Scenedesmus, Pyrobotrys, etc., are used for disposal of sewage.
A lot of researches in genetics and cytology have been carried out on Acetabularia. Chlorella has been intensively used for studying the path of carbon during photosynthesis.
Harmful Activities of Algae
Some algae such as Microcystis, Chrococcus, Oscillatoria, Anabaena, etc., cause water bloom in water reservoirs. Cephaleuros sp. are parasitic on leaves of tea and coffee.