GROWTH AND MOVEMENT 



131 



of osmotic control, and water filters out into the intercellular spaces. 

 The turgor of the cells on the sensitive lower side is thus diminished, 

 and the hinge falls. Protoplasmic continuity has been demonstrated 

 between the active cells of the hinge, which probably serves to convey 

 the stimulus to them all (compare Fig. 19, yl, p. 31). But this will not 

 explain its transmission for the longer distances in the leaf ; nor its 

 further conveyance from leaf to leaf of the shoot. A specific tissue is 

 beheved to carry this out in a purely mechanical way, not as a proto- 

 plasmic excitation. Long thin-v/alled tubes, filled with sap surrounded 

 by a delicate protoplasmic layer, are found in the phloem of the 

 Sensitive Plant. /j 



'^ ^ -> 



Leaves of Droscra roiundifoUa ; enlarged. A, in the receptive state before 

 stimulation. B, after stimulation, viewed from above, with tentacles partly 

 incurved. (After Darwin.) 



These are held to serve as the channels of the conveyance of pressure- 

 waves, consequent upon the movement in the hinge where the stimulus 

 was first received. The passage of waves of pressure along a rubber 

 tube filled with water can easily be illustrated ; if one observer pinches 

 one end of the tube, the stimulus is felt by an observer at the other 

 end. Either positive or negative waves of pressure may thus be 

 transmitted. It is believed that the stimulus in the Sensitive Plant 

 is conveyed in this way, as a negative wave of pressure. If this be 

 true the conveyance of the stimulus to a distance in the Sensitive 

 Plant would not involve any specialised tissue comparable with the 

 nerve of the animal body. 



The most sensational movements seen in Plants are those con- 

 nected with the Carni-vorous Habit, as in Sundew (Drosera) (Fig. 91), 



