EXPULSION OF WATER ON IRRITATION. 647 



motile organs are passed over, only to be put in motion later. If the plant is 

 left to itself, the leaflets and secondary petioles again expand after a few 

 minutes, the primary stalks become erect, and all the motile organs are then again 

 irritable. 



Just as a motile organ can be stimulated by cutting into a small leaflet, 

 or by burning it, so too it is possible, especially in the case of very turgescent, 

 and therefore highly irritable Mimosas, to stimulate the leaves from the inter- 

 nodes of the shoot-axis. Taking care that the shoot-axis is firmly fixed, and 

 then placing the edge of a very sharp razor carefully and without shaking 

 on the epidermis of the shoot-axis, and cutting gently into the succulent cortex 

 until the resistance shows that the razor is penetrating the wood, a drop of 

 water at once oozes out, especially if the razor is at once removed, and very 

 soon one of the neighbouring leaves (or even several of them) is set in motion 

 and relaxes. This experiment was known to the older vegetable physiologists, 

 Dutrochet and INIeyen, and is exceedingly instructive : it shows that the mere 

 movement of water within the tissue induces the position of the leaves produced 

 by irritation. This conclusion is the more certain, since the incision as far as the 

 wood only induces the effect described, when a drop of water wells out from the 

 wound : if the tissue is not sufficiently turgescent to exude water after the 

 wounding, no movement of the leaves in the neighbourhood is induced. 

 Evidently the incision of a leaflet with a pair of scissors, or burning it in the 

 focus of a lens, produces nothing more than a sudden movement of the water in 

 the tissues, which is propagated into the irritable organ, and induces the actions 

 yet to be described. In 1865, in my 'Handbook', I drew from these facts the 

 conclusion, that in the irritability of the Mimosse the only essential factor must be 

 the movement of water in the tissues, and corresponding changes in turgescence 

 in the motile organs.'. Further proofs of this, and a more exact insight into 

 the processes occurring on stimulation were obtained by Pfeff"er in 1872 ^ By 

 means of linear measurements on the organ, at first not stimulated and then 

 stimulated, he established in the first place that the volume of the lower parenchy- 

 matous half, which becomes concave in the curvature due to irritation, diminishes, 

 and that of the upper, as it elongates, increases ; but the increase in volume of the 

 upper half is much less than the decrease in volume of the lower one, whence it 

 follows that the whole motile organ becomes smaller — decreases in volume — as 

 it curves down in consequence of stimulation. From all the facts, it follows that 

 the decrease in volume of the lower parenchyma can only be due to the escape 

 of water from the tissue, and Pfeffer demonstrates this in the following manner, 



' I gave a comprehensive description of the irritable movements of the leaves ai Mimosa on the 

 basis of my own extended investigations and with reference to what was then known in my ' Expcri- 

 mcntal-physiologie' 1865 (p. 479), and there first laid stress on the fact that displacement of water 

 in the tissues is of importance in the matter. 



^ Very thorough investigations not only into the leaves of Mimosa and Oxalis, but also into the 

 stamens of the Cynarc(Z and Bcrhcris were given by Pfeffer in his ' P/tysiologischcti Uniersuchutigcn,'' 

 Leipzig (1873). With respect to his results mentioned in the text, I then gave a renewed 

 description of the phenomena of irritability generally in my ' Lc/ir/nnJi' IV Aufl., 1 874 (pp. 850-869), 

 and the preceding lecture contains essentially only an extract from that work ; to which I may refer 

 the reader as regards several points which can only be briefly touched on lu re. 



