40 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
led off lobe close to the distal and proximal hairs respectively, so that a straight line 
connecting the contacts would pass through both of them. On the opposite lobe they 
were applied opposite to each other in the space between the hairs. The observations 
with excitation on the opposite side were made first, so that the effect of “ exhaustion/’ 
if any, in increasing the delay, would tell against the result. Two observations were 
taken at each period, which coincided in most cases exactly. The weather was 
extremely warm and sunny. In other leaves the time interval between the two sets of 
results never exceeded that observed in the one recorded, so that I think I am justified 
in saying that the rate of transmission is at least 200 millims. per second. But it must 
be carefully borne in mind that this only applies to plants at hot-house temperature, 
as was strikingly shown in all of the experiments. A leaf, which in the chamber at 
32° C. responded to an excitation of the opposite lobe when the galvanic circuit was 
opened at 0'08" after excitation, gave no trace of a variation with a closing time which 
lasted to 0'20", when the experiment was repeated as soon as possible afterwards in the 
room at 21°. On shifting K 2 to O^O", so that the galvanometer circuit remained 
closed from one-fifth of a second after excitation onwards, the image shot off the 
screeD. 
PART V. 
r ■ 
Excitability. 
The investigation of excitability in Dioneea is a matter of great difficulty, partly on 
account of the rapid changes which it undei*goes, partly on account of the difficulty of 
so applying any excitant that its exciting action can be assumed to be constant. The 
results of our observations as to the behaviour of the leaf when excited will be best 
stated in relation to the different methods of excitation. 
1. Mechanical excitation. — It was observed by Mr. Dabwin (‘Insectivorous Plants/ 
p. 289) that when a single human hair was fixed in a handle, and cut off so that one 
inch projected, and “ the extremity of the hair was then brought by a slow movement 
laterally into contact with the tips of a filament, the leaf” excited “ instantly closed,” 
or, “ after two or more touches of the same kind; ” whence he concluded that they 
were extremely “ sensitive to a momentary touch ” which was best adapted to enable 
the trap to close on an insect. To this I have to add another observation, which is at 
first sight so improbable that I have become convinced of its truth with great reluc¬ 
tance, viz. : that the touch is much more efficacious when it is a living touch, that is, 
when communicated by an instrument (particularly a hair or a camel-hair pencil) held 
in the hand, of which the motion is muscular, than by any mechanical arrangement 
which I have been able to contrive. The two forms of mechanical excitation which 
have been used are (1) a lever, of which the axis works in a firm support of which the 
height can be varied at will, at the end of which a camel-hair pencil is fixed of which 
