Current Affairs 11th Class

The increase in thickness or girth due to the activity of the cambium and the cork cambium is known as secondary growth. (1) Secondary growth in stem : On the basis of the activities of cambium and cork-cambium, secondary growth in stem can be discussed under the following heads : Activity of cambium : The vascular cambium in between xylem and phloem is called intrafascicular or fascicular cambium which is primary in origin. At the time of secondary growth the parenchymatous cells of medullary rays between the vascular bundles become meristematic and form strip of cambium called as interfascicular cambium which is secondary in origin. Both inter and intrafascicular cambium joins together and form cambium ring which is partly primary and partly secondary in origin. By anticlinal divisions the circumference of the cambium increase. By periclinal division cambium produces the secondary xylem and phloem tissues on innerside and outerside. The amount of sec. xylem produced is 8-10 times greater than sec. phloem. The cambium has two types of cells : The fusiform initials : Which are elongated and form fibres, sieve cells, sieve tubes, tracheids. Ray initials : Which produce parenchyma cells of the rays in wood and phloem. Certain cells of cambium form some narrow bands of living parenchyma cells passing through secondary xylem and secondary phloem and are called secondary medullary rays. These provide radial conduction of food from the phloem, and water and mineral salts from the xylem. Annual rings : Activity of cambium is not uniform in those plants which grow in the regions where favourable climatic conditions (spring or rainy season) alternate regularly with unfavourable climatic conditions (cold water or dry hot summer). In temperate climates, cambium becomes more active in spring and forms greater number of vessels with wider cavities; while in winter it becomes less active and forms narrower and smaller vessels. The wood formed in the spring is known as spring wood and that formed in the dry summer or cold winter autumn wood or late wood. Both autumn and spring wood constitute a growth or annual ring. In one year only one growth ring is formed. Spring wood is light in colour while autumn wood is dark in colour. Activity of cork cambium : Cork cambium or phellogen develops from outer layer of cortex. It produces secondary cortex or phelloderm on innerside and cork or phellem on outerside. The cells of phellem are dead, suberized and impervious to water. Cells of phelloderm are thin walled, living and store food. Phellem, phellogen and phelloderm are collectively called as periderm. Periderm is secondary protective tissue. Due to pressure of secondary xylem, epidermis ruptures and cortex is largely lost after two or three years of secondary growth. Bark : All dead tissues lying outside the active cork-cambium are collectively known as bark. This includes ruptured epidermis, hypodermis and cork. When cork-cambium appears in the form of a complete ring, it is known as ring bark, e.g., Betula (Bhojpatra). If the cork cambium occurs as more...

Permanent tissues are made up of mature cells which have lost the capacity to divide and have attained a permanent shape, size and function due to division and differentiation in meristematic tissues. The cells of these tissues are either living or dead, thin-walled or thick-walled. Permanent tissues are of following types : Simple permanent tissues Simple tissues are a group of cells which are all alike in origin, form and function. They are further grouped under three categories : (1) Parenchyma : Parenchyma is most simple and unspecialized tissue which is concerned mainly with the vegetative activities of the plant.     The main characteristics of parenchyma cells are : (i) The cells are living, thin walled, soft, possess a distinct nucleus, having well developed intercellular spaces, vacuolated cytoplasm and cellulosic cell wall. (ii) The shape may be oval, spherical, cylindrical, rectangular and stellate (star shaped) in leaf petioles of banana and canna and some hydrophytes. (iii) This tissue is generally present in roots, stems, leaves, flowers, fruits and seeds. (iv) If they enclose large air spaces they are called as aerenchyma; if they develop chlorophyll, they are called as chlorenchyma and if they are elongated cells with tapering ends, they are called as prosenchyma. Functions : They perform the following functions : (i) Storage of food materials. e.g., Carrot, Beetroot etc. (ii) Chlorenchyma helps in photosynthesis. Aerenchyma helps in floating of the aquatic plants (Hydrophytes) and also helps in gaseous exchange during respiration and photosynthesis. e.g., Hydrilla. (iii) In turgid state they give rigidity to the plant organs. (iv) In emergency they behave like meristematic cells and help in healing of the various plant injuries. (v) Sometimes they store secretory substances (ergastic substance) such as tannins, resins and gums and they are called as idioblasts. (2) Collenchyma : The term collenchyma was coined by Schleiden (1839). It is the tissue of primary body. The main characteristics of collenchyma are given below : (i) The cells of this tissue contain protoplasm and are living without intercellular spaces. The cell walls are thickened at the corners and are made up of cellulose, hemicellulose and pectin. (ii) They are compactly arranged cells, oval, spherical or polygonal in outline. The tissue is elastic, extensible and have capacity to expand. (iii) Collenchyma occurs chiefly in the hypodermis of dicotyledonous stems (herbaceous, climbers or plants e.g. Cucurbita, Helianthus) and leaves. They are usually absent in monocots and in roots.     Types of collenchyma : Majumdar (1941) divided collenchyma into three types on the basis of thickening : (i) Angular collenchyma : When the thickening of the cells is confined to the corners of the cells. e.g., Tagetes, Tomato, Datura, Potato, etc. (ii) Plate or Lamellar collenchyma : When the thickenings are present in the tangential walls. e.g. hypodermis of sunflower stem. (iii) Lacunar or Tubular collenchyma : If the thickened cell wall is associated with intercellular spaces of more...

