PROPERTIES OF THE LEAF OF DIORAEA. 
47 
examples. If the leaf is expanded and not restrained, it may close at the first exci¬ 
tatory variation, i.e., one to one-and-a-half second after the closure of the current. 
But if the experiment is made at a low temperature (18° to 20° C.) this event does 
not happen until after the third, fourth, or fifth variation. To observe these facts 
the current of a Grove cell is led through one lobe, while the other is led off (in 
the usual way, to the electrometer, and the time after closure of the circuit noted by 
the watch, as in the following experiments :— 
I. Current of one Grove from above downwards for half a minute; excitatory 
variation at 3", 9", 13", 19", 23", 30". 
Same current directed upwards—variation at closure only. 
II. Current of one Grove from above downwards for one minute ; variation at 
1", 15", 25", 34", 38", 45" 
Same current directed upwards—variation at closure only. 
III. Same leaf. Current directed through contacts previously used for leading off; 
leaf led off by lobe previously excited. Variation at 2", 8", 22", 32" 
Same current in opposite direction—variation at closure only. 
Summation of stimuli. —To prove summation it must be shown that two equal 
excitations, each of which alone is inadequate to evoke a response, do so by summation, 
that is, when they follow each other at a short interval. If this is so, there is evidence 
that the first, although it is apparently without effect, yet produces a change in the 
excitable structure which renders it more excitable than it was before. The best 
method consists in making two alternating series of excitations, in one of which single 
opening induction shocks, just inadequate to produce excitatory effect, are used, while 
in the other, two induction currents of equal strength follow each other at a variable 
interval. 
1st Series. Distance of coil 7'6 centims. Temperature of chamber 32° C. 
Interval 0'02." Ten excitations, of which Nos. 2, 4, 6, 8 were single, 1, 3, 5, 
7, 9 were repeated. All of the latter were effectual; all of the former ineffectual. 
2nd Series. Distance of coil 7' 4 centims. Temperature 32° C. Interval 0T." 
Ten excitations; six single, five repeated (-fj"). All of the latter were effectual; 
all but one of the former ineffectual. 
3rd Series. Distance of coil 7*3 centims. Temperature 32° C. Interval 0*2." 
Sixteen excitations, of which half single, the rest repeated All of the 
latter were effectual; all except two of the former ineffectual. 
4th Series. Another leaf. Distance of coil 8 centims. Temperature 32° C. 
Four excitations; two single, two repeated at intervals of -J-". Both of the 
latter effectual; the other ineffectual. 
Four excitations; two single, two repeated at intervals of §". The latter 
effectual; the former ineffectual. In this series it could easily be seen that 
the excitatory effect followed the second excitation. 
