Current Affairs 11th Class

'The upward transport of water along with dissolved minerals from roots to the aerial parts of the plant is called Ascent of sap'. It is also called translocation of water. The water with dissolved minerals is called sap. Path of ascent of sap : It is now well established that the ascent of sap takes place through xylem. In herbaceous plants almost all the tracheary elements participate in the process, but in large woody trees the tracheary elements of only sap wood are functional. Further, it has been proved experimentally that sap moves up the stem through the lumen of xylem vessels and tracheids and not through their walls. Theories of ascent of sap : The various theories put forward to explain the mechanism of ascent of sap in plants can be placed in following three categories : (1) Vital force theories (2) Root pressure theory (3) Physical force theories (1) Vital force theories : According to these theories the forces required for ascent of sap are generated in living cells of the plant. These theories are not supported by experimental evidences hence they have been discarded. Some of the important vital force theories are mentioned below : According to Westermaier (1883), ascent of sap occurs through xylem parenchyma; tracheids, and vessels only act as water reservoirs. Relay pump theory (Clambering theory) : According to Godlewski (1884) ascent of sap takes place due to rhythmatic change in the osmotic pressure of living cells of xylem parenchyma and medullary rays and are responsible for bringing about a pumping action of water in upward direction. Janse (1887) supported the theory and showed that if lower part of the shoot is killed upper leaves were affected. Criticism (i) Strasburger (1891) and Overton (1911) used poisons (like picric acid) and excessive heat to kill the living cells of the plant. When such twigs were dipped in water, ascent of sap could still occur uninterrupted. This definitely proved that no vital force is involved in ascent of sap. (ii) Xylem structure does not support the Godlewski's theory. For pumping action living cells should be in between two xylem elements and not on lateral sides as found. Pulsation theory : Sir J.C. Bose (1923) said that living cells of innermost layer of cortex, just outside the endodermis are in rhythmatic pulsations. Such pulsations are responsible for pumping the water in upward direction. According to Bose, the pulsatory cells pump the water into vessels. Criticism : Dixon failed to verify the results of Bose. It has been estimated that sap should flow through 230–240 pulsating cells per second to account for normal rate of pulsations. This rate is several times higher as would be possible to the Bose theory (Shull, MacDougal, Benedict). (2) Root pressure theory : It was proposed by Priestley (1916). According to this theory the water, which is absorbed by the root-hairs from the soil collects in the cells of the cortex. The cortical cells become fully turgid. In such circumstances the elastic walls more...

Water is absorbed from soil by root system and specially by younger parts (i.e., root tips). In higher plants water is absorbed through root hairs. Soil water : The chief source of soil water is rain. In soil water is found in different forms. The total amount of water present in the soil is called holard, of this the available to the plant is called chresard and the water which cannot be absorbed by the plants is called echard. Water occurs freely deep in the soil and above the parent rock, it is called ground water. These are briefly described below : Gravitational water : When the water enters the soil and passes the spaces between the soil particles and reaches the water table, the type of soil water is called gravitational water. Capillary water : It is the water which is held around soil particles in the capillary space present around them due to force like cohesion and surface tension. This is the water which can be utilised by the plants. It is also called growth water. It occurs in the form of films coating smaller soil particles. The availability of capillary water to the plant depends upon its diffusion pressure deficit which is termed as the soil moisture stress. The plant cells have a DPD much more than the soil moisture stress for proper absorption of water. Hygroscopic water : This is the form of water which is held by soil particles of soil surfaces. The water is held tightly around the soil particles due to cohesive and adhesive forces. Cohesive and adhesive forces greatly reduce the water protential (yw) and thus this type of water in soil is not available to plants. Run-away water : After the rain, water does not enter the soil at all, but drained of along the slopes. It is called run-away water. Plants fail to avail this water. Chemically combined water : Some of the water molecules are chemically combined with soil minerals (e.g., silicon, iron, aluminium, etc.). This water is not available to the plants. Water vapour : That portion of the pore space in a soil which is not occupied by liquid water contain a soil atmosphere that always includes water vapour. Water holding capacity : The amount of water actually retained by the soil is called field capacity or water holding capacity of the soil. It is about \[2535%\] in common loam soil. The excess amount of water beyond the field capacity produces water logging. Soil atmosphere : In moderately coarse soils as well as in heavy soils (fine textured soil) that are with aggregated particles; there exists large interstitial spaces which facilitate the diffusion of gases. As a result the \[C{{O}_{2}}\]produced in a soil by respiration of soil organisms and roots is able to escape rather easily and oxygen used up in this process diffuses into the soil with corresponding case. Soil organisms : The soil fauna includes protozoans, nematodes, mites, insects, earthworms, rats. Protozoans alone are approximately more...