The word “Meristem” originated from “Meristos” (Greek = continuous division) and the term meristem was introduced by Nageli (1858). A group of cells which are much active and capable of showing continuous divisions and redivisions, is called as meristematic tissue. The various characteristic features of the meristems are discussed below : (1) They contain immature and young cells and are capable of repeated divisions. (2) Intercellular spaces are not present in meristematic tissue. (3) They contain a homogeneous thin cellulosic wall. (4) They contain large nuclei associated with abundant cytoplasm. (5) They are metabolically very active but they do not store food material and further no plastids in them. (6) Vacuoles are small or absent. (7) Meristematic cells are isodiametric in shape. (8) Undifferentiated tissue in which cells divides continuously           Types of meristems The meristems may be classified on the basis of their mode of origin, position or function : According to origin and development : On the basis of origin, meristematic tissues are of three types : (1) Promeristem or Primordial meristem : The promeristem originates from embryo and therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated. A group of initial cells that lay down the foundation of an organ or a plant part, is called promeristem. It occupies a small area at the tips of stem and root. The promeristem gives rise to all other meristems including the primary meristem. (2) Primary meristem : A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia. Primary meristem gives rise to the primary permanent tissue. (3) Secondary Meristem : They always arise in permanent tissues and have no typical promeristem. Some living permanent cells may regain the meristematic nature. This process in which permanent tissue regains meristematic nature is called dedifferentiation. The secondary meristems are so called because they originate from permanent cells. The phellogen or cork cambium arising from epidermis, cortex or other cells during secondary growth, is an important example of secondary meristem. The secondary meristems produce secondary tissues in the plant body and add new cells for effective protection and repair. According to position : On the basis of their position in the plant body meristems are classified into three categories : (1) Apical meristem : This meristem is located at the growing apices of main and lateral shoots and roots. These cells are responsible for linear growth of an organ. Solitary apical cells occur in ferns and other Pteridophytes while apical initials are found in other vascular plants. (2) Intercalary meristem : These are the portions of apical meristems which are separated from the apex during the growth of axis and formation of permanent tissues. It is present mostly at the base of node (e.g., Mentha viridis, Mint), base of internode (e.g., stem of many monocots viz., more...

Functions of different organs and tissues of a plant tissue system
  Roots Stems Leaves
(i) Functions (i) Absorb water and minerals. (ii) Anchor plant. (iii) Store materials. (i) Transport water and nutrients. (ii) Support leaves. (iii) Help to store materials. Carry on photosynthesis.
   
(ii) Tissues      
(a) Epidermis Root hairs absorb water and minerals. Protect inner tissues. Stomata carry on gas exchange.
(b) Cortex Store more...
"The loss of water in the form of vapours from the aerial parts of a plant is called transpiration". Maximum transpiration occurs in mesophytic plants. About 98 percent of the water absorbed by land plants evaporates from the aerial parts and diffuse in to the atmosphere.   Differences between transpiration and evaporation
S.No. Transpiration Evaporation
(1) It is a physiological process and occurs in plants. It is a physical process and occurs on any free surface.
(2) The water moves through the epidermis with its cuticle or through the stomata. Any liquid can evaporate. The living epidermis and stomata are not involved.
(3) Living cells are involved. It can occur from both living and non-living surfaces.
(4) Various forces (such as vapour pressure, diffusion pressure, osmotic pressure, etc) are involved. Not much forces are involved.
(5) more...
"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...

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...

'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...



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