Current Affairs 11th Class

The root is usually an underground part of the plant which helps in anchorage of plant in soil and absorption of water and minerals from the soil. The root with its branches is known as the root system. Characteristics of the root (1) The root is the descending portion of the plant axis and is positively geotropic and negatively phototropic. (2) It is non-green or brown in colour. (3) The root does not bear nodes, internodes, leaves and true buds. (4) Usually the root tip is protected by a root cap. (5) The root bears unicellular root hairs. (6) Lateral roots arise from the root which are endogenous in origin (arise from pericycle). Parts of the root (1) Region of root cap : The tip of the root is called calyptra or root cap. It is for protection of root tip against any injury. It is formed from meristem called calyptrogen. Pandanus is the only plant with multiple root caps. In the aquatic plants like Pistia, Lemna and Eicchornia instead of root caps, they have root pockets for buoyancy. The root caps are absent in parasites and mycorrhizal roots. (2) Region of cell formation or meristematic zone : This region of cell division lies protected below the root cap. It comprises of closely arranged, small, thin walled and isodiamatric cells which have dense protoplasm. Vacuoles of the cells are either reduced in size or absent. (3) Region of cell elongation : It lies behind the growing point. Cells of this region lose power of division. The cells elongate due to vacuolation i.e., formation of vacuoles. This region chiefly concerns with absorption of minerals along with some amount of water. (4) Region of cell differentiation or maturation (Root hair zone) : In this region elongated cells are differentiated into permanent tissues depending upon the functions they have to performs. It lies adjacent to the meristematic region some cell of the outermost layer of cells in this region develop root hairs. Most of the water absorption occurs through this region.       Types of root  Tap root : The tap root system develops from radicle of the germinating seed. It is also called the normal root system. The tap root system is present in dicotyledonous plants. Adventitious root : The root system that develops from any part of the plant body other than the radicle is called the adventitious root system. It is mostly seen in monocotyledonous plants. In grasses, fibrous root system is present.

The leaf is a green, flat, thin, expanded lateral appendage of stem which is borne at a node and bears a bud in its axil. It is exogenous in origin and develops from the leaf primordium of shoot apex. The green colour of leaf is due to presence of the photosynthetic pigment – chlorophyll which helps plants to synthesize organic food. The green photosynthetic leaves of a plant are collectively called foliage. Characteristics of leaf (1) The leaf is a lateral dissimilar appendage of the stem. (2) A leaf is always borne at the node of stem. (3) The growth of leaf is limited. (4) The leaves do not possess any apical bud or a regular growing point. Parts of a typical leaf The leaf consists of three parts namely, leaf base (usually provided with a pair of stipules), petiole and leaf blade or lamina. (1) Leaf base (Hypopodium) : Leaf base is the lower most part of the leaf meant for attachment. It acts as a leaf cushion. Some times leaf base shows different variations as follows : (i) Pulvinus leaf base : In members of leguminosae the leaf base is swollen. Such swollen leaf bases are called pulvinus leaf bases as seen in mango leaves. It helps in seismonastic movements (e.g., Mimosa pudica) and nyctinastic movements (e.g., Enterobium, Arachis, Bean).     (ii) Sheathing leaf base : In grasses and many monocots, the leaf base is broad and surrounds the stem as an envelope, such a leafbase is called sheathing leaf base. e.g., Sorghum, Wheat and Palms. In grasses (Sorghum, Wheat etc.) the sheathing leaf base protects the intercalary meristem. (iii) Amplexicaul : Leaf base completely encircles the stem. e.g., Polygonum. (iv) Modified leaf base : The leaf bases in few plants perform accessory functions and show modifications. In Allium cepa (Onion), the leaf bases store food materials and become fleshy. In Platanus and Robenia, the leaf bases protect the axillary buds and grow around them to form cup like structures. (v) Stipule : The stipules are the small lateral appendages present on either side of the leaf base. They protect the young leaf or leaf primordia. Leaves with stipules are called stipulate and those without them are called exstipulate. The stipules are commonly found in dicotyledons. In some grasses (Monocots) an additional outgrowth is present between leaf base and lamina. It is called ligule. The leaves having ligules are called ligulate. Types of stipules : Depending upon the structure and position various kinds of stipules are recognized. Free lateral stipules : A pair of freely arranged stipules present on either side of the leaf base are called free lateral stipules, e.g., Hibiscus and Cotton. Adnate stipules : The two stipules that fuse with the leaf base or petiole on either side are called adnate stipules, e.g., Arachis  and Rose. Inter petiolar stipules : Stipules present in between the petioles of opposite leaves, e.g., Ixora and Hamelia. Axillary stipules : more...

