Plant Growth Development

Plant Growth Response – Tropisms

A tropism indicates growth or the turning movement of an organism, usually a plant, in response to an environmental stimulus. Plants need to survive and grow so they use sources of water and light which they grow towards. In tropisms, this response is dependent on the direction of the species. The word tropism comes from the Greek word trope (“to turn” or “to change”). There are many different tropisms and are usually named for the stimulus involved, for example:

* Positive Tropism – A positive tropism is the turning towards the stimulus.

* Negative Tropism – A negative tropism is the turning away from the stimulus.

* Phototropism – A phototropism is the growth response of a plant in response to the light direction. Each different part of the plant reacts differently to light. Stems show positive phototropism while roots show negative phototropism.

* Geotropism – A geotropism is the growth response of plants response to gravity. Roots show positive geotropism whereas stems and leaves show negative geotropism.

* Thigmotropism – A thigmotropism is the growth response of a plant to physical contract. Plants that cling to physical structures, for example walls, show positive thigmotropism.

* Hydrotropism – A hydrotropism is the growth response of a plant water and roots show positive hydrotropism.

* Chemotropism – A chemotropism is the growth response of a plant to different chemicals. Roots grow towards useful minerals in the soil, but away from acids.

Controlling the direction of growth

Auxin is a plant hormone responsible for controlling the direction of growth of stem and root tips in response to different stimuli including light and gravity. Auxin is made at the tips of stems and rooms. It’s moved in solution to older parts of the stem and root, where it changes the elasticity of the cells. More elastic cells absorb more water and grow longer, causing bending in the stem or root. It’s thought that...