286 FIRST YEAR COURSE IN GENERAL SCIENCE 



The folding together, when touched, of the leaflets of the 

 sensitive plant is brought about by differences in amount of 

 water in the cells of the motor organs. Contact or even a 

 sudden jar of the plant is a stimulus to which protoplasm 

 responds by regulating the water in the motor organs. The 

 closed leaf has a smaller surface exposed to the wind or to 

 voracious enemies. 



It is thus seen that responses are made by the plants them- 

 selves, the external influence or force acting simply as a signal 



FIG. 152. LEAF OF A SENSITIVE PLANT 



1. Measure in mm. the length and width of one of the leaflets in the 

 open leaf and give an estimate of the area. 2. Give the total area of the 

 leaflets of one division of the four-parted leaf. 3. Compare it with 

 the exposed surface of one of the divisions of the closed leaf. 4. What would 

 be the advantage of the closed leaf in regard to heat, air, or plant enemies? 



or stimulus. The result is the bringing of organs into better 

 positions for doing their work or for protecting themselves 

 from injury. 



329. Internal Cause of Movement in Plants. The 

 movements of plants so far studied are brought about or 

 guided by some external force, although in all cases the living 

 protoplasm causes the useful movement. There is, however, 

 another class of movements which do not appear to result 

 from external conditions, and are hence thought to be due to 

 some internal cause although just what this cause is has 

 not yet been determined. 



