1876.] 



on the Leaf of Dionasii muscipula. 



429 



the number o£ double shocks which preceded the excursion increased as 

 follows : — 



18, 18, 20, 23, 21, 24, 24, 27, 33, 32, 74. 

 After the latter no further effect was producible. 



The result may be thus interpreted. Each excursion is an effect to 

 w^hich each of the induction-shocks which precedes it contributes. At 

 first the summation is rapid ; for the tissue traversed by the currents is 

 still prompt to enter into that molecular change of which the excursion 

 is the visible sign. As this promptitude to change or, as it might be 

 called, explosiveness, diminishes, the number of individual stimuli re- 

 quired in order to bring about the discharge becomes rapidly larger and 

 larger, until at last the result is indefinitely postponed. 



.When a leaf is excited at regular intervals by single shocks of such 

 intensity as to be just beyond the limit of adequacy, so that the slightest 

 diminution would render them futile, it is sometimes observed that the 

 effects become rhythmical. Thus in a series of 54 successive excitations 

 at. half -minute intervals, w^e obtained the follovidng results : — Excitations 

 1, 2, 3, and 4 were effectual; but of the sixteen excitations following, every 

 other was futile, the alternate ones being followed by excursions ; then 

 followed during 8 minutes a series of futile excitations, after which the 

 leaf was allowed to rest for 2 minutes. On resuming, the alternate 

 rhythm again appeared for six excitations, then becoming modified so 

 that an excursion followed every fourth instead of every third excitation, 

 a state of things which continued for a quarter of an hour. In other 

 instances the same tendency showed itself, but less distinctly, the usual 

 result being (as has been already stated) that no further effect was produced 

 so long as the same spot was acted upon by currents of the same in- 

 tensities. 



The fact that by changing the seat of insertion of the needle-points 

 the excursions could at auy time be reproduced we regard as of import- 

 ance, as showing that the excitability of the plant is a property possessed, 

 so to speak, independently by the protoplasm of every cell in the excitable 

 area. When, after repeated excitations at any particular point, effects 

 cease to manifest themselves, their absence denotes, not that the whole 

 leaf is exhausted (for if it were so, change of insertion would not renew 

 them), but merely that the excitability of the tissue in the immediate 

 contact with the needle-points has been blunted. 



Section 5. — The Electrical Disturbance considered in Melation to tJie Time 



ivhich it occiqnes. 



It has already been stated that the change of form consequent on 

 excitation does not begin until electrical disturbance is entirely over. In 

 other words, the latter occupies a period during which, while no ^dsible 

 changes are taking place, molecular changes must certainly be in progress 

 in the excited part — a period which, wath reference to muscle, Du Bois- 



2h 2 



