The chemical substance P is made and secreted by the meristematic tissue at the tip of stem (or shoot) of a plant. The chemical substance P is responsible for a phenomenon Q in plants in which the stem bends towards a source of light. The same chemical substance P has an opposite effect on the root of a plant. It causes the root of a plant to bend away from the source of light in a process called R.
(a) What is the chemical substance P?
(b) State whether P prefers to remains in the sunlit side of a stem or in shade.
(c) What is the effect of substance P on the rate of growth of (i) a root, and (ii) a stem?
(d) What is the name of process (i) Q, and (ii) R?
(e) What is the general name of chemical substances like P? Name another substance which belongs to this class of chemical substances.
(a) The chemical substance P is auxin.
(b) Auxin remains on the shaded side of the stem.
(c) Auxin increases the growth rate of shoots and decreases the growth rate of roots.
(d) Process Q is positive phototropism, whereas R is negative phototropism.
(e) The general name of chemical substances like auxin (P) is phytohormone. Cytokinins, gibberellins and abscisic acid are other examples of phytohormones.