In angiosperms, always the branches are produced by the growth of axillary buds or lateral buds. This type of branching is known as lateral branching. The lateral branching is classified into two kinds racemose and cymose. (1) Racemose branching : In this type of branching, the terminal (or apical) bud of the main stem grows indefinitely and the axillary buds grow out into lateral branches in acropetal succession. This type branching is also called monopodial branching. Due to monopodial branching the shoot system of plant appears conical e.g., Eucalyptus, Polyalthia (Ashoka tree). (2) Cymose branching : In cymose branching the terminal bud is active for a short period and becomes modified into some permanent structures like tendrils, thorns of flowers etc. Due to the terminal bud modification the growth of the main stem is definite. This is also called sympodial branching. Further growth in the plant is carried by one or more axillary buds. Cymose branching may be of three types : (i) Uniparous or Monochasial type : In uniparous type of branching only one lateral branch is produced at each time below the modified terminal bud. Here the successive lateral branches that are formed unite to form a stem. Such a stem is called false axis or sympodium. The uniparous branching is of two kinds, helicoid and scorpoid. (a) Helicoid branching : If the successive lateral branches develop on one side it is called helicoid branching. e.g., Saraca, Canna and Terminalia. (b) Scorpioid branching : If the successive lateral branches develop on either side alternately, it is called scorpioid branching, e.g., Cissus, Gossypium and Carissa.     (ii) Biparous or Dichasial type : When the activity of terminal bud stops, further growth of plant takes place by two lateral branches, e.g., Viscum (Mistletoe), Silene, Stellaria, Mirabilis jalapa (Four O’ clock), Dianthus (Pink), Carissa carandas (Karonda), etc. (iii) Multiparous or Polychasial type : When the activity of terminal bud stops, further growth of plant takes place by a whorl of three or more axillary branches. The axis is said to be multipodial, e.g., Euphorbia tirucalli, Croton, Nerium odoratum (Oleander).

Underground stem The underground stems lack green colour because of their geophillous nature. They can be identified as stems because of the presence of nodes, internodes, scale leaves, buds and branches. Based on the type of growth (transverse/vertical/oblique) and the part that stores food (main stem/ branch/ leaf base), the underground stems are classified into several types : (1) Sucker : This is a sub aerial branch that arises from the main stem. Initially it grows horizontally below the soil surface and later grows obliquely upward. They are shorter and stouter than the runners. e.g., Mentha arvensis (mint vern. Podina) and Chrysanthemum.     (2) Stem tuber : Stem tuber is the tuberous tip of an underground branch. It occurs beneath the soil at any depth. The axillary branches (stolons) that are produced near the soil surface grow into the soil and their tip become swollen due to accumulation of starch and proteins e.g., Solanum tuberosum (potato). In potato, the stem nature is evident by the presence of ‘eyes’ on its brownish corky surface. Each eye is a pit like structure and represents the node. Axillary bud is situated in the pit of the eye. The stem tubers are differentiated from the tuberous roots by the presence of vegetatively propagating eyes. (3) Rhizome : The rhizome is a thickened, underground dorsiventral stem that grows horizontally at particular depth within the soil. The rhizome is brown in colour. It can be distinguished from the modified root by the presence of nodes, internodes, terminal bud, axillary bud and scale leaves. The terminal bud develops aerial shoot that bears inflorescence. Adventitious roots develop on the ventral surface of the rhizome. The rhizomes are perennial and vegetatively propagating structures. It is of following types : (i) Rootstock : They are upright or oblique with their tips reaching the soil surface. e.g., Alocasia indica and Banana. (ii) Straggling : They are horizontal in position and generally branched (Sympodial or Monopodial), e.g., Nelumbo nucifera (Lotus), Zingiber officinale (Ginger), Curcuma domestica (Turmeric), Saccharum etc. (4) Corm : The corm is an underground modification of main stem. It grows vertically at particularly depth in the soil. The corm stores food materials and becomes tuberous. It is non green in colour and conical, cylindrical or flattened in shape. The corm bears scale leaves at each node. In the axils of these scale leaves axillary buds arise which grow into daughter corms. The terminal bud of the corm is large.     It grows into aerial shoot and bears leaves and flowers. Adventitious roots normally develop from the base or all over the body of the corm. With the help of some special adventitious roots called the contractile roots or pull roots, the corm remains constantly at a particular depth. The corm propagates vegetatively by daughter corms. e.g., Amorphophallus, Colocasia and Crocus (Saffron). (5) Bulb : A bulb is a specialized underground stem which more...

