Geotropism

Learn about geotropism, a plant's natural response to gravity! A sample experiment to create your own geotropism is provided.

A "tropism" is a plant movement triggered by stimuli. The term "geotropic" refers to a plant whose roots grow down into the soil as a response to gravity. Plants commonly exist in a state of "anisotropic growth," where roots grow downward and shoots grow upward. Anisotropic growth will continue even as a plant is turned sideways or upside down. In other words, no matter what you do to a plant within Earth's atmosphere, it will still grow roots down, stem up. The reason for this comes from the nature of a plant, and it's general response to gravity.

Geotropism was first experiemented upon in the early nineteenth century. Several experiments at this period proved that geotropism existed as a seperate plant response, and was not simply a response toward light. The assumption was held that root tips were pulled downwards by weight, or that the axis of the plant reoriented itself in the direction of the roots. Both of these assumptions were later refuted.

"Geotropism" was introduced as a term in the year 1869 by the scientist A.B. Franck. He identified three kinds of geotropisms: positive, negative and transversal. Positive and negative geotropism are the direction of the vertical growth of a plant's stem. Transversal geotropism is a direction of growth vertical to the shoot's axis. All varying directions are called plagiotropic. Therefore, a plant's axis and main stem are positive/negative geotropic, side roots and certain leaves are transversal, and leaves that branch from the main stem at angles are plagiotropic.



Until the year 1892, experiments were carried out on the stimulii that cause this reaction in plants. The commonly-held perception today is the "statholith theory," which postulates that certain small components in the tips of a plant roots push on the plasma bottoms of root cells. This local pressure stretches the cell and causes increased growth in cell plasma walls. These components are called "amnyoplasts." Therefore, when a plant is oriented in a position that defies its anisotropic tendencies, statholiths provide the pressure to bend the plant upwards again by elongating and growing the cell walls in question.

This thoery is not universally accepted by scientists. There are a few reasons why it is becoming very popular, however. Centrifugal forces usually cause predicted relocations of amnyoplasts in plant cells, indicating a stong response to gravitational forces. Mutant strains of plants with smaller amnyoplasts do not respond well to geotropism, and generally do not live as long as a normal plant. And finally, more and more evidence is coming forth that Golgi bodies, or cell components involved in cell growth, play an active part in cell elongation (bending) caused by amnyoplasts.

To create a geotropism of your own, you will need:

Two plants, in untainted soil

Two pieces of cellophane

Two small blocks of wood

An evenly lit room

Because this is an experiment, you will need to have a control plant and a varible plant, or the plant that will endure the experiment. Take the cellophane and wrap it snugly around both pots so that soil will not fall out. Place one plant right-side up in the evenly-lit room, and balance the other plant upside-down on the two wooden blocks. Within hours, examine the plant that you have placed upside-down. You should witness your geotropic effect.

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