434 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
very sluggish under fatigue; when excessively tired it temporarily 
loses its power of perception. In this condition the plant requires at 
least half an hour’s absolute rest to regain its equanimity. 
EXCITATORY IMPULSE IN MIMOSA, 
We next take up the question of the function of transmission of 
excitation. It has hitherto been supposed that in Mimosa the im- 
pulse caused by irritation is merely hydromechanical and quite dif- 
ferent from the nervous impulse in the animal. According to this 
hydromechanical theory, the turgid plant tissue is imagined to be like 
india-rubber tube filled with water. The application of mechanical 
stimulus is supposed to squeeze the tissue, in consequence of which 
the water forced out delivers a mechanical blow to the contractile 
organ of the plant. The propagation of mechanical disturbance is 
thus occasioned by the bodily transfer of fluid material in a pipe. 
In strong contrast to this is the transmission of nervous impulse, 
which is a phenomenon of passage of protoplasmic disturbance from 
point to point. The molecular disturbance, constituting excitation, 
passes along the conducting nerve, and this point-to-point propaga- 
tion of molecular upset is known as the transmission of excitatory or 
nervous impulse. If by any means the physiological activity of a 
portion of the nerve be enhanced, then excitation will pass through 
the particular portion with quickened speed. Such favorable condi- - 
tion is brought about by the application of moderate warmth. If a 
portion of nerve, on the other hand, be rendered physiologically slug- 
gish, then the speed of nervous impulse through that portion will be 
slowed down. There are certain agents which paralyze the nerve 
for the time being, causing a temporary arrest of the nervous impulse. 
Such agents are known as anesthetics. There may, again, be poison- 
ous drugs which destroy the conducting power. Under the action of 
such poisonous agents the nervous conduction is permanently abol- 
ished. 
We are now in a position to distinguish between mechanical and 
nervous transmission. ‘The mechanical conduction of water through 
a pipe will in no way be affected by warmth or cold; the pipe will not 
lose consciousness and stop the flow of watey, if 1t be made to inhale 
chloroform, nor will its conducting power be abolished by applying 
round it a bandage soaked in poison. These agents will, on the other 
hand, profoundly affect the transmission of excitation. The nature 
of an impulse may thus be discriminated by several crucial tests. 
If physiological changes affect the rate of conduction, then the 
impulse must be of a nervous character; absence of such effect, on the 
other hand, proves the mechanical character of the impulse. 
Of the various physiological tests, Pfeffer employed that of the nar- 
cotic drug. Chloroform applied on the surface of the stem of Mimosa 