Sometimes the root performs other functions other than fixation, absorption and conduction so get modified structurally. Both tap roots and adventitious roots may undergo such modifications. There are many types of root modifications. Modification of tap roots (1) Storage roots : In some plants, the primary tap roots are modified for storing reserve food materials. The secondary roots remain thin and they are absorptive in function. The storage roots are usually swollen and assume various forms: (i) Conical : The swollen root is broad at the base and tapers gradually towards the apex giving a shape of cone, e.g., Carrot. (ii) Fusiform : The root is swollen in the middle and narrow towards both its base e.g., Radish (Raphanus sativus). (iii) Napiform : The root is nearly globular or spherical in shape. The basal portion of root is much swollen which suddenly tapers towards the apex giving a top-shaped appearance, e.g., Turnip (Brassica napus, vern, Shalgam) and Beet (Beta vulgaris, vern. Chukandar).     (iv) Tuberous : The storage root having no definite shape is called tuberous, .e.g., Mirabilis jalapa (4 O’clock plant), (2) Branched roots (i) Nodular roots (Tuberculated roots) : The primary tap roots and its branches of leguminous plants, i.e., plants belonging to sub-family papilionatae of the family leguminosae (e.g., Pea, Gram, Ground nut, Beans etc.), bear nodule like swellings, called root nodules. They are red in colour due to the presence of leg-haemoglobin. The nodules are inhabited by nitrogen fixing bacteria called Rhizobium leguminosarum. It converts atmospheric nitrogen into nitrates and supply them to the plant. In turn Rhizobium gets nutrients and shelter from the plant. (ii) Pneumatophores or Respiratory roots : The roots of some plants growing in saline marshes (mangrove plants) suffer from the lack of oxygen. This is due to the water logged condition of the soil. To cope with this situation some root branches grow vertically upwards.     They become aerial and negatively geotropic. These roots bear many minute pores called pneumathodes (lenticels) towards their upper ends. Gaseous exchange takes place through pneumathodes. Such aerial, porous, roots which help in gaseous exchange are called breathing roots. e.g., Sonneratia, Heritiera, Rhizophora, Avicennia and Ceriops etc. and are found in sundarbans of West Bengal. For physiological or Vital functions (1) Storage roots : The roots where adventitious roots become swollen to store food. They are of following types : Tuberous roots : These adventitious roots are swollen without any definite shape e.g., Ipomoea batata or (Sweet potato). Fasciculated roots : These are tuberous roots arising in cluster from the base of the stem. e.g., Dahlia, Ruellia (Menow weed), Asparagus etc.     Nodulose roots : These roots become swollen at their tips due to accumulation of food e.g., Maranta sp. (Arrowroot), Curcuma amanda (Mango – ginger). Moniliform or Beaded roots : These adventitious roots are swollen at frequent intervals. more...

