204 



THE NERVOUS SYSTEM 



Problem. What is the response of plants to water f 



Prepare a pocket garden filled with cotton. Plant mustard seeds in a verti- 

 cal line across the middle of it. Water the seeds by moistening the cotton on 

 one side of the garden. Try to keep one part of the cotton always moist and 

 the other part dry. 



I. A. Describe the growth of the root and of the shoot. A response to 

 water is called hi/drotropism. 



B. What is the value of hydrotropism to the plant? 



C. Describe any evidence in the experiment that shows whether gravity 

 or water is the stronger stimulus. 



D. If you have ever seen willow trees growing along the bank of a river, 

 describe how hydrotropism tends to affect their growth ? 



The uses of tropisms. Plants make responses to other stiniuH. 



The response to cheniicalf^ is chemotropism, response to heat, therviO' 



tropism, and to touch or contact, thigmotropism. 



All the activities of living plants and animals involve a series of 



responses. As we have already learned, the responses of plants are 



definite for given stimuli. Leaves always grow toward the light, 



roots grow toward gravity, and 

 stems grow away from gravity. 

 Responses are divided into two 

 kinds of reactions, one toward 

 the force or stimulus, positive 

 tropisms, and the other reactions 

 away from the stimulus, nega- 

 tive tropisms. In general, tro- 

 pisms are protective. Without 

 sunlight, the leaves would not be 

 able to make starch ; without the 

 pull of gravity, roots would be 

 unable to anchor the plant in the 

 ground. Tropisms help the or- 

 ganism make the best possible 

 adjustments to its environment. 



3lirection 



Light influences the growth of plants. 

 Most stems and leaves turn toward the light 

 and a large area of leaf surface is exposed. 



