ESSAYS ON GROWTH AND DEVELOPMENT GROWTH AND DEVELOPMENT 1. (a) Name three parameters that can be used to measuring growth. (b) State the limitation in the use of the above parameters in the measurement of growth in organisms. a) Length, dry mass, volume b) - Limitation in measuring growth - Difficulty in choosing the right growth parameter - The use of a singe growth parameter does not take into account growth in other directions - Volume cannot be used for those organisms with irregular shape - Mass of an organism is usually affected by variation in the fluid content of the organism - Use of dry mass involved killing the organism - The use of mass or size may be inaccurate because different parts of an organism at different times - Irregularities in the growth of an organism due to fluctuation in the environment / diet 2. a) State the various causes of seed dormancy. b) Describe various factors that affect the process of seed germination. a) -Embryo may not be fully developed /; immature embryo; -Presence of chemical inhibitors; that inhibit germination in seeds e.g. abscisic acid. -Very low concentrations of hormones e.g. gibberellins; and enzymes reduce the ability of seeds to germinate -Hard and impermeable seed coat prevent entry of air and water in some seeds e.g. wattle -In some seeds the absence of certain wavelengths of light; make them remain dormant. -very low temperature /freezing of seeds during winter lowers their enzymatic activities; rendering them dormant b) i) Water - Activates the enzymes and provides the medium for enzymes to act and break down the stored food into soluble form. - Water hydrolyses and dissolves the food materials; - It is the medium of transport; of dissolved food substances through the various cells to the growing region of the radical and plumule. - It softens the seed coat to facilitate emergence of the radicle / plumule; ii) Oxygen - Necessary for respiration to provide energy; needed in germinating seeds for division and growth iii) Temperature - Seeds will not germinate at OC or above 47C - The optimum temperature; for seeds to germinated is about 30C. - High temperature denature enzymes and kill the protoplasm / destroys protoplasm /. - At very low temperatures the enzymes are inactive. - Rate of germination increase with temperature until it reaches an optimum. iv) Enzymes - Facilitates the oxidation of stored carbohydrates to release energy. - Hydrolyse carbohydrates to glucose, lipids to fatty acids glycerol, protein to Amino acids. 3. Discuss the various phases of growth. Lag phase/ slow growth; - Growth is slow due to - No. of cells dividing are few; - Cells not adjusted to environmental factors; Exponential phase/Lag phase; - Growth rapid - number of dividing cells has increased - Cells have adjusted to new environment; - Food and other factors such as space and diseases are not limiting; hence no competition for resources. - Rate of cell increase is higher that rate of cell death. Decelerating phase. - Slow growth. - Most cells are fully differentiated. - Fewer cells still dividing; - Shortage of oxygen and nutrients. - Space limited due to high number of cells. - Accumulation of metabolic wastes. Plateau phase. - Growth has ceased. - Cell division equals cell death. - Cells and tissues fully differentiated therefore no increase in number of cells. 4. Describe the role of growth hormones in plant growth and development (i) Indole Acetic acid (IAA)/Auxins Produced by apical meristems, young leaves and seed embryos. - Promote fruit formation/parthenocarpy - Promote cell division and elongation - Influences cell elongation/tropic responses - Cause apical dominance/inhibit growth of lateral branches/buds - Promote growth of adventitious roots - Cause tropic responses. (ii) Cytokinins/kinetin/zeatin Synthesized in roots - Stabilizes proteins and chlorophyll; - Promote flowering in some plant species; - Breaks seed dormancy in some plant species; - In low concentration it promotes leaf aging; - Promote cell division. - Stimulate formation of callus tissues in plants – healing of wounds. - Promote formation of adventitious roots. - Stimulate lateral bud development in shorts. - High concentration induce cell enlargement of leaves. (iii) Ethylene/ethane/ - Induces stem thicknening; - Inhibit stem elongation; - Induces ripening of fruits; - Promotes germination of certain seeds; - Promotes development of abscission layer leading in fruit/leaf fall; (iv) Gibberellins/GBA - Induce parthenocarpy - Initiate formation of IAA - Promote fruit development after fertilization - Inhibit growth of adventitious roots - Activates hydrolytic enzymes during seed germination - Promote cell division/cell elongation in dwarf varieties - Promote formation of side branches. - Breaks dormancy in buds. - Delays leaf fall. - Break seed democracy. (v) Abscisic Acid (ABA) - Causes seed dormancy/bud dormancy; - Causes abscission of leaves/fruit fall; - Inhibit seed germination; - In high concentration it leads to closure of stomata; (vi) Florigens - Promote flowering 5. Explain how meristematic tissues contribute to growth of higher plants. i) Apical meristems; Found near the root and shoot apex. They are responsible for primary growth (increase in length of shoot and root) at the apex. The tissue is divided into zones of cell division; cell elongation and cell differentiation ii) Vascular cambium Found between the xylem and phloem of woody plants. The cells decide to give rise to secondary xylem and phloem; resulting to increase in girth (diameter). Secondary parenchyma is formed between adjacent vascular bundles resulting to secondary growth. iii) Cork cambium; It is located beneath the epidermis. It divides to form secondary cortex and corky cells to the inside and outside respectively preventing rupturing of the stem and root when vascular cambium increases the girth. iv) Intercalary meristem Found at internodes and base of leaf sheath in grasses. They are responsible for increase in length of leaf and nodes. 6. Describe secondary growth in flowering plants Secondary thickening is facilitated by meristematic cells known as cambium which are located between phloem and xylem in vascular bundles of plants. The cambium cells divide radially to form a cambium ring. Those on the inner side differentiate to form secondary xylem while those on the outer side form secondary phloem. Interfascular cambium, between vascular bundles, divided to form secondary parenchyma thereby increasing the growth of medullary rays. Much more xylem is formed than phloem thus pushing phloem and cambium outwards. Pressure is exerted on the outer cells of the stem resulting to rupturing of the epidermis. In order to replace the protective epidermis, the cork cambium (phellogen) form in the cortex. The cork cambium cells divide and differentiate to form secondary cortex on the inner side and cork cells on the outer side. Cork cells are dead, thick-walled and filled with suberin. They form the bark which prevent water loss, prevents infection by fungi and insulate the stem. Lenticels (loose mass of cells) form in the bark to allow gaseous exchange to take place. The rate of secondary growth is dependent on the season/rainy resulting in annular rings. 7. Describe one method that can be used to measure the average growth rate of a single leaf. - Identify a young leaf which has just unfolded. Use the same leaf throughout. - Measure the total length of the whole leaf and record. - Repeat the procedure at regular interval until no change occurs in length. - The average rate of growth is equal to total increase in length divided by the period taken to achieve final length. OR - Identify a young leaf which has just unfolded. Use the same leaf throughout. - Trace the outline on a graph paper and estimate the area. - Repeat the procedure at regular interval until no change occurs in area. - The average rate of growth is equal to total increase in area divided by the period taken to achieve final area.
Posted on: Sat, 16 Aug 2014 09:17:08 +0000
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