Samacheer Class 8 Science - Plant Kingdom

Binomial Nomenclature is a system of naming organisms using two Latin words, which was developed by Carl Linnaeus. The first word represents the genus name and is written with a capital letter, while the second word represents the species name and is written in lowercase. Both words are italicized or underlined. For example, the binomial name of mango is Mangifera indica, where Mangifera is the genus and indica is the species. Similarly, the binomial name of humans is Homo sapiens. This system provides a universal scientific name that is recognized worldwide and helps in the proper classification and identification of organisms.

Seed has two cotyledons.
Plants have tap root system, leaves with reticulate venation.
Flowers are tetramerous or pentamerous. Calyx and corolla are well differentiated.
Pollination occurs mostly by insects.

Mushrooms contain rich protein, minerals and are edible. Example: Agaricus.
The antibiotic penicillin is got from the fungus penicillium notatum other antibiotics like gentamycin, erythromycin are also got from fungi.

The Natural System of Classification is a method of classifying plants based on several morphological characters rather than a single characteristic. Bentham and Hooker's classification is the most widely recognized example of this system. In this approach, plants are grouped based on their overall similarity in structure, form, and other observable features. This system of classification is more comprehensive and accurate than artificial systems because it considers multiple characteristics. The Natural System of Classification has been widely adopted in herbaria and botanical gardens throughout the world because it provides a more natural and practical arrangement of plants. Bentham and Hooker published their comprehensive Natural System of Classification in their famous work titled Genera Plantarum, which was published in three volumes and became a standard reference for plant classification.

Three important economic uses of algae are as follows. First, in agriculture, certain blue-green algae such as Nostoc and Anabaena are essential for fixing atmospheric nitrogen into the soil through a process called nitrogen fixation, which increases soil fertility and reduces the need for chemical fertilizers. Second, agar-agar is a gelatinous substance extracted from red algae species such as Gelidium and Gracillaria, which is widely used as a growth medium in laboratories for culturing microorganisms and in the food industry as a thickening and gelling agent. Third, iodine is obtained from brown algae such as Laminaria, commonly known as kelp, and is used in medicine and industry for various purposes including disinfection and as a nutritional supplement.

Algae and fungi differ in several fundamental ways. Algae are autotrophic organisms that contain photosynthetic pigments such as chlorophyll, enabling them to manufacture their own food through photosynthesis. Some algae are prokaryotic in nature, such as cyanobacteria including Nostoc and Anabaena, while others are eukaryotic. The reserve food material in algae is typically starch. In contrast, fungi are heterotrophic organisms that lack photosynthetic pigments and cannot produce their own food, instead obtaining nutrition by absorbing organic matter from dead or living organisms. All fungi are eukaryotic in nature, such as Agaricus. The reserve food materials in fungi are glycogen and oils rather than starch. These fundamental differences reflect their distinct evolutionary origins and ecological roles in nature.

Four important characteristics of pteridophytes are as follows. First, the main plant body is the sporophyte, which is the dominant phase in the life cycle of pteridophytes, and it is differentiated into true roots, stems, and leaves, making them more advanced than bryophytes. Second, pteridophytes reproduce by means of spores rather than seeds, and these spores are produced in structures called sporangia. Third, the leaves that bear sporangia are called sporophylls, and they may be similar to vegetative leaves or distinctly different. Fourth, most pteridophytes are homosporous, meaning they produce only one type of spore, which may be either microspore or megaspore, though some advanced pteridophytes are heterosporous and produce two different types of spores.

The outline of Bentham and Hooker's system of classification is organized as follows. The seeded plants are divided into three main classes. Class I is Dicotyledonae, which is further subdivided into three subclasses: Polypetalae containing series such as Thalamiflorae, Disciflorae, and Caliciflorae with approximately 36 families; Synpetalae containing series such as Inferae, Heteromerae, and Bicarpellatae; and Monochlamydeae. Class II is Gymnospermae, which contains approximately 3 families of plants with naked seeds. Class III is Monocotyledonae, which is organized into series and families similar to dicots, containing approximately 36 families. This hierarchical classification system was based on morphological characters and became widely used in herbaria and botanical institutions worldwide.

Monocots and dicots differ in several key characteristics. First, monocot seeds contain a single cotyledon while dicot seeds have two cotyledons. Second, monocot leaves display parallel venation where veins run parallel to each other, whereas dicot leaves show reticulate or net-like venation. Third, monocots possess a fibrous root system consisting of many thin roots of equal size, while dicots have a tap root system with one main root and smaller lateral roots. Fourth, monocot flowers are trimerous, meaning their floral parts occur in multiples of three, whereas dicot flowers are tetra or pentamerous with floral parts in multiples of four or five. Fifth, the vascular bundles in monocot stems are scattered throughout the tissue, while in dicots they are arranged in a ring. Examples of monocots include paddy, wheat, and maize, while examples of dicots include mango, bean, and sunflower.

Gymnosperms and angiosperms are two major groups of seed-bearing plants with distinct differences. Gymnosperms reproduce through the production of cones and are naked seed-bearing plants where seeds are not enclosed in a fruit, whereas angiosperms produce flowers and are closed seed-bearing plants with seeds enclosed within a fruit. Pollination in gymnosperms occurs primarily through wind, while in angiosperms it can occur through wind, water, insects, or other agents. The vascular tissue differs significantly: gymnosperm xylem lacks vessels and phloem lacks sieve tubes, while angiosperm xylem and phloem are well-developed with vessels and sieve tubes respectively. In terms of life cycle, gymnosperms have two distinct phases, the sporophyte and gametophyte, both of which are prominent. In contrast, angiosperms show a dominant sporophytic phase with a much-reduced gametophytic phase. Gymnosperms are generally more ancient and include conifers and cycads, while angiosperms are more recently evolved and represent the most diverse group of plants today.

Gymnosperms have significant economic importance in various industries and applications. The wood of many conifers such as Pinus and Agathis is extensively used in the paper and pulp industry for manufacturing paper and cardboard products. Conifers provide soft wood that is valuable for construction purposes, packing materials, and plywood manufacturing, with examples including Cedrus and Agathis. Turpentine, an essential oil extracted from the resin of Pinus, is widely used in paint preparation and has medicinal applications for pain relief and treatment of bronchitis and respiratory conditions. The seeds of Pinus gerardiana are edible and consumed as food in certain regions. Ephedrine, an alkaloid alkaloid extracted from Ephedra plants, is used pharmaceutically to treat asthma and various respiratory problems. Additionally, Araucaria bidwillii and other gymnosperms serve as ornamental plants in landscaping and gardens. Some gymnosperms also provide resins, gums, and other chemical compounds used in various industrial and medicinal applications, making them economically valuable resources.

Medicinal plants and their uses:
1. Acalypha indica (Kuppaimeni):
The paste obtained from the leaves of this plant is used to cure the bums on the skin.
The juice of this plant leaves is mixed with lemon juice to cure ringworm.
2. Aegle marmelos (Vilvam):
The unripe fruit of this tree is used to treat indigestion.
It is used to cure chronic, diarrhoea and dysentery.
3. Solanum trilobatum:
The leaves and fruits of this plant cure cough and cold.
It is widely used in the treatment of tuberculosis and bronchial asthma.
4. Phyllanthus amarus (Keezhanelli):
The entire plant is used for the treatment of jaundice.
It gives additional strength to human liver and used to treat other liver disorders.
5. Aloe vera (Sothu katrazhai):
Leaves of this plant is used to cure piles and inflammations on the skin.
It cures peptic ulcer.