The flowers are arranged in some definite manner on the plant in each species of the flowering plants. The mode of arrangement of flowers on a specialised branch on top of the plant which bears flowers is called inflorescence. The stalk of the inflorescence is called peduncle. Depending upon the arrangement of flowers, inflorescence is classified as follows :  

It can be defined as modified dwarf shoot which is meant for sexual reproduction. It is characteristic feature of angiosperm. Parts of a typical flower : A typical flower of an angiosperm consists of four types of floral parts namely calyx, corolla, androecium and gynoecium. General description of a flower The flowers are termed pedicellate if they possess stalks and sessile if they lack them. The flower may be described as complete if it bears all the floral parts and incomplete, when one or more floral parts are absent. Flowers are called bisexual if they bear both androecium and gynoecium. The unisexual flowers have either androecium or gynoecium. The unisexual flowers may be male flowers or female flowers. The male flower are also called staminate flowers as they have stamens only. The female flowers have only the carpels and hence called pistillate flowers. Flowers with sterile sex organs are described as neutral flowers. According to the distribution of male, female and bisexual flowers, various patterns are recognized. Monoecious : Presence of male and female flowers on the same plant, e.g., Acalypha, Cocos and Ricinus. Dioecious : Presence of male and female flowers on different plants, namely, male plants and female plants. e.g., Cycas, Carica papaya and Vallisneria. Polygamous : Presence of unisexual and bisexual flowers on the same plant, e.g., Mangifera and Polygonum. Symmetry of flower The number, shape, size and arrangement of floral organs in a flower determines its symmetry. On the basis of symmetry flowers can be of the following types : Actinomorphic (Regular = Symmetrical) : Actinomorphic flowers can be divided (passing through center) by any vertical plane into two equal and similar halves. e.g., Mustard, Brinjal, Catharanthus roseus. Zygomorphic (Monosymmetrical) : Zygomorphic flowers can be divided into two equal halves by only one vertical division e.g., Pea, Larkspur, Ocimum. Asymmetrical (Irregular) : Asymmetrical flowers can not be divided into two equal halves by any vertical division. e.g., Canna, Orchids.     Arrangement of floral organs On the basis of arrangement of floral organs, three types of flowers are recognized. They are : Acyclic : Here the thalamus is conical or convex and the floral parts are spirally arranged, e.g., Water lily and Magnolia. Cyclic : Here the floral organs are arranged in regular whorls at the nodes of the thalamus, e.g., Hibiscus and Datura. Hemicyclic (Spirocyclic) : Here some floral parts (sepals and petals) are arranged in regular whorls and the remaining parts (stamens and carpels) are arranged spirally. e.g., Annona  and Polyalthia. Number of floral parts in whorl is called the merosity. There are two kinds of flowers based on the merosity of the flower. They are isomerous flowers and anisomerous flowers. If the number of sepals, petals, stamens and carpels of flower is equal, such flowers are called isomerous flowers. Dimerous : Two floral parts in each whorl. e.g., Poppy flower. Trimerous : Three floral parts in each whorl. e.g., Monocot flowers (Liliaceae). Tetramerous : Four more...

A bud is a compact undeveloped young shoot consisting of a shoot apex, compressed axis and a number of closely overlapping primordial leaves arching over the growing apex. Buds which develop in to flower are called floral buds. Nature of buds : According to nature they are following types : (1) Vegetative buds : These buds grow to form only leafy shoots. (2) Floral buds :  These buds grow to form flowers. (3) Mixed buds : They produce both vegetative and floral branches. Position of buds : They are of following types : (1) Normal buds : These buds are borne on stems either terminally or laterally. Since they are borne in normal positions, they are called normal buds : Apical buds : They are borne at the apex of the main stem or a branch. They are also called terminal buds. Cabbage is a large apical bud. Lateral buds : The buds, which are borne in any other place except at the apices of main stem and its branches, are called lateral buds. (2) Adventitious buds : When a bud grows from a position other than normal, it is called adventitious bud. Bulbils or Specialised buds : Modification of whole buds into swollen structures due to storage of food materials are called bulbils. e.g., In Lilium bulbiferum and Dioscorea bulbifera, the bulbils develop in axil of leaves; in Agave, floral buds of inflorescence transform into bulbils; In Oxalis, they develop just above the swollen roots.

Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “Tissue System''. According to Sachs (1975) there are three major tissue systems in plants as follows : (1) Epidermal tissue system : The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment. Epidermis : Epidermis is composed of single layer of cells. These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some cases epidermis may be multilayered e.g. Ficus, Nerium, Peperomia, Begonia etc. The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces. Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types : (i) In aerial roots, the multiple epidermal cells are modified to velamen, which absorbs water from the atmosphere (e.g., Orchids). (ii) Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. It is hygroscopic in nature. e.g., Ammophila. They are thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves. (iii) Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells. Cuticle and Wax : In aerial parts, epidermis is covered by cuticle. The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all walls. It helps in reducing the loss of water by evaporation. Usually the cuticle is covered with wax which may be deposited in the form of granules, rods, crusts or viscous semiliquid masses. Other substances deposited on the cuticle surface may be oil, resin, silicon and salts (cystoliths are crystals of calcium carbonate, e.g., Ficus. Druse and Raphides, e.g., Pistia are crystals of calcium oxalate). Thick cuticle are found in leaves of dry habitats plants. Stomata : Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard cells. Stomata are absent in roots. In xerophytes the stomata are sunken in grooves due to which rate of transpiration is greatly reduced (e.g. Nerium). Usually there is a large air cavity below each aperture, it is called substomatal cavity. In some species the guard cells are surrounded by subsidiary cells or accessory cells which differ morphologically as well as ontogenitally from the other epidermal cells. In monocots subsidiary cells and guard cells originated from same cell. e.g., Doob, Maize guard cells are dumb bell shape. Stomata are scattered in dicots leaves but they are arranged in rows in monocots. Trichomes : These are epidermal outgrowths present temporarily or permanently on almost all plant parts. They may be unicellular or multicellular and vary in size more...

These tissue perform special function in plants, e.g., secretion of resins gum, oil and latex. These tissues are of two types : (1) Laticiferous tissues : They are made up of thin walled, elongated, branched and multinucleate (coenocytic) structures that contain colourless, milky or yellow coloured juice called latex. These occur irregularly distributed in the mass of parenchymatous cells. Latex is contained inside the laticiferous tissue which is of two types : (i) Latex cells : A laticiferous cell is a highly branched cell with long slender processes ramifying in all directions in the ground tissue of the organ. They do not fuse and do not form network. Plants having such tissues are called simple or non-articulated laticifers. e.g., Calotropis (Asclepiadaceae) Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae). (ii) Latex vessels : They are formed due to fusion of cells and form network like structure in all directions. At maturity, they form a highly ramifying system of channels full of latex inside the organ. Plants having such tissues are called compound or articulated laticifers. e.g., Argemone, Papaver (Papaveraceae), Sonchus (Compositae), Hevea, Manihot (Euphorbiaceae). (2) Glandular tissue : This is a highly specialized tissue consisting of glands, discharging diverse functions, including  secretory and excretory. Glands may be external or internal. (i) External glands : They generally occur on the epidermis of stem and leaves as glandular hair in Plumbago and Boerhaavia, stinging hair secrete poisonous substance in Urtica dioica, nectar secreting glands in flowers or leaves. e.g., Rutaceae and Euphorbiaceae. Digestive enzyme secreting glands in insectivorous plants e.g., Drosera (Sundew), Nepenthes (Pitcher plant). (ii) Internal glands : These are present internally and are of several types. e.g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus, mucilage canals in Cycas. Water secreting glands (hydathodes) in Colocasia (present at the tip of leaves), Tropaeoleum (along margin), etc. The glands which secrete essential oil are called osmophores (osmotrophs).



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