CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 319 



speaking, the down movement is more rapid, yet at times the 

 up and down movements approximate to each other in this 

 respect. I have occasionally found similar instances in the 

 pulsatory movement of Desmodium, when there would be 

 only a slight difference between the rates of the up and down 

 movements. The normal rates again may even become 

 reversed under the influence of external agencies. 



Responsive significance of up and down movements. — 

 A question of some difficulty and importance arises here as 

 to the significance, whether of contraction or relaxation, of 

 these up and down movements. We saw in the cases of 

 Mimosa and Biophytum that the down movement was due 

 to the relatively greater contraction of the lower half of the 

 pulvinus, and the up movement to recovery or relaxation. 

 It must be borne in mind that the upper half of the pulvinus 

 of Mimosa, like the lower, is also excitable, though in a minor 

 degree, as will be shown in a subsequent chapter. We have 

 further seen that the more excitable half responds earlier to 

 stimulus. The depression of the leaf, then, is due to the 

 earlier and greater contraction of the lower half of the organ ; 

 and its subsequent erection, to a natural expansive recovery, 

 possibly aided by the later and feebler contraction of the 

 upper half of the pulvinus. Arguing from analogy, we may- 

 regard the movement of the Desmodium leaflet as essentially 

 similar to this. For here too we find, generally speaking, 

 that the down movement is the more energetic, and the up 

 movement relatively the slower, of the two. Hence we 

 may infer that in Desmodium the down movement is due 

 to contraction and the up movement to relaxation. 



Test by increased internal hydrostatic pressure. — It 

 will be shown on page 349 that when Desmodium in a sub- 

 tonic condition undergoes arrest of pulsation, an increase of 

 internal hydrostatic pressure is found to renew the rhythmic 

 activity. With fairly high internal pressure, the frequency of 

 the pulsation is increased, though the amplitude is decreased. 

 In order now to test the responsive significance of the up and 

 down movements respectively, I tried the effect of an increase 



