46 PLANT RESPONSE 



former, or vacuum-pump, water is sucked away, or expelled 

 from the plant and its organs ; when, on the contrary, the 

 pressure is increased by connection with the force-pump, 

 water is forced in. From the curve given below (fig. 31), 

 it will be seen that the expulsion of water from the organ 

 actually causes the fall of the leaf, and that the forcing of 

 it back brings about erection. From the processes involved 

 in this artificial response and recovery, we can see clearly 

 how in the true response to stimulation we have a two-fold 

 process of (1) the expulsion of water caused by stimulus, 

 bringing about the depression of the leaf, and (2) the return 



Fig. 31. Artificial Hydraulic Response of Mimosa 



The plant was subjected to diminished pressure up to a, and to normal 

 pressure to b, after which the pressure was increased. The effect 

 of diminished pressure, in the depression of the leaf, continues for a 

 while. The ordinate represents movement of tip of leaf in cm., 



abscissa re-presents time. 



of water into the organ, bringing about the restoration of the 

 leaf to its original position, or recovery. And since the 

 lower half of the organ is the more contractile, it is evident that 

 in this lower half there must be relatively greater expulsion 

 and absorption during response and recovery. 1 



Two possible types of response. — We must bear in 

 mind that the entire response consists of these two alternating 

 processes, and that the recovery, or restoration of turgidity, 



1 As a normal type I have taken the pulvinus of Mimosa, the excitability of 

 the lower half of which is greater, and where the responsive movement is 

 down. Put there may be other cases. If the excitability of the upper half be 

 relatively the greater, excitation will in' such cases cause upward responsive 

 movement. In what follows, unless the contrary be staled, I shall speak of the 

 normal type. 



