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 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 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 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 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., 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 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 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 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.