Kinesis (biology), a movement or activity of a cell or an organism in response to a stimulus.

1. In ecology, the extent to which an organism is well adapted to its environment. The fitness of an individual animal is a measure of its ability, relative to others, to leave viable offspring.

Kinesis and taxis Some organisms have innate behaviors in which they change their movement in response to a stimulus, such as high temperature or a tasty food source. In kinesis, an organism changes its movement in a non-directional way—e.g., speeding up or slowing down—in response to a cue.

taxis (taxic response; tactic movement) The movement of a cell (e.g. a gamete) or a microorganism in response to an external stimulus. Certain microorganisms have a light-sensitive region that enables them to move towards or away from high light intensities (positive and negative phototaxis respectively).

Phototaxis is one of the different forms of taxis. Examples of phototrophic organisms exhibiting phototaxis are the phytoflaggellates, e.g. Euglena, and photosynthetic bacteria. A negative phototaxis is one in which the organism moves away from the light source as demonstrated by certain insects such as cockroaches.

Gravitropisms. Phototropism is a response to the stimulus of light, whereas gravitropism (also called geotropism) is a response to the stimulus of gravity . When the stem grows against the force of gravity (upwards), this is known as a negative gravitropism.

The growth response of a cell or an organism to gravitational field is called gravitropism. Gravitropism is a feature exhibited by many organisms, such as most plants and fungi. For instance, the roots of the plants grow towards the gravitational field whereas the stem grows away from the gravitational field.