PROPERTIES OE THE LEAP OE DiONAEA. 49 
second, after which it gradually declines, and that the total effect consequent on each 
excitation is very variable, being dependent on accidental differences in the way in 
which the sensitive hair is touched. 
The mirror experiment .—The effect of series of frequently repeated mechanical 
excitations on leaves at relatively low temperatures may be observed with greater 
accuracy by another method. The leaf is attached to the rotating cork in exactly the 
same way as in the experiment last described, but instead of a lever a very light glass 
mirror, such as is used for reflecting galvanometers, is cemented to the external surface 
of the free lobe, near the margin. By means of this mirror the image of a horizontal 
slit is thrown on a vertical scale which is so graduated that the angular movement of the 
lobe can be measured with great delicacy. An experiment of this kind gives the 
following results. The leaf having been led off to the electrometer by electrodes 
applied to the upper surface of the attached lobe and to the midrib, was subjected to 
22 excitations, each of which consisted in touching very gently one of the hairs of the 
attached lobe. The total angular movement (rotation) of the mirror was 167°. 
This was accomplished in 20 excitations, all of which were effectual in the sense that 
each produced a normal variation. 
Number of excitation. 
Effect. 
Sum of effects. 
O 
O 
1 
•o 
•o 
2 
•o 
•o 
3 
0-5 
0-5 
4 
0*4 
0-9 
5 
0-4 
1-3 
6 
0-8 
2-1 
7 
1-0 
3T 
8 
4T 
4-5 
9 
3-5 
8-0 
10 
4-0 
12-0 
11 
5-5 
17-5 
12 
7-5 
25-0 
13 
13-0 
38-0 
14 
15-0 
53-0 
15 
42-0 
95-0 
16 
34-0 
129-0 
17 
10-0 
139-0 
18 
11*0 
150-0 
19 
13-0 
163-0 
20 
4-0 
167-0 
The fact that in this and in other similar experiments it is possible to excite the 
leaf once, twice, or a greater number of times without any appreciable movement of 
the image, at first sight seems to indicate that the electrical effect is independent of 
the mechanical. But before accepting such an inference it must be remembered that 
we cannot be at all sure that the interstitial movement of liquid, which in the leaf as in 
all the moving organs of plants is the efficient cause of change of form, may not 
begin without making itself visible by any change in the curvature of the lobe, how- 
MDCCCLXXXIL H 
