48 
PROFESSOR B, SANDERSON ON THE ELECTROMOTIVE 
5th Series. Eighteen double excitations at intervals of half a second; of these eight 
were fruitless. Of the ten responses, half occurred at the first excitation, the 
remainder at the second. 
These experiments show that at all intervals between 0*02" and 0*4" summation 
occurs, and that at 0'5" the result becomes uncertain. 
Another method may be used which consists in subjecting a leaf to repeated excita¬ 
tions by successive series of inadequate induction shocks at a fixed interval— e.g., at a 
twentieth of a second, gradually increasing the number in each series until a response 
is obtained. In an experiment of this kind a leaf responded in a room at 20° C. when 
the number of opening shocks amounted to 23. In the chamber at 32° C. it responded 
after 7. It is to be noted that, in an experiment of this kind, the distance of the 
secondary coil from the primary must very slightly exceed that at which single shocks 
are responded to without fail. This method is useful as a means of testing excitability 
under different conditions. 
PART VI. 
Relation between the Excitatory Process and the Mechanical Effect. 
The moment at which the mechanical effect begins can be easily measured graphically 
by the following method. A cork is prepared, with a hole across its axis for the 
reception of the round horizontal arm of a support resembling that shown in fig. 4 
(electrodes). On this the cork can be rotated. The cork is cut so as to present 
a surface somewhat smaller than that of a lobe of the leaf. To this a leaf is cemented 
with plaster of Paris by the external surface of one lobe, leaving the midrib free. To 
the border of the opposite lobe a very light straw lever is fixed by the same means, in 
the position it would assume if it were a prolongation of the middle marginal hair. 
This having been done, the lever is made horizontal by rotating the cork which, 
with the leaf, is brought into such a position that its motions are inscribed on a 
blackened cylinder, of which the horizontal movement should be 1 centim. per second. 
The leaf is excited mechanically by a camel-hair pencil of which the motion breaks 
a circuit. The moment at which this happens is recorded on the same cylinder by 
a Deprez’ time marker. It is thus learnt that the mechanical response, at tem¬ 
peratures from 15° to 20° C., begins at an interval of from a minute-and-a-half to two 
minutes after excitation, so that in the fundamental experiment it coincides with 
the close of the first phase of the variation. If this observation is made at a low 
temperature, as e.g., 15° C., and the hairs are touched with great care, the first excita¬ 
tion produces no perceptible motion. At the fifth or sixth the lever slowly rises and 
continues to do so for several seconds (see Proceedings 1876, p. 415). To record this 
effect, the best plan is to leave the lever free and record its position at the end of 
every half second on a smoked glass plate. The result is shown in the figure. It is 
seen that in each case the rate of ascent of the lever is greatest during the first half 
