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Figure 11.12 Tobacco seedlings grown in the dark. The source of gravity is at the bottom of the pictures. Normal "wild type" seedlings in the top row are all more or less perpendicular to the gravity. The seedlings in the bottom row are mutants with much less starch than normal plants. The mutant seedlings are disoriented, suggesting that any amyloplasts of typical mass function as statoliths in the perception of gravity. However, mutants of other species lacking amyloplasts do respond to gravity, suggesting that parts of cells other than amyloplasts may be involved in the perception of gravity, x0.5. (Courtesy John Z. Kiss)

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Parts Amyloplast

Figure 11.13 A clinostat, which is a tool used by plant biologists to negate the effects of gravity. Growing plants or seedlings are slowly rotated so that the statoliths in cells that perceive gravity do not settle to the bottom, and typical growth or bending of stems or roots away from gravity does not occur.

Figure 11.13 A clinostat, which is a tool used by plant biologists to negate the effects of gravity. Growing plants or seedlings are slowly rotated so that the statoliths in cells that perceive gravity do not settle to the bottom, and typical growth or bending of stems or roots away from gravity does not occur.

sensors) nor other organelles can settle while the plant is moving, and this apparently prevents transport of auxin, which would bring about cell elongations that produce curvatures of root and stem.

Other Tropisms A plant or plant part response to contact with a solid object is called a thigmotropism. One of the most common thigmotropic responses is seen in the coiling of tendrils and in the twining of climbing plant stems (see Figs. 7.14 and 11.8). Such responses can be relatively rapid, with some tendrils wrapping around a support two or more times within an hour. The coiling results from cells in contact becoming slightly shorter while those on the opposite side elongate.

Roots often enter cracked water pipes and sewers. In fact, roots have been known to grow upward for considerable distances in response to water leaks. Some scientists have called such growth movements hydrotropisms, but most plant physiologists today doubt that responses to water and several other "stimuli" are true tropisms. Other external stimuli that produce tropic responses designated as tropic by some scientists include chemicals (chemotropism), temperature (thermotropism), wounding (traumotropism), electricity (electrotropism), dark (skototropism), and oxygen (aerotropism).

Greater concentrations of roots tend to occur on the north and south sides of wheat seedlings, and it has been suggested that magnetic forces may be involved; the term geomagnetotro-pism has been proposed for this phenomenon. Some of these tropic or tropic-like responses have been artificially induced, but others take place regularly in nature. For example, germinating pollen grains produce a long tube that follows a diffusion gradient of a chemical released within a flower; this is considered a chemotropism. Thermotropic responses to cold temperatures may be seen in the shoots of many common weeds, which grow horizontally when cold temperatures prevail and return to erect growth when temperatures become warmer.

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