Current Affairs 11th Class

"The movement of organic food or solute in soluble form, from one organ to another organ is called translocation of organic solutes." The process of translocation requires expenditure of metabolic energy and the solute moves at the rate of 100 cm/hr. Directions of translocation Downward translocation : It is of most important type, i.e., from leaves to stem and roots. Upward translocation : From leaves to developing flowers, buds, fruits and also during germination of seeds and tubers, etc. Radial translocation : From pith to cortex and epidermis. Path of translocation (1) Downward translocation of organic solutes : Phloem is the path for downward translocation of organic food. Following evidences are in support of it : (i) Elimination of other tissues : Xylem is responsible for upward movement of water and minerals, so it cannot account for downward translocation of solute at the same time. Thus only phloem is left (where there is end to end arrangement of sieve tubes united by sieve pores). Which is responsible for translocation of solutes in downward direction. (ii) Chemical analysis of phloem sap and xylem sap : Chemical analysis of sieve tube sap proves that concentrated solution of sucrose is translocated from the place of synthesis to other parts of the plant body. Glucose and fructose are sometimes found in traces only. The amount of sucrose is more in phloem sap during the day and less in night. In xylem the amount of sucrose is in traces and also there is no diurnal fluctuation. (iii) Blocking of phloem : Blocking of sieve pores by 'callose' during winter blocks translocation of solutes. (iv) Ringing or Girdling experiment : It was first performed by Hartig (1837). On removing the ring of bark (phloem + cambium) above the root at the base of stem, accumulation of food occurs in the form of swelling just above the ring, which suggests that in absence of phloem, downward translocation of food is stopped. (v) Structure of phloem : The structure of phloem tissue is well modified for conduction of solutes. Phloem tissue of an angiosperm consists of sieve tubes, companion cells several kinds of parenchyma cells, fibres and scleroids. Of these sieve tubes are involved in sugar translocation. (2) Upward translocation of organic solutes : According to Curtis upward conduction of foods also takes place through phloem. Mechanism of translocation Diffusion hypothesis : Mason and Maskel (1928) working on cotton plant demonstrated that the translocation of foods occurs from the place of high concentration (place of manufacture or storage) to the place of lower concentration (place of consumption) but it is very slow process so Mason and Phillis (1936) modified this concept and proposed activated diffusion hypothesis. According to this concept the food particles are first energy activated then translocated. This hypothesis is not accepted due to lack of experimental evidence. Protoplasmic streaming hypothesis : This concept was proposed by de Vries (1885). According to him the food is transported across by streaming current of protoplasm. The cell protoplasm more...

'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...

Water is mainly absorbed by the roots of the plants from the soil, then it moves upward to different parts and is lost from the aerial parts, especially through the leaves. Before taking up the absorption and movement of water in plants, it is worthwhile to understand the phenomenon of imbibition, diffusion and osmosis involved in the water uptake and its movement in the plants. Imbibition (L. imbibere – to drink) : The process of adsorption of water by hydrophilic surfaces of a substance without forming a solution is called 'imbibition'. It is a type of diffusion by which movement of water takes place along a diffusion gradient. The solid particles which adsorb water or any other liquid are called imbibants. The liquid which is imbibed is known as imbibate. Agar, cellulose, pectic substances, protoplasmic protein and other organic compound in plant cells show great power of imbibition. Characteristics of imbibition : The phenomenon of imbibition has three important characteristics : Volume change : During the process of imbibition, imbibants increase in volume. It has been observed that there is an actual compression of water. This is due to arrangement of water molecules on surface of imbibant and occupy less volume than the same molecules do when are in free stage in the normal liquid. e.g., If a dry piece of wood is placed in water, they swell and increases in its volume. Production of heat : As the water molecules are adsorbed on the surface of the imbibant, their kinetic energy is released in the form of heat which increases the temperature of the medium. It is called heat of wetting (or heat of hydration). e. g., during kneading, the flour of wheat gives a warm feeling due to imbibition of water and consequent release of heat. Development of imbibitional pressure : Imbibition pressure can be defined as the maximum pressure that an imbibant will develop when it is completely soaked in pure water. Imbibition pressure is also called as the matric potential because it exists due to the presence of hydrophilic substances in the cell which include organic colloids and cell wall. Factors influencing the rate of imbibition Nature of imbibant : Proteins are the strongest imbibants of water, starch less strong, cellulose being the weakest. Surface area of imbibant : If more surface area of the imbibant is exposed and is in contact with liquid, the imbibition will be more. Temperature : Increase in temperature causes an increase in the rate of imbibition. Degree of dryness of imbibant : If the imbibant is dry it will imbibe more water than a relatively wet imbibant. Concentration of solutes : Increase in the concentration of solutes in the medium decreases imbibition. pH of imbibant : Proteins, being amphoteric in nature, imbibe least in neutral medium. Towards highly acidic or highly alkaline pH, the imbibition increases till a maximum is reached, there after it starts slowing down. Significance of imbibition (1) The water is first imbibed by walls of root 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.