. Principles of modern biology. Biology. 264 - Multicellular Plants. Fig. 14-7. Positive chemotropism of pollen tubes, which are growing toward a crushed stigma (central dark area). auxin produces a curvature toward the source of light; whereas in the root, the inhibiting effect gives a bending in the opposite direc- tion. Many other tropisms also depend upon the auxins. Chemotropisms, such as the growth of pollen tubes through the style (Fig. 14-7), or the growth of roots toward water (Fig. 14-8); and thigmotropisms, such as the twin- ing of the tendrils of climbing plants around solid suppor
. Principles of modern biology. Biology. 264 - Multicellular Plants. Fig. 14-7. Positive chemotropism of pollen tubes, which are growing toward a crushed stigma (central dark area). auxin produces a curvature toward the source of light; whereas in the root, the inhibiting effect gives a bending in the opposite direc- tion. Many other tropisms also depend upon the auxins. Chemotropisms, such as the growth of pollen tubes through the style (Fig. 14-7), or the growth of roots toward water (Fig. 14-8); and thigmotropisms, such as the twin- ing of the tendrils of climbing plants around solid supporting objects—all appear to be typical growth movements, although these reactions have not been studied very inten- sively. However, some plant tropisms—such as the turning of leaves in following the course of the sun—appear to be turgor move- ments, involving receptor-conductor-effector structures that are generally similar to those of Mimosa. Characteristics of the Auxins, fn distinct contrast to the hormones of higher animals (Chap. 22), the auxins do not display a very high degree of specificity. Under proper con- ditions, in fact, very similar effects are ob- served when one auxin is substituted for an- other. Each single compound, on the other hand, displays a surprising versatility of action when applied in different concentra- tions and to different parts of the same plant. In the stem, for example, the same concen- tration of an auxin may stimulate growth in the cells directly behind the main terminal bud, but inhibit the development of the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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