302 



Why JJv'mg Things Behave As They Do unit v 



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 



