=Tropisms= A tropism is a growth movement whose direction is determined by the direction of the stimulus '''A directional response [growth] to a directional stimulus''' A tropism can be: * '''Positive''' - when the plant, or a part of it, grows in the direction from which the stimulus originates. * '''Negative''' - when the growth is away from the stimulus. ==Examples:== [image:] '''Phototropism''' • Stems demonstrate a positive phototropism (grow towards light) • Roots demonstrate a negative phototropism (grow away from light) [image:] '''Geotropism''' (or Gravitropism) • Stems demonstrate a negative geotropism (grow against the force of gravity) • Roots demonstrate a positive geotropism (grow with the force of gravity) [image:] '''Positive Hydrotropism''' A positive growth towards water in plants is known as a hydrotropism. This is a very strong response and takes precedence over the geotropism. One theory is that increasing water concentrations help dissolve the amyloplasts, thus reducing the response to gravity. Auxin inhibits root cell growth / elongation. [image:] '''Heliotropism –Solar Tracking''' Heliotropism is the motion of plant parts (flowers or leaves) in response to the direction of the sun. Heliotropic flowers track the sun's motion across the sky. During the night, the flowers may assume a random orientation, while at dawn they turn again towards the east where the sun rises. This is why sunflowers are said to “always face the sun”. [image:] '''Positive Thigmotropism -Twining''' Climbing vines need to find a suitable support on which to grow. Shortly after germinating, they begin what appears to be a hunting motion (an irregular motion / wobble). This movement is known as Nutation and is not actually a directional response to any external factor. Most plants exhibit this behaviour, but it is much more extreme in climbing vines. Once the shoot touches a suitable support, the contact induces a '''positive thigmotropism''' and the shoot begins to grow toward the surface it's touching, usually curling around it. ==Why?== Each of these behaviours has a clear adaptive advantage. For instance roots growing down and/or away from light are more likely to find the soil, water, and minerals they need. Stems growing up and toward the light will be able to expose their leaves so that photosynthesis can occur, their flowers will be more visible and they will be better able to spread seed (a reproductive advantage).
Credit: Ben Himme