William Sutherland Gravitropism


Gravitropism, also known as geotropism, is the growth or movement of plant parts in response to gravity.


stems/shoots exhibit negative gravitropism.


Gravitropic movement, which likely has more influence on a plant than any other tropism, enables it to remain oriented relative to the source of gravity. Thus, “if a plant is knocked down, the shoot will grow faster on the lower side until it is more-or-less standing up again.”5 Furthermore, gravitropic movement ensures that no matter which way a seed falls, the roots will grow downward and the shoot upward.


The primary benefit of gravitropic movement is to help plants to flourish by optimizing the growth of all of their parts. First, plants develop well-branched root systems through positive gravitropic growth, in which roots generally grow downward towards the source of gravity. Such a root system anchors the plant, and attains water and minerals, to name a few functions.


Second, negative gravitropic stem growth positions a plant to maximize light absorption for photosynthesis by elevating its leaves, which are oriented at set angles, also through gravitropic movement.


Last, negative gravitropic growth also “positions pollen-producing flowers at the top of [a] plant where [their pollen] can be carried by the wind to… female flowers”6 or optimize attraction and entry of pollinating organisms. Basically, plants generally exist in a state of “anisotropic growth,” unless knocked down, where roots grown downward and stems and flowers upward.


For simplicity, gravitropic movement, either the bending or curving of roots and stem occurs in the following way:


1. Starch-filled plastids called amyloplasts found in specialized cells in the rootcap of the tip of roots and starch sheath in the growing zone of stems sense gravity.


2. When cell orientation changes due to gravitational effects, the heavy mass of the amyloplasts causes them to sink to the lower end of the cell. 3. The sinking amyloplasts trigger auxin production.


105 In plants, roots exhibit positive gravitropism and


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