14 Macfarlane. — Contributions to the History of 



of each is prepared for and aided by its predecessors ; so that 

 although, as in the present instance, a, b, c, d may seem to 

 produce no effect whatever, each of them really produces a 

 change in the excited structure, and each contributes when 

 summed with its predecessors and successors to the bringing 

 about of the visible effect which follows e. During the re- 

 mainder of the process the operation of the same law shows 

 itself in the gradual augmentation of the increments— the 

 last contraction— that by which the leaf closes, being the re- 

 sult of the summation of the excitation which immediately 

 preceded it with all the previous excitations. Our conception 

 of the nature of the process may be otherwise expressed by 

 saying, that under the influence of successive excitations the 

 latent excitability of the leaf gradually increases, for whereas 

 before it either made no response, or postponed its response 

 indefinitely, it now answers to the same stimulus by a visible 

 motion, of which the promptitude and the extent increase 

 together." 



It is noteworthy that when a leaf closes, which has been 

 twice stimulated, with short time interval between the first 

 and second shocks, the closing movement is a rapid one, 

 being limited within two seconds. When a leaf closes which 

 has received five or six shocks with a pretty wide time in- 

 terval between each, the period of actual closure movement 

 of each leaf-half may extend over ten to fifteen seconds, while 

 one which has been frequently stimulated may even require 

 forty-five to fifty seconds to effect final closure of half the 

 movement limit. Now this is what we would expect from a 

 contractile tissue which has been fatigued by repeated stimuli; 

 a reduction of the high degree of contractility which it nor- 

 mally possesses. 



It has hitherto been supposed that only the sensitive hairs 

 and the triangular area between these are irritable on surface 

 stimulation, but both outer and inner leaf surfaces exhibit a 

 marked degree of sensitivity, though greatly inferior to that 

 of the sensitive hairs. The latter are, therefore, localized 

 centres of irritability, set apart in an otherwise sensitive or 

 irritable leaf for receiving external impressions that would be 



