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Quick Lab

In some plant stems, phototropism is not caused by auxin movement. In these instances, light may cause the production of a growth inhibitor on the lighted side. Negative phototropism is sometimes seen in vines that climb on flat walls where coiling tendrils have nothing to coil around. These vines have stem tips that grow away from light, toward the wall. This growth brings adventitious roots or adhesive discs in contact with the wall on which they can cling.

Solar tracking, also called heliotropism, is the motion of leaves or flowers as they follow the sun's movement across the sky, as shown in Figure 31-6. By continuously facing toward a moving light source, the plant maximizes the light available for photosynthesis. These movements are not actually a tropism but a related kind of movement.

Thigmotropism

Thigmotropism is a plant's growth response to touching a solid object. Tendrils and stems of vines, such as morning glories, coil when they touch an object. Thigmotropism allows some vines to climb other plants or objects, increasing the vine's chance of intercepting light for photosynthesis. It is thought that ethylene and a type of auxin are involved in this response.

Visualizing Phototropism

Materials 2 in. pots (2) containing potting soil, 4 bean seeds, cardboard box

Procedure

1. Plant two bean seeds in each pot. Label each pot for your group. Place one pot in a window or under a plant light.

2. Cut a rectangular window in the box, and place the box over the second pot so that the window faces the light. Place the box in a different location from the open pot. Keep both pots moist for several days.

3. Remove the box 2-3 days after the seedlings have emerged from the soil. Compare the seedlings grown in the light with the ones grown in the box. Sketch your observations.

Analysis How are the seedlings different? How do you account for the difference? Describe or draw what you think the cells inside the curved part of the stem look like.

figure 31-6

Word Roots and Origins thigmotropism from the Greek thiga, meaning "touch," and tropos, meaning "turning"

figure 31-6

These sunflower plants face the sun as it sets in the evening. In the morning, the plants face the sun as it rises, then they follow it as it moves across the sky. This movement is called solar tracking.

Gravitropism

Gravitropism is a plant's growth response to gravity. A root usually grows downward and a stem usually grows upward—that is, roots are positively gravitropic and stems are negatively gravitropic.

Like phototropism, gravitropism appears to be at least partly regulated by auxins. One hypothesis proposes that when a seedling is placed horizontally, auxins accumulate along the lower sides of both the root and the stem. This concentration of auxins stimulates cell elongation along the lower side of the stem, and the stem grows upward. A similar concentration of auxins inhibits cell elongation in the lower side of the root, and the root grows downward, as shown in Figure 31-7.

Chemotropism

Plant growth that occurs in response to a chemical is called chemotropism. An example is the growth of a pollen tube after a flower is pollinated. The pollen tube grows out from the pollen grain and down through the stigma and style to the ovule, in the direction of increasing concentrations of figure 31-7

This photograph was taken seven days after cuttings of the inch plant, Zebrina pendula, were placed in growing tubes in different orientations. Notice that the two lower cuttings have started to grow upward. The upward growth of the lower cuttings is caused by the plants' response to gravity and is called negative gravitropism. When the roots turn and begin to grow in a downward direction, the growth is called positive gravitropism.

figure 31-8

(a) Mimosapudica, also called the sensitive plant, is a small shrub that has leaflets. (b) When a leaflet is touched, it folds together. This rapid movement is a thigmonastic movement.

figure 31-7

This photograph was taken seven days after cuttings of the inch plant, Zebrina pendula, were placed in growing tubes in different orientations. Notice that the two lower cuttings have started to grow upward. The upward growth of the lower cuttings is caused by the plants' response to gravity and is called negative gravitropism. When the roots turn and begin to grow in a downward direction, the growth is called positive gravitropism.

chemicals produced by the ovule.

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