Other Free Encyclopedias » Science Encyclopedia » Science & Philosophy: Gastrula to Glow discharge

Geotropism

auxin roots darkness root

Plants can sense the Earth's gravitational field. Geotropism is the term applied to the consequent orientation response of growing plant parts. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of the Earth. In contrast, shoots are negatively geotropic, that is, they will bend and grow upwards, or away, from the surface.

These geotropisms can be demonstrated easily with seedlings grown entirely in darkness. A seedling with its radicle (or seedling root) and shoot already in the expected orientation can be turned upside down, or placed on its side, while kept in darkness. The root will subsequently bend and grow downwards, and the shoot upwards. Because the plant is still in darkness, phototropism (a growth movement in response to light) can be eliminated as an explanation for these movements.

Several theories about the manner by which plants perceive gravity have been advanced, but none of them is entirely satisfactory. To account for the positive geotropism of roots, some researchers have proposed that under the influence of gravity, starch grains within the cells of the root fall towards the "bottom" of the cell. There they provide signals to the cell membrane, which are translated into growth responses. However, there have been many objections to this idea. It is likely that starch grains are in constant motion in the cytoplasm of living root cells, and only "sink" during the process of fixation of cells for microscopic examination. Roots can still be positively geotropic and lack starch grains in the appropriate cells.

A more promising hypothesis concerns the transport of auxin, a class of plant-growth regulating hormones. Experiments since 1929 have shown that auxin accumulates on the "down" side of both shoots and roots placed in a horizontal position in darkness. This gradient of auxin was believed to promote bending on that side in shoots, and to do the opposite in roots. Confirmation of the auxin gradient hypothesis came in the 1970s. When seeds are germinated in darkness in the presence of morphactin (an antagonist of the hormonal action of auxin), the resulting seedlings are disoriented—both the root and shoot grow in random directions. Auxin gradients are known to affect the expansion of plant cell walls, so these observations all support the idea that the transport of auxin mediates the bending effect that is an essential part of the directional response of growing plants to gravity.

User Comments

Your email address will be altered so spam harvesting bots can't read it easily.
Hide my email completely instead?

Cancel or

Vote down Vote up

over 2 years ago

This information was very useful but the voice was creepy.

Vote down Vote up

about 3 years ago

highly educational..i like it.

Vote down Vote up

over 2 years ago

i might use this info for my research paper on gravity's change of direction on plants. :D

Vote down Vote up

over 3 years ago

interesting!

Vote down Vote up

almost 6 years ago

i felt very nice reading ur article.thanks

Vote down Vote up

about 3 years ago

abelity ofscience project of class X cbse

Vote down Vote up

over 2 years ago

I learned a lot from reading this. I'm boing an experament on Geotropism for school, are their any sites that you guy could tell me about?

Vote down Vote up

about 3 years ago

abelity ofscience project of class X cbse

Vote down Vote up

almost 2 years ago

This is a good article.Very helpful for my project.

Vote down Vote up

over 1 year ago

Great article, thank you very much for the help!

Vote down Vote up

almost 3 years ago

moo

Vote down Vote up

almost 3 years ago

sex teehee :3

Vote down Vote up

almost 3 years ago

FART.

mmmmmm geotropism.

Vote down Vote up

almost 2 years ago

this is great but it needs to demonstrate by atleast showing a picture to demonstrate geotropism because i need a picture to do a model ..... *sigh :-/

Vote down Vote up

about 1 year ago

its very informative, thanks.