. Adventures with animals and plants. Biology. solid object with which they come in contact and wind themselves about the support in tight coils. In this way they hold the plant up. This is called positive thigviotropisv^, which means the turning toward an object that is touched. The stems of a number of plants twine around objects. Pole beans and morning glories are good examples. This is also thigmot- ropism. See Figure 280. Chemical substances may act as the stimulus which causes some part of a plant to turn. The unequal distribution of water, minerals, gases, or other sub- stances in the n
. Adventures with animals and plants. Biology. solid object with which they come in contact and wind themselves about the support in tight coils. In this way they hold the plant up. This is called positive thigviotropisv^, which means the turning toward an object that is touched. The stems of a number of plants twine around objects. Pole beans and morning glories are good examples. This is also thigmot- ropism. See Figure 280. Chemical substances may act as the stimulus which causes some part of a plant to turn. The unequal distribution of water, minerals, gases, or other sub- stances in the neighborhood of the plant may result in che?iiotropism. One very common example Of this is the response of most roots to water. This special kind of chemotrooism is known as hydrotro-. FiG. 281 (above) A young simftower plant was blown down by the wind. The tip of the stem continued to grow. Can you give two possible reasons why it grew up? (schneider and Schwartz) Fig. 280 (left) Squash tendrils attached to a dead pine stem. After its tip has caught on an object the tendril tighteiis 7tp iji a corkscrew. (SCHNEIDER AND SCHWARTZ) pimi. As you can see, plant protoplasm is sensitive to a variety of stimuli. Try Exercise 4. Not all responses of plants are tropisms. Light — its intensity, the kind of light rays — the relative length of periods of light and darkness, temperature, gravity, water, the presence of other chemical substances, and, to a lesser degree, elec- tricity and other stimuli bring about re- sponses in plants. The responses are not always tropisms. For example, some flowers close at ni^ht. They respond to a stimulus, but it is not a turning toward or awav from the source of a stimulus as you can see in Figure 282. If tulips arc available it will be interesting for you to try the effects of temperature on open- ing and closing of the petals. See Exer- cise 5. Why plant responses are slow. You can jerk your hand awav from a hot stove very rapidly because you have.
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