Growth of an organ toward anenvironmental stimulus is called apositive tropism; for example, stems growing toward light are positivelyphototropic. Conversely, curvature ofan organ away from a stimulus is called a negative tropism. Roots, which usually grow away from light, are negatively phototropic.
Plant shoots- positive phototropism
Plant roots- negative phototropism
Response of plants towards light. Sunflower shows positive phototropism
In order to stay alive, plants and animals need to be able to detect, and respond to, changes in their external environment. They also need to find ways to obtain food, shelter and a mate so that they can reproduce. All animals have adaptations - characteristics - that benefit the individual and help it to survive and reproduce in their environments
A tropism is a response that anorganism makes to a stimulus. An example of a common tropism inplants is phototropism (or light response). Plants grow towards light sources, and if the direction of light is changed, the plant will also change its direction of growth to accommodate for survival.
It rains, the plant absorbs the water. The sun shines too much and the leaves curl to protect themselves from drying out or because they're too dry. Watch Maple leaves turn upside down just before a storm! When the wind blows, the roots grow further into the soil to provide a firmer grip. Plants are known to do all sorts of things to protect themselves from insects.
Animals cannot all detect the same stimuli - for example dogs can detect high sound frequencies whereas humans are insensitive to them. Elephants can detect very low frequencies which again, humans can't!
Being able to detect particular stimuli plays a huge part in the survival of that animal. For example, when muscles contract there is a very weak electric current produced. Sharks can detect the electric fields set up by their potential prey based on the prey's muscle movement in the water.