<P> In botany, the Cholodny--Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the stalk to grow towards light (phototropism) and the roots to grow downward (gravitropism). In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone . </P> <P> in plants (and bacteria) </P> <Ul> <Li> Aerotropism, growth of plants towards or away from a source of oxygen </Li> <Li> Chemotropism, movement or growth in response to chemicals </Li> <Li> Electrotropism, movement or growth in response to an electric field </Li> <Li> Exotropism, continuation of growth "outward," i.e. in the previously established direction </Li> <Li> Geotropism (or gravitropism), movement or growth in response to gravity <Ul> <Li> Apogeotropism, negative geotropism </Li> </Ul> </Li> <Li> Heliotropism, diurnal motion or seasonal motion of plant parts in response to the direction of the sun, (e.g. the sunflower) <Ul> <Li> Apheliotropism, negative heliotropism </Li> </Ul> </Li> <Li> Hydrotropism, movement or growth in response to water; in plants, the root cap senses differences in water moisture in the soil, and signals cellular changes that causes the root to curve towards the area of higher moisture <Ul> <Li> Prohydrotropism, positive hydrotropism </Li> </Ul> </Li> <Li> Hygrotropism, movement or growth in response to moisture or humidity </Li> <Li> Magnetotropism, movement or growth in response to magnetic fields </Li> <Li> Orthotropism, movement or growth in the same line of action as the stimulus </Li> <Li> Plagiotropism, movement or growth at an angle to a line of stimulus such as gravity or light </Li> <Li> Phototropism, movement or growth in response to lights or colors of light <Ul> <Li> Aphototropism, negative phototropism </Li> <Li> Skototropism, negative phototropism of vines </Li> </Ul> </Li> <Li> Thermotropism, movement of growth in response to temperature </Li> <Li> Thigmotropism, movement or growth in response to touch or contact </Li> </Ul> <Li> Aerotropism, growth of plants towards or away from a source of oxygen </Li>

What are the different types of tropic movement