Quiz 32: Animal Tissues and Organ Systems


Hormones influence plant growth whether it is primary or secondary growth. Plant hormones interact with one another. They are influenced by environmental factors. There are basically five plant hormones. Auxin promote cell division, shorten internodes, prevent lodging of plants, promote root initiation and apical dominance, initiates flowering of plants, prevents the development of abscission, induces parthenocarpy and helps in the eradication of weeds. Gibberellin helps in plant elongation, promotes flowering, parthenocarpy and helps in breaking dormancy. Abscisic acid induces dormancy in buds, twigs, suppresses seed germination and growth. It promotes stomatal closure during stress and thus prevents water loss. Cytokinin promotes cell division, stimulates the growth of lateral buds, it helps in breaking dormancy and delays senescence. Ethylene promotes the ripening of fruits and vegetables, seed germination, induces flowering and helps in the development of root hairs. Primary growth takes place when the roots and shoots lengthen through the activity at apical meristems. Apical meristems are usually present at the tips of root and stems, comprising of undifferentiated cells of embryonic origin which retain the capacity to divide. Cells of apical meristem are responsible for primary growth of the plants. The new cells formed from the activity of apical meristem differentiate into various tissues. The 3 types of cells which are basically produced due to the activity of apical meristems are protoderm which gives rise to plant epidermis, ground meristem which gives rise to ground tissues and procambium which gives rise to primary vascular tissues. Lateral meristem is responsible for secondary growth of the plant. The tissue present here divides and thickens the plant stems and roots. The activity of lateral meristem results in the formation of vascular cambium and cork cambium. Vascular cambium is produced when the lateral meristem becomes active in older stems and roots and produces secondary xylem and secondary phloem. Secondary xylem is formed on the inner side of lateral meristem whereas secondary phloem is produced on the outer side of the lateral meristem. The inner region on the secondary xylem thickens and displaces the meristematic cells towards the outer surface of the stem forming a ring of vascular cambium. The expansion of the inner region of the secondary xylem puts pressure towards the stem and root surface. This in turn ruptures the cortex and outer part of secondary phloem. The parenchyma cells starts dividing forming cork cambium.

Seed germination is over when the primary root pokes out of the seed coat. Germination is a process by which the seed containing the embryo sporophyte resumes growth under appropriate environmental conditions. Seeds germinate when the temperature is right in between 25 tO30 degrees centigrade, sunlight, soil with right amount of ions and minerals and proper amount of moisture. The dry seed coat absorbs the water molecule. The storage proteins in the endosperm absorb the water molecules by the hydrophilic groups. This causes the seed to swell which in turn ruptures the seed coat. This causes oxygen to infiltrate and reach within the embryo and aerobic respiration is carried out producing ATP. ATP is utilized for providing energy for plant growth. Division of meristematic cells starts and primary root penetrates out of the seed. Hence, the correct answer is option (c).

Seedlings that were germinated in dark can grow tall due to application of hormones. Plant hormones are active in plant embryos within seeds. The application of auxin promotes cell division, shortens internodes, prevents lodging of plants and promotes root initiation and apical dominance. Gibberellin helps in plant elongation, promotes flowering and helps in breaking dormancy. Cytokinin promotes cell division, stimulates the growth of lateral buds, it helps in breaking dormancy and delays senescence. Ethylene helps in the development of root hairs. Auxins and gibberellins promote stem elongation. Both these hormones together promotes cell enlargement.

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