that tardily and feebly. There can be no doubt that it is the sensitive cells of 

 the under side that extrude this liquid, which then finds its way into the inter- 

 cellular spaces and from them to the cut surface. 



As the air in the intercellular spaces becomes compressed by the excreted 

 liquid, another phenomenon makes its appearance which was first observed by 

 LINDSAY and, later, correctly interpreted by BRUCKE. At the moment of 

 stimulation the under side of the pulvinus assumes a darker colour, such as 

 follows upon injection of water under the air-pump. This dark coloration takes 

 place even although the articulation be mechanically prevented from curving 

 (PFEFFER, 1873 a), and it follows that the change in colour is not caused by the 

 contraction of the pulvinus and consequent approximation of the chloroplasts. 

 The sudden reduction in the expansive power of the lower half of the pulvinus 

 is obviously associated with an alteration in its turgidity, and hence arise two 

 possible explanations of the phenomenon (compare PFEFFER, 1890, 326) ; either 

 the elasticity of the membrane increases, or the pressure in the interior dimin- 

 ishes ; either phenomenon would result in the excretion of water from the cells. 

 Since, if there be no alteration in the pressure of the cell contents, the size of the 

 cell depends on the elasticity of the cell-wall, obviously a contraction of the cell 

 might be produced by a decrease in the elasticity of the membrane. This possi- 

 bility is not precluded in the case of Mimosa, but since it is certainly not applic- 

 able to the very similar movements exhibited by the stamens of the Cynareae 

 and, at the same time, is not very probable in itself, we need not consider it 

 further. When the pressure in the cell decreases we naturally assume this to 

 be due to a decreasing osmotic pressure, a decrease which may well amount to 

 2 to 5 atmospheres, and may be due either to the transformation of osmotic- 

 ally active substances into bodies with larger molecules, or to alterations in 

 the permeability of the plasma, and an excretion of materials from the cell. As 

 evidence of the excretion of material we may quote the fact that PFEFFER 

 (1873 a) observed crystals of unknown nature appearing on evaporation of the 

 liquid expressed from the intercellular spaces. Still there are several reasons 

 for doubting this conclusion. It is a remarkable fact that plasmolytic research 

 (HILBURG, 1881) affords no evidence of any decrease in osmotic pressure (com- 

 pare p. 506). Further it is not very probable that the original turgor relation- 

 ships would be at once reinstated by cutting the articulation. 



As PFEFFER has suggested, it is possible that a decrease of the pressure as 

 affecting the membrane may also be induced by alterations in the protoplasm, 

 more especially alterations in its capacity for swelling. 



No complete insight into the mechanism of the stimulus movement in 

 Mimosa has as yet been obtained, although one thing is certain, viz., that there is 

 a decrease in expansive power on the under side of the articulation. Expansion on 

 the upper side arises only from the removal of the opposing pressure below : 

 at the same time the weight of the leaf helps to squeeze the under side. 

 Again, when the plant is placed horizontally or inverted, that is, when the 

 weight of the leaf is rendered non-effective, contraction of the sensitive half of 

 the pulvinus in response to stimulus still takes place, showing that the weight of 

 the leaf is not necessary for performance of the movement. 



The articulations at the bases of the secondary petioles and of the leaflets 

 behave, so far as we know, in a precisely similar way to that just described for 

 the pulvinus of the chief petiole. 



The depression of the chief petiole takes place, as has been already pointed 

 out, with considerable rapidity. BERT (1870) found that, in a plant laid hori- 

 zontally, the movement was completed in 4-7 seconds, and that it was more 

 rapid still when the weight of the leaf was allowed to operate at the same time. 

 As soon as the petiole had reached the position of maximum depression it began 

 again to raise itself, and in the course of 10-15 minutes the original position was 

 again attained, so that the leaf became once more fully capable of responding 



L 1 2 