Plant and their parts develop continuously from germination until death. The production of flowers, fruits and seeds in annuals and biennials leads to senescence. The latter part of the developmental process, which leads from maturity to the ultimate complete loss of organization and function is termed senescence. Several workers equate ageing and senescence as same process. Ageing is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant. The study of plant senescence is called phytogerontology.  Types of senescence : Plant senescence is of four types- whole plant senescence, shoot senescence, sequential senescence and simultaneous senescence. The last three are also called organ senescence.     (1) Whole plant senescence : It is found in monocarpic plants which flower and fruit only once in their life cycle. The plants may be annual (e.g., rice, wheat, gram, mustard etc.), biennials (e.g., cabbage, henbane) or perennials (e.g., certain bamboos). The plant dies soon after ripening of seeds. (2) Shoot senescence : This type of senescence is found in certain perennial plants which possess underground perennating structures like rhizomes, bulbs, corm etc. The above ground part of the shoot dies each year after flowering and fruiting, but the underground part (stem and root) survives and puts out new shoots again next year. e.g., banana, gladiolus, ginger etc. (3) Sequential senescence : This is found in many perennial plants in which the tips of main shoot and branches remain in a meristematic state and continue to produce new buds and leaves. The older leaves and lateral organs like branches show senescence and die. Sequential senescence is apparent in evergreen plants e.g., Eucalyptus, Pinus, Mango. (4) Simultaneous or synchronous senescence : It is found is temperate deciduous trees such as elm and maple. These plants shed all their leaves in autumn and develop new leaves in spring.  Because of this shedding of leaves, autumn season is also called fall. e.g., Dalbergia, Elm, Mulberry, Poplar. Theories of senescence (1) Wear and tear : According to this theory, senescence occurs due to loss of activity and cells undergo wear and tear due to disintegration of organelles. (2) Toxicity : It is viewed that senescence takes place due to accumulation of toxic and deleterious substances in all. (3) Loss of metabolites : It is assumed that senescence leads to gradual depletion of essential metabolites in a cell. (4) Genetic damage Characteristics of ageing and senescence (1) There is general decline in metabolic activities, decline in ATP synthesis and also decreased potency of chloroplast. (2) Decrease in RNA and DNA (3) Decrease in semipermeability of cytoplasmic membranes. (4) Decrease in the capacity to repair and replace wornout cells. (5) There may be accumulation of chromosomal aberrations and gene mutations with advancing age as a result of these changes protein synthesis becomes defective. (6) Increased production of hydrolytic enzymes such as proteases and nucleases. (7) Deteriorative change in cell organelles and membranes. (8) Decrease in the internal content of auxin more...

Plants show movements in response to a variety of stimuli. Stimulus can be defined “as a change in external or internal environment of an organism that elicits response in the organism”. The reaction of plant to a stimulus is known as response. The power or ability of a plant to respond to a stimulus is called sensitivity or reactivity or irritability. The movements which occur without the effect of external stimulus are called autonomic or spontaneous movements. Thus spontaneous movements are brought by definite internal stimulus, and if the movements are produced in response to external stimulus, they are known as paratonic or induced movements. The area which perceives a stimulus is called perceptive region, while the plants part showing the response is known as responsive region. The minimum duration or time required for a stimulus to be applied continuously on the perceptive region to produce visible response is called presentation time. The duration between the application of stimulus and production of visible response is called latent time or reaction time. Classification of plant movements Plants movements are broadly classified into two types: (1) Movements of locomotion: In this case, plant moves physically from one place to another. The movements of locomotion are of two type-autonomic (occurs spontaneously) or paratonic (induced by external stimuli). (i) Autonomic movement of locomotion : These movement of locomotion are due to internal stimuli they are of following types : (a) Ciliary movements : Certain motile algae (e.g., Chlamydomonas, Volvox, etc). Zoospores and gametes of lower plants move from one place to another by means of cilia or flagella. (b) Amoeboid movements : It is the movement of naked mass of protoplasm by means of producing pseudopodia like process e.g., members of Myxomycetes (slime fungi).     (c) Cyclosis : These are movements of cytoplasm with in a cell (also called protoplasmic streaming).  These are of two types :
  • Rotation : When the protoplasm moves around a single central vacuoles in either clockwise or anticlockwise direction e.g., leaf cells of Hydrilla, Vallisneria.
  • Circulation : When the movement of protoplasm accurs around different vacuoles in different directions within the cell e.g., staminal hair of Tradescantia, shoot hairs of gourds.
(d) Excretory movements : Apical part of Oscillatoria is like a pendulum. It is considered that such movements are due to excretion of substances by the plants. (movements opposite to the side of excretion). (ii) Paratonic movement of locomotion (Tactic movement) : These movements take place in whole small plants.  e.g., chlamydomonas or small free ciliated organs e.g., gametes. These movements are due to external factors like light, temperature or chemicals and are of following types : (a) Phototactic movements or phototaxisms : It is the movement of free living organism towards or away from light. e.g., movement of Chlamydomonas, Ulothrix, Cladophora, Volvox etc. towards suitable light intensity. Three types of arrangement present in columular cells in more...

