CONDITIONS FAVOURABLE TO MECHANICAL RESPONSE 5 I 



brought about by the greater contraction and resultant con- 

 cavity of the more excitable. 



Differential contraction effect in Mimosa magnified 

 by the petiolar index. — We are impressed with the magni- 

 tude of the responsive movement of the Mimosa leaf. It is 

 worth while to remember, however, that the fundamental 

 differential contraction of the organ by which this is brought 

 about is very inconspicuous. If the petiole be amputated, 

 we shall hardly notice the responsive action of the pulvinus. 

 But the petiole, with its attached secondary petioles, acts as 

 a long index, by which the curvature produced at the 

 pulvinus on excitation is highly magnified. Many plant- 

 movements, which now pass unnoticed, would have arrested 

 our attention had there been in their case any such magnify- 

 ing index. 



Summary 



An anisotropic organ is unequally excitable on its two 

 differentiated sides. 



In a dorsi-ventral organ, like the pulvinus of Mimosa, 

 lateral response is brought about by the differential con- 

 traction of the two halves. 



In such dorsi-ventral organs, owing to the differentiation 

 of the two halves, an increase of turgidity causes erection, 

 and a diminution the depression, of the leaf. 



Hence two opposite kinds of responses are possible, 

 (a) that which is usually seen, due to the negative variation 

 of turgidity, followed by recovery to the normal ; and (/;) the 

 counter-variation, that is to say, a variation of turgidity 

 above the normal — causing abnormal erectile response — and - 

 recovery. 



The ordinary mechanical response of dorsi-ventral organs 

 being dependent on (a) the difference in excitability of the 

 two halves, and {b) on the expulsion of water from the ex- 

 cited organ, it follows that any condition which dimimsnes 

 or abolishes this difference, or prevents the expulsion of 

 water, will render the mechanical response impossible. A 



ii 2 



