B2: Green Plants as Organisms Control of Growth (H)

Control of growth Plants must be able to detect the directions of gravity, of moisture and of the greatest light intensity so that their roots and shoots can grow in the direction which give the plant the best chance of surviving. Thus plants are sensitive to light, moisture and gravity. Plant shoots grow towards light and away from gravity. Plant roots grow towards gravity and towards moisture but away from light. Plant growth substances, e.g. auxins influence cell division and elongation of cells at the root and shoot tips. If a shoot is illuminated from one side only, auxin, produced in the tip, tends to be more concentrated on the darker side so that greater cell elongation is stimulated on the darker side. Thus the shoot tends to grow towards the light.

Control of growth Narcissus plants which have been illuminated from one side only for 24 hours. Narcissus plants which have been left on their side for 24 hours. The shoots grow away from gravity The shoots grow towards light The responses of plant roots and shoots to light, gravity and moisture are the result of unequal distribution of hormones, causing unequal growth rates.

Control of growth The hormones which control the processes of growth and reproduction in plants can be used by humans to: produce large numbers of plants quickly by stimulating the growth of roots from cuttings; regulate the ripening of fruits on the plant and during transport to customers; kill weeds by disrupting their normal growth patterns. Rooting hormone powder Cutting taken from mature plant. Cut end dipped into rooting hormone powder and planted. After a few days roots form from the cut end. For example, broad leaf plants, such as plantains will absorb more growth hormone (through the leaves) than narrow leaf plants, such as grass. The hormones makes the plantains ‘bolt’ (grow too big, too quickly) and die, but will not harm the grass. Ethene gas, given off by ripe fruits, stimulates nearby fruits to ripen. Thus ethene can be added to a greenhouse or added to boxes of fruit to cause ripening.

Control of growth Try to complete the table, using the words ‘light’, or ‘gravity’ or ‘water’ or ‘hormone’. Factor Plant activity Regulates the ripening of fruits. Stems grow towards it. Stimulates the root growth of cuttings. Stems grow against the direction of it. Plants wilt when too much is lost through stomata. Roots grow to where there is more of it. Hormone. Light. Hormone. Gravity. Water. Water. Which two of the following are correct plant responses? A. Roots grow in the direction of the force of gravity. B. Shoots grow towards moisture. C. Shoots grow in the direction of the force of gravity. D. Shoots grow towards light. E. Roots grow away from moisture. Answer: A and D.