Flowering in a plant occurs at a particular time of the year and controlled by many morphological and environmental conditions. Two important controlling factors are photoperiod or light period, i.e., photoperiodism, low temperature i.e., vernalization. (1) Photoperiodism (Light period) : The effects of photoperiods or daily duration of light periods (and dark periods) on the growth and development of plants, especially flowering is called photoperiodism. The role of photoperiodism in the control of flowering was demonstrated for the first time by W.W Garner and H.A. Allard (1920). They observed that Maryland Mammoth variety of tobacco could be made to flower in summer by reducing the light hours with artificial darkening. It could be made to remain vegetative in winter by providing extra light. On the basis of length of photoperiod requirements of plants, the plants have been classified into following categories. (i) Short day plants (SDP) : These plants initiate flowering when the day length (Photoperiod) become shorter than a certain critical period. Most of winter flowering plants belong to this category e.g., cocklebur (Xanthium), Chrysanthemum, sugarcane, tobacco (Mutant Maryland Mammoth), soyabean, strawberry etc., (ii) Long day plants (LDP) : These plants begin flowering when the day length exceeds a critical length. This length too differs from species to species. The long day plants fail to flower, if the day length is shorter than the critical period. e.g., spinach (Spinacea oleracea), henbane (Hyoscymus niger), radish, sugar-beet, wheat, lattuce, poppy, larkspur, maize etc. (iii) Day neutral plants : These plants can flower in all possible photoperiods. The day neutral plants can blossom throughout the year. e.g., cucumber, cotton, sunflower, tomato, some varieties of pea, etc.     (iv) Intermediate plants : These plants flower only under day lengths within a certain range usually between 12-16 hours of light but fail to flower under either longer or shorter photoperiods. e.g., Mikania scandens, Eupatorium hyssopifolium and Phaseolous  polystacous.  (a) Amphiphotoperiodic plants : Such plants remain vegetative on intermediate day length and flower only on shorter or longer day lengths. e.g., Media elegans. (b) Short long day plants : These plants require short photoperiods for initiation of flowering and long photoperiods for blossoming. e.g., Triticum vulgare, Secale cereale. (c) Long short day plants : These plants require long photoperiods for initiation of flowering and short photoperiods for blossoming. e.g., Bryophyllum, Cestrum. Critical period : Critical photoperiod is that continuous duration of light, which must not be exceeded in short day plants and should always be exceeded in long day plant in order to bring them to flower. There is no relation with the total day length. Thus, the real distinction between a SDP and LDP is whether flowering is induced by photoperiods shorter or longer than the critical period. The critical day length for Xanthium (a short day plant) is 15. 6 hours and that for Hyoscymus niger (a long day plant) is about 11 hours, yet the former is more...

