560 LECTURE XXI. 



being suddenly abolished, there is nothing to oppose the 

 pressure exercised by the turgid cells of the outer longi- 

 tudinal half, consequently the filament moves upwards and 

 inwards. In this case also there is evidence that the irritable 

 cells give up water on stimulation, for a drop of water ap- 

 pears at the cut surface of a filament, though, inasmuch as 

 there are no intercellular spaces it is not clear how exactly 

 the water travels. 



The mechanism of the movements of the leaf of Mimosa 

 pudica on stimulation is essentially the same as that of the 

 stamens of Berberis. With regard first to the movement of 

 the primary petiole. From his observation that when he 

 removed the lower half of the pulvinus the petiole remained 

 directed almost vertically downwards, Lindsay concluded 

 that the depression of the petiole on stimulation is due to 

 an increase of the pressure of the upper half. This conclu- 

 sion, which was held also by Dutrochet and by Burnett and 

 Mayo, was shewn by Briicke to be erroneous. It is clear 

 that if Lindsay's view be correct, stimulation of the pulvinus 

 must be accompanied by an increased turgidity of its cells, 

 and therefore also by an increased rigidity of the whole 

 organ. Briicke's observations, however, clearly prove that 

 this is not the case. He found, namely, that, on stimulation, 

 the rigidity of the pulvinus is markedly diminished, that 

 stimulation induces flaccidity and not turgidity, and that 

 therefore the fall of the petiole which follows on stimulation 

 of its pulvinus is due, not to an increase of the downward 

 pressure of the upper half of the organ, but to the abolition of 

 the turgidity, and with it the upward pressure, of the lower 

 half of the organ. 



Briicke's method of estimating the rigidity of the pulvini has been so 

 universally adopted in experiments with Mimosa that it is important to 

 describe it. The plant is placed in such a position that the petiole to be 

 observed is horizontal, and then the angle is measured which the petiole 

 makes with the internode below its insertion. The plant is then turned 

 upside down, the petiole under observation is again brought into a 

 horizontal position, and the angle which it now makes with the same 

 internode is measured. Inasmuch as the pulvinus is not perfectly rigid 

 the two angles are not the same ; the difference between them indicates 



