Plants, like animals, use hormones to coordinate and control their growth and their responses. Two important responses are to light, called phototropism, and to gravity, called gravitropism (or geotropism). Shoots normally grow towards light and away from gravity. Roots normally grow towards gravity.
These responses are controlled by a plant hormone called auxin. An unequal distribution of auxin causes unequal growth rates in the shoots and roots. In a shoot lit from one side, auxin moves to the shaded side, where it makes the cells grow faster. The shoot therefore bends towards the light. In a root, the effect of auxin is different, so the root grows downwards.
Higher tier only: gibberellins are important in starting seed germination, and ethene controls cell division and the ripening of fruits. You do not need to know how gibberellins or ethene work.
Required practical 8: to investigate the effect of light or gravity on newly germinated seedlings, you grow seedlings under different conditions, for example with light from one side only, or turned on their side. You measure the length of the shoots or roots and make careful, labelled biological drawings to show the effects.
