Tropism is a reaction of living organisms, expressed in a change in the direction of growth towards a physical or chemical stimulus (positive tropism) or in the opposite direction (negative tropism). From tropism, taxis should be distinguished - reactions of directed movement of mobile organisms caused by external physical or chemical factors. According to the nature of acting stimuli, geotropism, i.e., orientation with respect to the earth's surface, thermotropism, to temperature factor, chemotropism, to chemical stimuli, etc. are distinguished. For example, green plants that need light (as a result of perpendicular to the light rays). The roots of plants, finding positive geotropism, grow deep into the soil .