In botany the orientation or directed movement of plants in response to an external stimulus is called a tropism. The effective light is normally from the blue region of the spectrum. The significance of phototropism to the plant is to assist in making the plant grow toward the light that it requires. Growth movements in response to gravity are called gravitropism. Roots are gravitropic, growing in the direction of the Earth's gravitational field.
Primary roots are more positively gravitropic than secondary roots.
Shoots are negatively gravitropic, growing away from the gravitational field. Gravitropisms are caused by different auxin concentrations, with more auxins collecting on the lower side of the shoots or roots. How higher concentrations on the lower sides of both shoots and roots produce opposite effects are not clear. Biological Clocks Biological clocks, physiological systems that enables organisms to live in harmony with the rhythms of nature, trigger certain cells and hormones that are crucial in the growth and development if plants. These clocks aren't perfect though they are necessary to all plants.