Tag: biology

'The power of movement of plants' is a book by Charles Darwin on phototropism and other types of movement in plants. This book contains his works in producing evidence for his theory of natural selection. The power of movement of plants was published on 6th November 1880, and 1500 copies were quickly sold by publisher John Murray. This book stands at the culmination of a long line of study in plants and is immediately preceded by 'The different forms of flowers on Plants of the same species' (1877).

Charles Darwin studied phototropism by covering different parts of oat coleoptiles and subjecting them to light from one direction. When he covered or removed coleoptile tip, phototropic response did not occur and concluded that phototropic bending in the zone of elongation was controlled by the tip. The movements which allow organs of plants to move in or the other directions is called movements of curvature. His demonstration that the site of photoreception at the shoot tip and the location of curvature are separable. From his observations, Darwin was able to propose that a transmissible substance produced in the tip is responsible for inducing curvature in lower regions of the plant. This insightful discovery eventually lead to the discovery of first plant hormone - auxin.

A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. Tropisms are typically associated with plants (although not necessarily restricted to them). Where an organism is capable of directed physical movement (motility), movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a taxis (directional response) or a kinesis (non-directional response).
Therefore, the correct answer is option A.

Phototropism was first studied by Charles Darwin in 1880 in Canary Grass and oat coleoptiles. Later on F.W. Went in 1923, suggested the involvement of auxins in this phenomenon. Experimental approach to demonstrate positive phototropism in shoot tips is a darkened black box. The black box is having a small window at one side. A well-watered potted plant is placed inside the box. The is referred to as a phototropic chamber or heliotropic chamber. The window is kept closed for about 24 hours and the plant shows normal growth. If the window is kept opened, it is found after two days, that the shoot tip bends and grows towards light proving that it is positively phototropic.

The movement of plants in response to light is referred to as phototropism and to gravity is called geotropism. Arachis hypogea or peanut or groundnut belonging to the family Fabaceae or Leguminoseae, this is a herbaceous plant with yellowish organ flowers borne on auxillary clusters with ovary positioned at the base of the floral cup. The stigma is supported on a long style and before fertilization this shows positive phototropism.

Most vines and other climbing plants have tendrils that coil rapidly around supports. These grasping organs usually grow straight until they touch something; the contact stimulates a coiling response caused by differential growth of cells on opposite sides of the tendrils, the directional growth in response to touch is called thigmotropism, and it allows the vine to take advantage of whatever mechanical supports it comes across as it climbs upward toward a forest canopy.

Pneumatophores are negatively geotropic roots found in mangroves. They have pores for gaseous exchange. These pores are important because the soil is waterlogged and there is deficiency of air in the soil.

The Cholodny-Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the shoot to grow towards light (phototropism) and the roots to grow downward (gravitropism). In terms of phototropism, the Cholodny-Went theory proposes that:

Plants of Family Cucurbitaceae have weak stems supported by tendrils. Tendrils are sensitive to touch and show thigmotropism, i.e., growth movement in response to touch. Haptotropism is a related term and essentially has same meaning, i.e., the movement of parts of a plant in response to a touch stimulus. In case of family Cucurbitaceae, these terms are used to describe growth of tendrils which support weak stem of the plants of this family.