76 5 
Moto-excitability in Mimosa. 
for nearly fifteen seconds ; it then begins to recover slowly. As the succes- 
sive dots of the Oscillating Recorder are at intervals of a minute, the maxi- 
mum fall of leaf is accomplished between two successive dots. The dotted 
response record here obtained exhibits the recovery from maximum fall 
under stimulation (cf. Fig. n). The recovery of the leaf in one minute is 
less than one-tenth the total amplitude of the fall, and is proportionately 
the same in all the response records. Hence the successive amplitudes of 
response curves that are recorded at different hours of the day afford us 
measures of the relative variations of excitability of the plant at different 
times. This enables us to demonstrate the reality of diurnal variation of ex- 
citability. In my experimental investigations on the subject I have not been 
content to take my data from any particular method of obtaining response, 
but have employed both types of recorders, the Resonant and Oscillating. It 
will be shown that the results given by the different instruments are in 
complete agreement with each other. 
II. Effects of External Conditions on Excitability. 
Before giving the daily records exhibiting periodicity of excitability, 
I will give my experimental results on the influence of various external con- 
ditions in modifying excitability. The conditions which are likely to affect 
excitability and induce periodicity are, first, the effects of light and dark- 
ness : under natural conditions the plant is subjected in the morning to the 
changing condition from darkness to light ; then to the action of continued 
light during the day ; and in the evening to the changing condition from 
light to darkness. A second periodic factor is the change in the condition 
of turgidity, which is at its maximum in the morning, as evidenced by the 
characteristic erect position of the petiole. Finally, the plant in the course 
of day and night is subjected to a great variation of temperature. I will 
now describe the effects of these various factors on excitability. It should 
be mentioned here that the experiments were carried out about the middle 
of the day, when the excitability, generally speaking, is found to remain 
constant. 
(i) Effects of Light and Darkness. 
I have frequently noticed that a depression of excitability occurred 
when the sky was darkened by passing clouds. This is clearly seen in the 
following records obtained with the Resonant Recorder. Uniform sub- 
maximal stimuli had been applied to a specimen of Mimosa at intervals of 
fifteen minutes. The dotted up-line represents the responsive fall, and the 
continuous down-line, the slow recovery. The first four are the normal 
uniform responses (Fig. 4). The next three show the depressing effect of 
relative darkness due to cloudy weather. The sky cleared after forty-five 
minutes, and we notice the consequent restoration of normal excitability. 
