The movement of a plant in the direction of a stimulus or away trom it is called tropic movement or tropism It is said to be positive if it is directed towards the source of stimulus and negative if directed away from the source stimulus. e.g. geotropism, the tropic respose towards gravity.

Forms of tropism include phototropism (response to light), geotropism (response to gravity), chemotropism (response to particular substances), hydrotropism (response to water), thigmotropism (response to mechanical stimulation), traumatotropism (response to wound lesion), and galvanotropism, or electrotropism (response …

A tropism is the innate ability of an organism to turn or move in response to a stimulus. In a positive tropism the animal will move toward the stimulus. In a negative tropism, the animal will move away from the tropism.

Hydrotropism– It is the movement of a plant towards the water. For example, plant root growing in humid air bending toward a higher relative humidity level. Chemotropism – It is the movement of plants in response to a chemical stimulus. For example, the growth of the pollen tube towards the ovule.

Phototropism is the growth of an organism in response to a light stimulus. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the furthest side from the light.

The best known early research on phototropism was by Charles Darwin, who reported his experiments in a book published in 1880, The Power of Movement in Plants. Darwin demonstrated that these coleoptiles are phototropic in that they bend toward a light source.

Seeds push little leaves up from the ground into the light. A house plant in a dark room will grow toward the light. This movement in response to light is called phototropism. Sunlight reduces auxin, so the areas of the plant that are exposed to sunlight will have less auxin.

The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. Kenneth V. Thimann (1904-1997) became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA).

From his family home, Down House, Darwin studied many plant organs such as young growing roots, emerging shoots, leaves and their stalks, flower peduncles. Timelapse photography was used to capture the slow movement of the plant, undectable by the human eye alone, over a 24 hour period.

Darwin proposed that evolution could be explained by the differential survival of organisms following their naturally occurring variation—a process he termed “natural selection.” According to this view, the offspring of organisms differ from one another and from their parents in ways that are heritable—that is, they …

There are four principles at work in evolution—variation, inheritance, selection and time. These are considered the components of the evolutionary mechanism of natural selection.