11 



tioally eliminated. The source of friction in such arrangements 

 arises from permanence of this contact. In this instrument, how- 

 ever, the writing lever is virtually free, except for the brief inter- 

 vals in which the smoked glass surface is brought into periodic 

 contact with it. For these records, the glass surface moves in a 

 vertical plane by means of clockwork, and a minute oscillation 

 to and fro is given to it by the agency of an electro-magnetic ar- 

 rangement. The period of this oscillation is, say, one fifth of a 

 second, and the record is thus made to consist of a series of dots, 

 separated by time-intervals of one fifth of a second. Thus we can 

 see the time-relations of the curve at a glance. 



For responsive movements of minute leaflets the speaker em- 

 ployed the optical lever. By use of this a very high magni- 

 fication is possible. The record is made on a traveling photo- 

 graphic plate by the spot of light reflected from the optical lever, 

 connected with the responding plant. 



For the instant detection of the effect of stimulus on the rate of 

 growth, the balanced crescograph is used. Here a balanced and 

 stationary point of light undergoes a sudden movement up or 

 down, according as the rate of growth is enhanced or depressed 

 by the action of an external agent. 



In order to study the effects of external agencies on physio- 

 logical excitability, it is first necessary to obtain a series of normal 

 responses under stimuli of uniform intensity and duration, applied 

 at regular predetermined intervals. This is accomplished by 

 means of the automatic stimulator, in which an expansible fan 

 periodically closes the exciting circuit. The intervals between 

 successive applications and the period of stimulation are, in this 

 instrument, capable of adjustment at will. 



In a complete curve of response of the sensitive leaf or leaflet 

 oi Mimosa or Biophyhini sensitivmn , we find (i) a short horizontal 

 line representing the latent period, (2) an up-curve showing attain- 

 ment of maximum reaction ; followed by (3) a down-curve repre- 

 senting the recovery. The latent period in a vigorous Mimosa is 

 about .24 of a second. The effect of fall of temperature or fatigue 

 results in the prolongation of this latent period to .3 of a second 

 in the former, and .58 in the latter case. The maximum fall of 