The term hormone used by first Starling (1906). He called it stimulatory substance. The growth and development in plants is controlled by a special class of chemical substances called hormones. They are needed in small quantities at very low concentrations as compared to enzyme. They are rarely effective at the site of their synthesis. Thus, growth hormones also called phytohormones term given by Thimann (1948), it can be defined as ‘the organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes’. A group of plant hormones including auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to regulate growth. Auxins : Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the roots. Among the growth regulators, auxins were the first to be discovered. Discovery : Julius Von Sachs was the first to indicate the presence of organ forming substances in plants. The existence of first plant growth hormone came from the work of Darwin and Darwin (1881). Darwin described  the effects of light and gravity in his book, “Power of movements in plants”. Darwin and his son found that bending movement of coleoptile of Canary grass (Phalasis canariensis) was due to exposure of tip to unilateral light. Boysen-Jensen (1910; 1913) found that the tip produces a chemical which was later named auxin. Paal (1914, 1919) removed coleoptile tip and replaced it asymmetrically to find a curvature. Auxin was first collected by Went (1928) from coleoptile tip of Avena. Went also developed Avena curvature test for bioassay of auxin. Types of auxins : There are two major categories of auxins natural auxins and synthetic auxins. (1) Natural auxins : These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.     The first naturally occurring auxin was isolated by Kogl and Haagen-Smit (1931) from human urine. It was identified as auxin-a (auxenotriolic acid,\[{{C}_{18}}{{H}_{32}}{{O}_{5}}\]). Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, auxin, called auxin-b (auxenolonic acid,\[{{C}_{18}}{{H}_{30}}{{O}_{4}}\]) from corn germ oil (extracted from germinating corn seeds), and heteroauxin from human urine. Heteroauxin (C10H9O2N) also known as indole-3-acetic acid (IAA), is the best known natural auxin, Besides IAA, indole-3-acetaldehyde, indole-3-pyruvic acid, indole ethanol, 4-chloro-idole actic acid  (4-chloro-IAA) etc., are some other natural auxins. Natural auxins are synthesized (Young) in physiologically active parts of plants such as shoot apices, leaf primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in stem more...

The process of shedding of leaves, fruits or flowers by a plant is called abscission. The shedding of plant parts takes place by the formation of a special layer of cells called abscission layer, within the region of attachment. The middle lamella between certain cells in this layer in often digested by polysaccharide hydrolyzing enzymes such as cellulase and pectinases.

The name "thyroid" was introduced by Thomas Wharton (1656). It is derived from Greek "Thyreos" a shield. Location : This is the largest endocrine gland of our body. It is located in our neck upon the ventral aspect of larynx (sound box or Adam's apple) and a few anteriomost tracheal rings. It is a dark brown and H-shaped/butterfly bilobed gland. Origin : It is endodermal in origin and arises in the embryo as a midventral process from the floor of the tongue in pharyngeal region between the first and second pharyngeal pouches. Later, the duct-like connection (thyroglossal duct) of the process degenerates, so that the process is separated from the tongue and becomes endocrine. Probably, the gland is homologous to the endostyle of lower chordates. Structure of thyroid gland : In adult human beings, thyroid gland measures about 5 cm in length and 3 cm in width. It's average weight is 30 grams. It is somewhat larger in women. In old age, it becomes somewhat smaller as age advances. Its two lobes are connected by a narrower isthmus formed of nonglandular connective tissue. A small, conical pyramidal lobe is often found extended forwards from the isthmus. The whole gland is enveloped by a fibrous capsule. Thin septa or trabeculae, extending inwards from the capsule, divide the gland into a number of lobules. Each lobule, in turn, consists of a large number of small and hollow, spherical follicles (acini) embedded in a small amount of a loose connective tissue that forms the stroma of the gland. The wall of each thyroid consists of a single-layered cuboidal epithelium suspended from a basal lamina, while its cavity is filled with a yellowish, jelly-like and iodinated colloid glycoprotein substance, called iodothyroglobulin. Besides containing a dense network of blood capillaries, the stroma contains small clusters of specialized parafollicular or 'C' cells. The latter are remnants of ultimobranchial bodies derived from the fifth pharyngeal (branchial) pouches in the embryo.     Synthesis and storage of iodothyroglobulin : Synthesis of a glycoprotein thyroglobulin (TGB) - occurs continuosly in the follicular cells under genic control. The cells keep extruding thyroglobulin in follicular cavity by exocytosis. Each molecule of thyroglobulin contains about 500 amino acid momoners of which 123 monomers are of tyrosine at fixed places. Soon as the molecules of iodine and thyroglobulin come out of follicular cells, these interact in such a way that 15 tyrosine monomers of each thyroglubulin molecule at fixed places become iodinated. Certain tyrosine monomers bind with single atoms of iodine, forming monoiodotyrosine (MIT or \[{{T}_{1}}\]). Other tyrosine monomers bind with two atoms of iodine, forming diiodotyrosine (DIT or \[{{T}_{2}}\]). This is called organification of thyroglobulin. Molecules of iodothyroglobulin keep accumulating in follicular cavity, forming the jelly-like colloid. Within the colloid, molecules of iodothyroglobulin undergo conformational changes and may even interact with each other. This results in a coupling of most of the iodinated tyrosine monomers in pairs. This more...

