14 PLANT RESPONSE 



unequally contractile, we obtain a DIFFERENTIAL RESPONSE, 

 the more contractile becoming concave ; and it is evident 

 that such movements must take place in a direction perpen- 

 dicular to the plane of separation. 



Typical cases of mechanical response in plants are 

 obtained from pulvinated organs. A good example of this 

 is found at the insertion of the petiole in Mimosa pudica. 

 When such an organ is stimulated, it is the lower half that 

 undergoes the greater contraction, and the leaf is depressed 

 by the concavity thus produced. It is generally assumed 

 that the upper half of the pulvinus is not excitable, but this, 

 as I shall show later, is an error. The responsive movement, 

 however, is due to the differential contraction of the two 

 halves, and, as already explained, takes place in a direction 

 perpendicular to the plane which separates them. Such 

 differential response will be found characteristic of all organs 

 possessing dorsi-ventral differentiation. 



Whenever the plant is subjected to any sudden dis- 

 turbance, the sensitive leaf reacts by a fall, which is brought 

 about by the hinge-like mechanism at the pulvinus. The 

 sudden disturbance which induces the fall constitutes the 

 stimulus. The leaf responds when it is shaken, or cut, or 

 when a prick is applied to it, or when a sudden variation of 

 temperature is produced, as by touching it with a hot wire, 

 or with ice, or when an electrical shock is passed through it, 

 or if it be acted on by certain chemical reagents, or a beam 

 of strong light be thrown on it. All these constitute the 

 various forms of stimuli — mechanical, thermal, electrical, 

 chemical, and photic. 



We have next to study the relation between the intensity 

 of the stimulus and the extent of response under varying 

 conditions ; that is to say, we have to determine the 

 ' threshold of response,' in other words, the minimum intensity 

 of stimulus that will be just sufficient to initiate reaction. We 

 have then to observe the repeated response of the plant to 

 repeated stimulation, whether uniform or gradually increasing. 

 We have to detect the signs of fatigue if there be any, and 



