PROBLEM 



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4 Hoiv Do Plants Respond to Their 

 Environment? 



A common response in plants. Surely 

 you have noticed that some plants placed 

 near a window turn the broad side of 

 their leaves toward the light. The turn- 

 ing may take a few days or sometimes 

 only a few hours. Light is the stimulus; 

 the response is bending of the stem in 

 the direction of the source of the light 

 or the bending or twisting of the leaf 

 stalk. By doing Exercises i and 2 you can 

 determine how both roots and stems re- 

 spond to light. When the response is the 

 turning of the organism or some part of 

 the organism toward or away from the 

 source of the stimulus, the behavior is 

 called a tropimi (troh'pism). The word 

 means "turning." When the stimulus is 

 Ught, the tropism is called phototropisj/i 

 (fo-tot'row-pism), or light turning. The 

 stem and leaf stalks, since they turn to- 

 ward the light, are said to show positive 

 phototropism. See Figure 279. The turn- 

 ing of a part away from the light is nega- 

 tive phototropism. 



In some animals, too, there are reflexes 

 which can be called tropisms. That is, 

 the animals turn toward or away from 

 the source of a stimulus. You have seen 

 moths fly directly into the Hght. Is this 

 positive or negative phototropism? Pro- 

 tozoa, also, make some reflex responses 

 that are tropisms. 



Other tropisms. It is very easy for you 

 to discover the effect of gravity on the 

 parts of a plant by doing Exercise 3. 

 Gravity is a stimulus to which plants are 

 sensitive but to which most animals do 

 not respond. The main root of a plant 

 turns down toward the center of the 

 earth, the main stem grows away from it. 

 The root is therefore said to have posi- 

 tive geotropism (jee-ot'ro-pism), earth 

 turning, and the stem is said to have neg- 

 ative geotropism. See Figure 281. 



Mechanical stimuli also cause striking 

 tropisms in some plants. The grapevine, 

 the pea plant, and others which grow tall 

 and have stems without much wood 

 have sensitive stemlike structures known 

 as tendrils. The tendrils grow toward any 



Fig. 279 Where was the source of light to 

 which these nastiirtimn plants responded? How 

 did they respond? (blakiston) 



