IRRITABILITY. 561 



the extent to which the pulvinus is flaccid, so that calling the angles 

 respectively a and a t , a x being greater than a, the flaccidity of the pulvi- 

 nus is represented by c^-a. If now the same measurements be made 

 after stimulation, it will be found that the difference between the two 

 angles, which we may now term /3 and /3 t respectively, is greater than 

 that between a and a l5 that is, that the flaccidity of the pulvinus has in- 

 creased. The following is an example of Briicke's observations. 



^=120 

 136 = 80 



a 1 -a= 14 i-= 40 



With regard to the nature of the changes induced in the 

 cells of the lower half of the pulvinus, Lindsay observed that, 

 on stimulation, this portion of the organ assumes a deeper 

 colour. Briicke suggested that this change of colour is due 

 to a replacement of the air in the intercellular spaces by 

 water which has been driven out of the irritable cells, a view 

 which is fully confirmed by Pfeffer who has further observed 

 that, on stimulating the pulvinus of a petiole which has been 

 cut off short, an escape of water at the cut surface may be 

 observed under appropriate conditions. Further evidence, if 

 any is needed, that currents of water are set in motion in the 

 pulvinus in consequence of stimulation, is afforded by the 

 electrical phenomena, described in a previous lecture (p. 324), 

 to which stimulation gives rise. There can be little doubt 

 that the positive variation of the normal current is due to 

 electrical disturbances set up by the travelling of the water 

 through the tissue. 



The mechanism of the induced movements of the primary 

 petiole of Mimosa is then this, that, on stimulation, the paren- 

 chymatous cells of the lower, irritable, half of the pulvinus 

 give up water, so that they become flaccid and no longer 

 exercise an upward pressure ; consequently the petiole sinks 

 down, partly under the downward pressure of the still rela- 

 tively turgid cells of the upper half of the pulvinus, and 

 partly under the weight of the secondary petioles and leaf- 

 lets. This is also, mutatis mutandis, the mechanism of the 

 induced movements of the secondary petioles and of the 

 leaflets, and probably not of these only but of all induced 

 V. 36 



