194 HOW BODY CONTROL IS BROUGHT ABOUT 



meter in length is most sensitive to gravity, as the turning response 

 takes place there (see page 67). Some tips of stems show a simi- 

 lar sensitiveness, as do certain parts of growing leaves. 



The Mechanism of Responses in Plants. — Some of the results 

 of these responses are easily seen in plants, but the method by 



which the responses are 

 brought about is not so 

 easy to see. For example, 

 we say leaves place them- 

 selves so as to get as much 

 hght as possible. But this 

 movement is different from 

 that found in animals which 

 have an internal skeleton 

 with muscles attached. The 

 changes in position in parts 

 'ulvinus of plants are often produced 

 by a more rapid growth of 

 the cells on one side of a 

 structure than on the other, 

 this growth having been ex- 

 cited by an external stimu- 

 lus, such as gravity, water, 

 light, or heat. Such are 

 the curving movements of roots or stems. The placing of the 

 leaves in a horizontal position is brought about by the more rapid 

 growth of tissues on one side of the leaf 

 stalk than the other. 



Changes in the position of leaves are 

 often brought about by special structures 

 at the base of the petiole, as may be seen 

 in the bean plant. These structures, called 

 pulvi'ni (sing, pulmnus), contain thin- 

 walled cells filled with water, and the po- 

 sition of the leaf depends on the relative 

 amount of water in these cells. The more rapid movements 

 of the opening and closing of flower petals ; the changes in po- 

 sition of leaflets of the pea, clover, alfalfa, oxalis, and other plants 



Pulvinus 



Pulvinus of a bean plant : A, external view ; 



B, 1 and 2, section seen through a microscope ; 



C, turning leaves upward ; D, turning leaves 

 downward. Explain from text. 



Clover leaf : 1, in the morn- 

 ing ; 2, at night. 



