Februaky 19, 1909] 



SCIENCE 



319 



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 arrangement. The perio of this oscilla- 

 tion is, say, one fifth of a second, and the record 

 is thus made to consist of a series of dots, sepa- 

 rated 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 employed the optical lever. By use of 

 this a very high magnification is possible. The 

 record is made on a traveling photographic plate 

 by the spot of light reflected from the optical 

 lever, connected with the responding plant. 



For the instant detection of the effect of stimu- 

 lus on the rate of growth, the balanced cresco- 

 graph 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 physiological excitability, it is first necessary 

 to obtain a series of normal responses under 

 stimuli of uniform intensity and duration, ap- 

 plied at regular predetermined intervals. This 

 is accomplished by means of the automatic stimu- 

 lator, in which an expansible fan periodically 

 closes the exciting circuit. The intervals between 

 successive applications and the period of stimula- 

 tion are, in this instrument, capable of adjustment 

 at will. 



In a complete curve of response of the sensitive 

 leaf or leaflet of Mimosa or Biophytwm sensitivum, 

 we find (1) a short horizontal line representing 

 the latent period, (2) an up-curve shovring attain- 

 ment of maximum reaction; followed by (3) a 

 down-curve representing the recovery. The latent 

 period in a vigorous Mimosa is about .24 second. 

 The effect of fall of temperature or fatigue re- 

 sults 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 the leaf was attained 

 in 1.5 seconds. Complete recovery takes place in 

 6 minutes in summer and in 18 minutes in winter. 

 In a leaflet of Biophytum, the maximum fall is at- 

 tained in .5 of a second and full recovery is 

 reached in 3 minutes. The excitatory fall of the 

 leaf takes place when stimulus is applied at or 

 near the responding point. Seen from different 

 points of view, this reaction will appear as a 

 diminution of turgor in the pulvinus, constituting 

 a negative turgidity-variation, or a shortening or 

 contraction of the more excitable lower half of the 

 pulvinus. Electrically speaking, this reaction will 



have its concomitant in an electrical variation of 

 galvanometric negativity. It is convenient to in- 

 clude all these excitatory symptoms together 

 under the single term negative response. Here, 

 however, we may describe a responsive change of 

 precisely opposite character, which takes place 

 under definite conditions. This positive response 

 consists of an erectile movement of the leaf; a 

 positive turgidity-variation, expansion, and an 

 electrical change of galvanometric positivity. The 

 occurrence of this positive response may be de- 

 monstrated, in Mimosa, by applying stimulus at a 

 point distant from the responding organ. In a 

 certain experiment, this positive or erectile re- 

 sponse occurred .6 second after the application of 

 a stimulus, and was followed, 2.8 seconds later, by 

 the normal excitatory fall of the leaf. Here we 

 have a response which is diphasic, positive followed 

 by negative. When stimulus is moderate, and ap- 

 plied at a still greater distance, the response 

 evoked is positive alone. These facts obtain uni- 

 versally, and from them we derive the following 

 law of direct and indirect stimulation: The effect 

 at the responding-region of a strong stimulus 

 transmitted to a short distance, or through a good 

 conducting channel, is negative. The effect trans- 

 mitted to a great distance, or through a semi- 

 conducting channel, is positive. 



Responsive movements, like those of the " sensi- 

 tive " plants so-called, can be detected also in 

 ordinary plants. It will be noticed, in Mimosa, 

 that the responsive movement is made possible by 

 the unequal excitability of the upper and lower 

 halves of the pulvinus, the movement being deter- 

 mined by the greater shortening or contraction of 

 the lower. If now we take a hollow tubular organ 

 of some ordinary plant, say the peduncle of daffo- 

 dil, it is clear that the protected inner side of the 

 tube must be the more excitable. When this is cut 

 into the form of a spiral strip, and excited by 

 means of an electrical shock, we observe a respon- 

 sive movement to take place by curling, due to the 

 greater contraction of the inside of the strip. 

 This mechanical response is at its maximum at 

 that season which is optimum for the plant. 

 When the plant is killed, its response disappears. 



In Mimosa, under continuous stimulation, there 

 is a fatigue-reversal, the responsive fall being con- 

 verted into a movement of erection. The same 

 thing happens in the response of ordinary plants, 

 when the first contractile movement of the spiral, 

 for instance, is reversed, under continuous stimu- 

 lation, to an expansive uncurling. 



An important series of observations is that oj 

 the modification of response by the tonic condition 



