CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 9(a)

(a) Plant hormones (phytohormones) are chemical substances that coordinate and control the growth, development and responses of a plant to its environment, such as cell division, cell enlargement, bending of shoots and roots, flowering, ripening of fruits and the wilting of leaves. They are made at one place and diffuse to the area where they act. The plant hormone essential for cell division is cytokinin. (b) The hormone is auxin, which is synthesised at the tip of the shoot. When light falls on the shoot from one side, auxin diffuses towards the shaded side. The higher concentration of auxin there makes the cells on the shaded side grow longer than those on the lit side, so the shoot bends towards the light (phototropism).