1876.] 



on the Leaf of Dionsea muscipula. 



415 



The apparatus (fig. 2) employed for this purpose consists of : — 1, a 

 7'ecording-cylinder ; 2, an electro-magnetic chronograph made for us in 

 Paris by M. Yerdin, the perfection of which we owe to the kindness of 

 our friend Prof. Marey ; 3, signalling-keys, of which one closes, the other 

 opens, the signal-circuit — that of a battery of two Daniells. The cylinder, 

 of \\^hich the surface is blackened in the usual way, reyolves with great 

 regularity five times per minute, and has a circumference of half a meter, 

 so that the rate of horizontal movement of its surface is two and a half 

 meters per minute (4*166 centims. per second); consequently a hundredth 

 of a second (0*4166 millim.) is readily measurable. 



In the present experiment it was necessary that the touching of the 

 sensitive hairs should be accomplished with great gentleness, care, and 

 exactitude, and always in the same manner. The touch in each experi- 

 ment was made by one observer at an expected signal from the other, 

 the arrangement being that A should count aloud 1, 2, 3, 4, 5, that B 

 should touch at the moment 5 is said, and that at the same moment A 

 should close the signal-circuit, having his eye on the lever, and being 

 ready to break the circuit at the first perceptible movement. Consider- 

 ing that the periods to be measured were of several seconds' duration, 

 this method was quite accurate enough for the purpose. 



The time-measurements, as will be seen at a glance, stand in a remark- 

 able relation to the mechanical effects, showing that the delay between 

 excitation and effect diminishes as the extent of the effect increases, both 

 facts having the same meaning — namely, that in the plant, as in certain 

 cases well known to the animal physiologist, inadequate excitations when 

 repeated exercise their influence by what has been termed summation, 

 i. A. that when any number of such stimulations, say a, 6, c, e, &c., 

 follow each other in succession, the effect of each is prepared for and 

 aided by its predecessors : so that although, as in the present instance, 

 rt, 6, c, d, may seem to produce no effect whatever, each of them really 

 produces a change in the excited structure, and each contributes, when 



