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Phototropism
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In biology, phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hormone called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the furthest side from the light. Phototropism is one of the many plant tropisms, or movements, which respond to external stimuli. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Negative phototropism is not to be confused with skototropism, which is defined as the growth towards darkness, whereas negative phototropism can refer to either the growth away from a light source or towards the darkness. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The combination of phototropism and gravitropism allow plants to grow in the correct direction.
The Thale Cress (Arabidopsis thaliana) is regulated by blue to UV light
Phycomyces, a fungus, also exhibit phototropism
Phycomyces, a fungus, also exhibit phototropism
Example on a Phalaenopsis
Example on a Phalaenopsis
Ravenalas growing between two buildings in Kinshasa, Democratic Republic of Congo. The plane (here perpendicular to the north–south axis) of these two
Ravenalas growing between two buildings in Kinshasa, Democratic Republic of Congo. The plane (here perpendicular to the north–south axis) of these two plants is orientated to maximize daylight absorption
Gravitropism
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Gravitropism is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing upwards. Herbaceous (non-woody) stems are capable of a degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.
Example of gravitropism in a tree from central Minnesota. This tree has fallen over and due to gravitropism exhibits this arched growth.
Example of gravitropism in a tree from central Minnesota. This tree has fallen over and due to gravitropism exhibits this arched growth.
Gravitropism maintains vertical orientation of these trees. These trees, typical of those in steep subalpine environments, are covered by deep snow in
Gravitropism maintains vertical orientation of these trees. These trees, typical of those in steep subalpine environments, are covered by deep snow in winter. As small saplings, they are overwhelmed by the snow and bent nearly flat to the ground. During spring growth, and more so as larger trees, gravitropism allows them to orient vertically over years of subsequent growth.
Banana fruit exhibiting negative geotropism.
Banana fruit exhibiting negative geotropism.