128
LIFE MOVEMENTS IN PLANTS
in the records exhibit chauged rates of conduction. The
middle record is the normal, in the absence of any current. The upper record, denoted by the left-hand arrow, shows the action of a heterodromoas current in shortening the period of transmission and thus enhancing the velocity above the normal rate. The lower record, denoted by the right-hand arrow, exhibits the effect of a homodrom- ous current in retarding the velocity below the normal rate. I find thit a very feeble heterodromous current is enough to induce a considerable increase of velocity, which soon reaches a limit. For inducing retardation of velocity, a relatively strong homodromous current is necessary. I give below a table showing the results of several experiments.
TABLE V. — EFFECT OF HETERODROMOUS AND HOMODROMOUS CURRENT OF FEEBLE IXTEN'SITY OX VELOCITV OF TRANSMISSION.
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Speci- men.
iutengity of
heterodromous
current.
Acceleration
above normal
Intensity of
homodromous
current.
Retardation
below normal.
microampere
per cent
microamperes.
per cent.
1
0-35
16
I
20
2
0-7
13
1-5
19
3
0-8
18
2-0
14
4
0-8
11
20
13
5
10
18
2-5
12
6
lo
15
3-0
40
1
VARIATION OF INTENSITY OF TRANSMITTED EXCITATION UNDER HETERODROMOUS AND HOMODROMOUS CURRENTS.
In the next method of investigation, the induced varia- tion of intensity of transmitted excitation is inferred from the varying amplitude of response of the terminal muscle. Testing stimulus of sub-maximal intensity is applied at the middle of the nerve, v^^here the constant current induces no variation of excitability. Stimulation is eflEected either by single break-shock or by the summated effects of a definite number of equi-alternating shocks, or by chemical stimulation