Origin and position : The thymus is bilobed gland, is located in the upper part of the thorax near the heart in the mediastinum. It is endodermal in origin, arising in the embryo from the epithelium of outer part of third branchial pouches. Structure : Structurally, it is like lymph gland enveloped by a thin, loose and fibrous connective tissue capsule. Septa, or trabeculae extending inwards from the capsule, divide the two lobes of the gland into a number of small lobules. Each lobule is distinguished into a cortical parenchyma containing numerous lymphocytes, and a medullary mass of large, irregularly branched and interconnected epithelial cells (reticular cells), a few lymphocytes and some phagocytic cells called macrophages or Hassal's corpuscles.     Function of thymus glands (1) Thymus is haemopoietic, as well as, an endocrine gland. Thymus is the "seedbed" of "thymic lymphocytes (T-lymphocytes). Certain "stem cells", originating in yolk sac and liver in early embryo, but only in bone marrow in late embryo, migrate into the thymus and proliferate to form a large number of lymphocytes. (2) The major function of thymus is to secrete thymosin hormone, thymic humoral factor (THF), thymic factor (TF), thymopoietin. These compounds induce, not only the proliferation of lymphocytes, but also their differentiation into a variety of clones differently specialized to destroy different specific categories of antigens and pathogens likely to get into the body. This is called maturation of lymphocytes. (3) As is clear from above account, thymus is essential in neonatal (newly born) infant and postnatal child for normal development of lymphoid organs and cellular immunity. That is why, the thymus, small at birth, progressively grows in size about three or four-folds upto about the age of puberty. By this time lymphoid organs and tissues are well-developed. The thymus, therefore, starts gradually diminishing in size and its tissue is progressively infiltrated by yellowish adipose tissue. This is known as the "immunity theory of ageing". By the old age, the thymus is reduced to quite a thin, yet functional chord of tissue.

Pituitary is known as hypophysis cerebri, its name pituitary was given by Vesalius. Muller’s gland of amphioxus and subneural gland of hardmania is homologous to pituitary of vertebrates. Weight of pituitary is 0.5 gm. Removal of pituitary is knows as hypophysectomy. Position and origin : Pituitary gland is the smallest (about 1 to 1½ cm in diameter) endocrine gland of the body. It is pea-shaped, ovoid, radish brown gland situated at the base of the brain in a cavity, hypophyseal fossa of the sella turcica of sphenoid bone. It is connected by a short stalk called Infundibulum, to the ventral wall (Hypothalamus) of diencephalon. That is why it is also called hypophysis cerebri. It weight about 0.5 to 1 gm. It control most of the endocrine glands. Hence, it is also called leader of endocrine orchestra or master gland. Pituitary gland is closely related with hypothalamus, hence, it is also called hypothalamo-hypophyseal gland, pituitary is ectodermal in origin. Parts and component Adenohypophysis       Structure of pituitary gland : Pituitary gland is comprised of two main lobes – Adenohypophysis and Neurohypophysis. Adenohypophysis is arises as hypophysial or Rathke's pouch from dorsal wall of embyronic stomodeum. It is the anterior lobe of pituitary. The neurohypophysis (Pars nervosa or Posterior lobe) form as an outgrowth from the infundibulum of the floor of hypothalamus.     In pituitary following types of cells are found : (1) Chromophobes cells : Found in adenohypophysis of pituitary. These are not stained by acid and base dye. Pigment granules are absent. These are colourless may change into chromophils. (2) Chromophil cells : Found in adenohypophysis of pituitary. These are stained by acid and base dye. Pigment granules are filled in these cells. These may be two types : (i) Acidophils : It is also known as a-cells synthesize and secretes growth hormone and prolactin. (ii) Basophils : It is also known as cyanophils or b-cells synthesize and secretes TSH, ACTH, FSH, LH and MSH hormones. (3) Pituicyte cells : These cells found in neurohypophysis of pituitary. These are supporting neuroglia cells and gives support to herring bodies. (4) Herring bodies : Herring bodies Are dilated terminal portion of Neurosecretory axon constituting hypothalamohypophyseal tract. They are hormone precursors for oxytocin and vasopressin.     Blood supply to pituitary or Hypophyseal portal system : A pair of posterior hypophysial arteries and a pair of anterior hypophysial arteries provide blood to the pituitary gland. Posterior arteries supply blood to the pars nervosa, and anterior arteries supply blood to the hypothalamus and pars distalis. Adenohypophysis has dual blood supply by means of a "circle of willis". The anterior hypophysial artery which bring blood into this circle big ureates in to two branches outside the lobe. One more...


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