IRRITABILITY. 569 



superficial resemblance to that which they take up when stimu- 

 lated mechanically, but Briicke's observations shew that the 

 condition of the pulvinus is entirely different in the two cases. 

 In the first place, when the petiole has sunk in the evening, it 

 is still irritable and will sink still lower on stimulation, and 

 secondly, the rigidity of the pulvinus is well-marked, whereas, 

 when the petiole has sunk in consequence of stimulation, it 

 will not sink lower on further stimulation, and the rigidity of 

 the pulvinus is much diminished. But we also learned in the 

 last lecture that the primary petiole has a daily periodic 

 movement, and we will now endeavour to understand the 

 mechanism of it. Briicke ascertained that the rigidity of the 

 pulvinus is less during the day than during the night, and it 

 might be concluded from this that the gradual fall of the 

 petiole during the afternoon is simply to be attributed to 

 the diminished rigidity, and the rise during the night to 

 the increased rigidity of the pulvinus. But Pfeffer's observa- 

 tions on the pulvinus of Phaseolus, and we may infer that 

 what is true in this case holds good in the case of Mimosa, 

 shew that this is not the case. He found, namely, that when 

 either the upper or the lower half of the pulvinus was re- 

 moved, the petiole rose during the night and sank during the 

 day-time. The significance of these facts is this, that, the fall 

 of the petiole during the day is due to the diminishing tur- 

 gidity of the cells of the lower half of the pulvinus and to 

 the increasing turgidity of those of the upper half; and con- 

 versely, that the rise of the petiole during the night is due 

 to the increasing turgidity of the cells of the lower half of 

 the pulvinus and to the diminishing turgidity of those of the 

 upper half. Inasmuch as the rigidity of the pulvinus is less 

 during the day-time than it is at night, we conclude that the 

 turgidity in the lower half of the pulvinus during the day is 

 less than the gain in the upper half, and conversely, that the 

 gain of turgidity in the lower half during the night is more 

 considerable than the loss in the upper half. Clearly, then, 

 the daily periodic movement of the petiole is not due to 

 uniform variations taking place simultaneously in all the 

 cells of the pulvinus, but to variations of an opposite nature 



