THE DISPLAY OF ENERGY 



.17:} 



of position in space when they come to rest on the sensory hairs which 

 Hue tiie httle pits, or otocysts. A somewhat similar explanation has 

 been advanced to account for the 

 response to gravity in plants. 

 Cells of plants are filled with fluid, 

 but they also have in them various 

 solid bodies, some of which are 

 starch grains, and others tiny crys- 

 tals of calcium oxalate, or other 

 minerals. It is thought that the 

 movement of these bodies within 

 the cell may give the stimulus for 

 the turning movements attributed 

 to gravity. The twining move- 

 ment and spiral growth of stems 

 also seems to be related to the 

 stimulus of gravity, for if such 

 plants are placed on a rotating clinostat, the twining movement ceases. 

 There are many other kinds of responses, but the mechanism of the 

 response is not always clear. Roots travel for long distances toward a 

 source of water. A case is cited in California of a eucalyptus tree 

 which sent out its roots over 100 feet underneath a boulevard, the 

 fine roots ultimately clogging a cement water pipe on the other side 



Perceptive region 

 of Roripa amphibia; 

 of the granules in 

 Nemec.) 



in the root cap 



with the position 



the cells. (After 



Wrhjhl I'iirct' 



The Sensitive Plant (Mimosa pudica) before and after stitmilation. Time 

 required for reaction can be measured in seconds. 



