PULSATIONS Of DESMODIUM GYRANS 295 



the ' up ' movement is executed somewhat slowly, and 

 comes to what may be called a pause at the extreme position 

 for a certain length of time. There is in reality no cessation 

 of movement ; but the rate becomes extremely slow at the 

 turning-point where ' up ' is reversed to ' down ' movement 

 and vice versa. After reaching the extreme ' up ' position the 

 ' down ' movement is commenced, and this is accomplished 

 in a much shorter time. After reaching the lowest position 

 there is again a pause, when the cycle is again repeated. 

 These normal movements and their rates are, however, 

 subject to modification under the influence of external 

 conditions. 



These movements of Desmodium leaflet are brought 

 about, as in the case of Mimosa, by the contraction or 

 expansion of the pulvinus. Here, also, it is the lower half 

 of the motile organ that is predominant in its action. A 

 question now arises as to the significance, whether of 

 contraction or relaxation, of the up and down movements. 

 The question may be settled in three different ways : The 

 contractile movement, generally speaking, is quicker ; 

 hence the quicker down movement of the leaflet may be 

 regarded as that due to contraction. Again, I have found 

 that a leaflet in a state of standstill exhibits under stimu- 

 lation an excitatory contractile movement which is down- 

 wards. And lastly, by means of internal hydrostatic 

 pressure, we may induce expansion of tissue. This has the 

 effect, as we shall see in a later chapter, of shifting the 

 pulsatory movements upwards. All these different con- 

 siderations point to the conclusion that in Desmodium the 

 ' down ' position of the leaflet represents contraction and 

 the ' up ' position denotes expansion of the more effective 

 lower half of the motile organ. The up-and-down move- 

 ments of the leaflet thus correspond to the diastolic and 

 systolic movements of the animal heart. In the records 

 of the pulsation of Desmodium, up-curve represents down 

 movement and vice versa. 



It would be interesting to know the absolute rate of 



