PROPERTIES OP THE LEAP OF DIOH/EA, 
41 
the point is downwards, and so constructed that when it is depressed by the finger 
the signalling circuit is closed^ and 'the tip of the"" camel-hair pencil brought into 
contact at the same moment; and (2) a similar lever of which the motion is due to 
the action of an electro-magnet included in the signalling circuit. Whichever of these 
forms was employed, and whatever care was taken in setting the pencil so as to ensure 
its hitting the hair to be excited without fail, it was invariably found that after a few 
excitations no further response was obtained; whereas it is always easy to repeat an 
experiment which I made with Dr. Francis Darwin in 1873, in which we succeeded 
in exciting a leaf during an hour at two minutes’ intervals, without a single failure of 
the electrical response. This may, I think, be due to the relative rudeness (if such a 
word may be applied) of the mechanical motion as compared with the other. I 
thought at first that it might be attributed to the sameness of the motion—-he., to the 
circumstance that when the blows are inflicted mechanically, however gently, they are 
all in the same direction and each acts on a part which has been acted on before. As, 
however, I have not found that altering the direction of the motion makes any per¬ 
ceptible difference in the result, this explanation fails. What is to be learned from 
the observation is this : the very slightest touch is an “ adequate ” stimulus ; any 
beyond this is more than sufficient and injurious. Its exhausting influence is, however, 
evidently perfectly local; for the loss of sensibility of one hair fails to determine any 
similar loss in others which have not been excited. If in any leaf a single hair, 
excited repeatedly by mechanical means, has ceased to respond, you can at once repro¬ 
duce the result by acting on another hair, which will respond just as readily as if the 
leaf were intact. 
Electrical excitation. 
1. Single induction shocks .—If dry metallic electrodes are applied to the internal 
surface of the leaf, induction shocks conducted through them are without effect, how¬ 
ever near the electrodes may be—the reason being, no doubt, that the surface is so 
perfectly dry that but little current passes between the platinum points. This diffi¬ 
culty may be overcome, either by using the pointed wires so that by piercing they can 
be brought into contact with the cells beneath, or by covering the surface with a layer 
of conducting material such as kaolin, made into paste with half per cent, solution of 
common salt. I used the former of these methods in my first experiments in 1873, as 
well as in those of which the results were published in 1876. For obvious reasons 
it is preferable to use non-polarizable electrodes, to which the form described in 
Part II. is the best that can be given. I have always used the small induction 
apparatus of dtj Bois-Reymond without removing the core, and one Daniell cell in 
the primary circuit. In all experiments opening induction shocks have been exclusively 
used. As a rule, the electrodes are applied to opposite points of the upper and under 
surface of one lobe. With reference to the action of such currents, so applied, it has 
been ascertained : (l) That when in a succession of excitations the secondary coil is 
MDCCCLXXXII, G 
