The taxis, a type of movement that involves either the entire plant or its reproductive cells, occurs in several groups of plants and fungi but not among flowering plants. In response to a stimulus, the cell or organism, either propelled or pushed by flagella (whiplike appendages) or cilia (short, whiplike appendages), moves toward or away from the source of the stimulus.
Figure 11.19 Water conservation movement in a grass leaf when insufficient water to maintain normal turgor is available. A. The leaf when adequate water is available. B. The leaf after it has rolled up. C. Enlargement of a cross section of a rolled leaf showing the location of the large, thin-walled bulliform cells, which partially collapse under dry conditions and thus bring about the rolling of the leaf blade.
Stimuli for taxic movements include chemicals, light, oxygen, and gravitational fields. In ferns, for example, the female reproductive structures produce a chemical that prompts a chemotaxic response in the male reproductive cells (sperms)—that is, the sperms swim toward the source of the chemical. Certain one-celled algae exhibit phototaxic responses, swimming either toward or away from a light source. Other similar organisms exhibit aerotaxic movements in response to changes in oxygen concentrations.
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