Growth in Plants
Plants respond to stimuli slowly by growing in a particular direction.
As with animals, plants also use a variety of hormones to control their growth and development. We have already seen that a family of hormones called auxins is commonly found in plants and promotes growth. Auxins are produced in the meristems of plants.
Auxins are responsible in promoting cell elongation, a process that is required before differentiation of a cell. This cell elongation is carried out by promoting the intake of water, increasing the elasticity of the cell to cope with the increase of water taken in by the cell.
Phototropism is a growth movement induced by a light stimulus. Growth towards a source of light is called positive phototropism that away from the source is termed negative phototropism. The tips of shoots are usually positively, that of roots negatively phototropic.
Originally phototropism was called heliotropism, because the plant grows towards the sun. The name was altered when it became clear that plants react towards artificial sources of light also.
Geotropism is a similar occurrence to phototropism where the plant exhibits directional growth in response to gravity. The shoot tip illustrates negative geotropism (grows against force of gravity) while the root tip exhibits positive geotropism (grows in the same direction as gravity).