Gravitropism

This is an image taken of a tree from Central Minnesota. The tree was on the face of a hill and had blown over in a storm or fell over due to erosion in the soil surrounding it. The tree continues to grow however, and because it was horizontal, its growth exhibits gravitropism which can be seen in its 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 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.

Gravitropism (also known as geotropism) 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.[1] That is, roots grow in the direction of gravitational pull (i.e., downward) and stems grow in the opposite direction (i.e., upwards). 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 (biologists say, turning; see tropism) 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.[2] Although the model has been criticized and continues to be refined, it has largely stood the test of time.[citation needed]

  1. ^ Darwin, Charles; Darwin, Francisc (1881). The power of movement in plants. New York: D. Appleton and Company. Retrieved 24 April 2018.
  2. ^ Haga, Ken; Takano, Makoto; Neumann, Ralf; Iino, Moritoshi (January 1, 2005). "The Rice Coleoptile Phototropisim1 Gene Encoding an Ortholog of Arabidopsis NPH3 Is Required for Phototropism of Coleoptiles and Lateral Translocation of Auxin(W)". Plant Cell. 17 (1): 103–15. doi:10.1105/tpc.104.028357. PMC 544493. PMID 15598797.