i8 



PLANT RESPONSE 



^ \A 



ft i\ ft fi !| 



The period of the complete vibration was in this case 

 3"5 minutes, of which the down movement was accom- 

 pHshed in the course of I'S minute. The up movement 

 was relatively slower, and was accomplished in two minutes. 

 The mean amplitude of pulsation — that is to say, the 

 vertical distance travelled by the tip of the leaf — was 

 25 mm. The fact that the down movement is, generally 

 speaking, relatively the quicker is seen visually demonstrated 

 in the photographic record of uniform pulsations obtained 



with another specimen 

 (fig. 127). In connection 

 with this, certain pecu- 

 liarities of photographic 

 action should be borne 

 in mind. It is found 

 that a short exposure 

 gives an image which 

 in the case of a line is 

 very thin and sometimes 

 consists of only the 

 faintest impression; but 

 when the exposure is 

 prolonged the line is 

 much thickened. Hence, 

 in the records, the faitit or more sharply outlined portions 

 indicate responsive down movements which were relatively 

 rapid. In fig. 127, therefore, these differences of line afford 

 us a graphic representation of the various rates of movement, 

 and durations of pause, in the different parts of the curve. In 

 all the photographic records given here and elsewhere, we 

 are thus able to distinguish the down mov^ements by the 

 relative thinness of the recording line. In fig. 134, at the 

 end of this chapter, will be found a spark-record of the 

 pulsation of Desmodiuin in which the different rates of move- 

 ment at different stages of the response can be distinguished 

 at a glance. 



In the multiple responses o'i BiopJiytuiii^ though, generally 



i y y y ii y i y y ^ 



Fk 



>7. Photographic Record of Uniform 

 Pulsations in Desmodiiim 

 Period of each'pulsation = 27 minutes. 



