PROPERTIES OE THE LEAF OF BIOFTEEA. 
53 
primary, may for many purposes be used more advantageously than any other mode of 
excitation, for by means of them a leaf may be subjected to repeated excitations for 
several hours without failure or appreciable exhaustion. 
The minimum effectual strength of an opening induction current depends on its 
direction, weaker currents being responded to when they are directed downwards than 
in the contrary case. Induction currents which are inadequate to evoke a response 
are yet followed by a local change in the electrical state of the surfaces through which 
they are led which resembles that produced by weak voltaic currents. 
11. Summation of stimuli .—Two inadequate opening induction currents which 
follow one another at any interval greater than too"> or ^ ess than -f^-ths of a second, 
may evoke a response. In this case the response follows the second excitation. When a 
leaf is subjected to series of inadequate induction currents at short intervals (e.g., - 2 - 0 Th 
of a second) the response may occur after a greater or less number of excitations, 
according to the temperature at which the experiment is made and the strength of 
the current. 
12. Summation of effects .—In a series of mechanical excitations each of which is 
just adequate to produce an electrical response, those which occur earliest are followed 
by no visible change of form. Of the later members of the series each produces 
a measurable movement, the extent of which becomes greater each time that the 
excitation is repeated until eventually the leaf closes.] 
COHCLUSIOK 
In the preceding paragraphs all reference has been omitted to the physiological 
meaning of the electromotive phenomena which have been described. The moment 
has now arrived at which it appears necessary to offer such an explanation as the 
knowledge acquired justifies, as to their relation to the vital processes with which 
they are associated. 
According to Professor Murk the electromotive properties of the leaf of Dionsea 
may be accounted for on the supposition that in each cell the ends of the cell are, in 
the resting state, positive to the middle, and that in excitation the difference of 
potential between the ends (poles) of the cell and its middle (equator) undergoes a 
sudden diminution. This theory must be rejected at once, on the ground that it 
fails to explain the fundamental experiment. The electromotive forces it supposes 
to be in operation act in directions parallel to the surface of the leaf—at right angles, 
therefore, to the path of the currents which show themselves when the two opposite 
surfaces of the leaf are led off to the galvanometer. Under these conditions it is 
impossible that the latter can be the expression of the former. 
On the other hand, I accept as fundamental the doctrine that whatever physiological 
properties the leaf possesses, it possesses by virtue of its being a system of living cells. 
The first question, therefore, to be determined is that of the electromotive endow¬ 
ments of the individual cell. 
