222 



applied to the tip of a branch 

 at A. in Fig. 4, the leaflets 

 nearest the flame close at once 

 by an almost instantaneous 

 movement, then the motion 

 is taken up by each pair of 

 leaflets in turn until the base 

 is reached. Then an impulse 

 starts towards the tip of the 

 other secondary petioles clos- 

 ing the leaflets in succession. 

 Meanwhile an impulse has 

 been travelling toward the 

 base of the leaf stalk, and as it 

 reaches the thick cushion of 

 tissue — the pulvinus — which 

 joins it to the stem, the pul- 

 vinus contracts on the lower 

 side and the entire leaf sinks 

 downward. Impulses then ■ — t. 

 travel downward and upward 

 along the stem and branches, 

 and out to the tips of the 

 leaves, producing a move- 

 ment similar to that of the first leaf. 



"7T"f 



A 



> 



rig. 



5. Section of pulvinus of Mimosa 

 a. Fibres and vessels, b. Upper side. 



Fig. 4. Diagram showing direction of impulses 

 in Mimosa from a flame applied at A. 



It may interest you to know that 

 the rate at which impulses travel' 

 has been accurately measured and 

 that it is found to be about four- 

 teen feet per minute, though 

 dependent upon the temperature, 

 age of the plant, etc. At the rate 

 given, an impulse might traverse 

 the entire body of the plant in a 

 few seconds. All of the move- 

 ments of the Mimosa are effected 

 by means of pulvini at the bases 

 of the primary and secondary 

 petioles, and of the leaflets. The 

 structure and action of a pulvinus 

 may be illustrated by reference 

 to the diagram. (See Fig. 5.) 

 The portion of the stalks consti- 

 tuting the pulvinus consists of a 

 central strand of very flexible 

 hard tubes, vessels and fibres sur- 

 rounded by a cylinder of thin 

 walled cells which are capable of 

 almost instantaneous changes in 

 size and form. When an im- 

 pulse from a shock reaches a pul- 

 vinus, the cells on the lower side 

 at once lose a part of their con- 



