Tropisme are reactions of living organisms, which is expressed in the change of a direction of growth toward physical or chemical irritant (positive tropisme) or in the opposite direction (negative tropisme). From trapistov should be distinguished taxis - reaction directional move motile organisms caused by an external physical or chemical factors. Accordingly, the nature of existing stimuli are distinguished: geotropism, i.e. orientation towards the earth's surface, thermochromism - to the temperature factor, geotropism to chemical stimuli and other Positive phototropism have, for example, green plants that need the light (as result of their leaves are perpendicular light rays). The roots of plants, detecting positive geotropism, grow deep into the soil.