THE MOVEMENTS A CONSEQUEXCE OF THE EXPULSION OF WATER. 649 



the parenchyma and the axial vascular strand, even in the stimulated organ which 

 has lost water, and that the tension in this condition is greater between the paren- 

 chyma of the upper side and the strand, than between that of the lower side and 

 the strand. 



If an organ prepared as above and still attached to the petiole is laid in water, 

 in order to replace the loss of water consequent on the operation, and thus to 

 produce a condition which approaches the normal one, the downward curva- 

 ture of the upper half becomes still more pronounced; but now the lower 

 half curves strongly upwards, and its tissue, previously flaccid, becomes very 

 tense and almost as hard as cartilage, as in the other half. This shows that 

 the turgescence of the parenchyma of the lower side was more diminished in the 

 operation causing the loss of water than that of the upper side, and that by 

 the re- absorption of water it increases in a higher degree than that does. In 

 other words, the irritable lower side gives up its water more easily than the 

 upper side, and takes it up again with greater energy ; the upper parenchyma is 

 alwa}'S tending to press down the axial strand, while the lower only tends to 

 curve it strongly upwards when it is very full of water. The latter is the case 

 with the parenchyma, however, only w'hen it is not irritated ; the stimulation 

 consists simply in the strongly turgescent parenchyma of the lower side expelling 

 water. 



Lindsay noticed a long time ago that the irritated side of a motile organ 

 becomes darker in colour; Pfeffer fastened the non-irritated petiole so that the 

 organ could not become curved on stimulation : on then touching a spot on the 

 irritable side he saw a darker colour extend with lightning-like rapidity from that 

 spot. He concludes from this that the air is driven from the intercellular spaces, 

 and replaced by water expelled from the stimulated parenchyma-cells, since the 

 darkening only appears to be thus explicable. 



Putting together all that has been mentioned, Pfeffer also eventually comes to 

 the conclusion already arrived at by Dutrochet and myself, that the propagation 

 of the stimulation in Mimosa is effected by means of the vascular bundles. So 

 long ago as 1865 I supposed the manner of this to be somewhat as follows. The 

 water contained in the irritable parenchyma as well as in the vascular strand and 

 in the wood of the shoot-axis, is to be regarded as a continuous mass, which, in 

 the non-irritated condition of the plant is in a state of relative rest ; every dis- 

 turbance of this equilibrium, every movement of a portion of this water, brings 

 about an expulsion of water, chiefly from the lower half of the motile organ, and 

 it is obvious that the movement of a leaf must necessarily effect a disturbance of the 

 equilibrium mentioned, and even over wide distances, and each organ entering into 

 the irritable condition must in its turn at once cause a new- disturbance of this 

 unstable equilibrium. 



Most of the other leaves which are irritable to shocks and vibrations are far 

 less sensitive than those of Mimosa pudica. In the case of the leaves of the 

 False Acacia {Robitu'a) and of the Wood Sorrel {Oxalis acetoselhi) really powerful 

 vibrations are needed to produce movements. However, so far as the mechanics 

 have been investigated in these cases, they depend in all essential points on 

 exactly the same processes as in I\fi??iosa. Among the organs most irritable 



