648 LECTURE XXX VIT. 



and I have myself repeated the experiment several times. After cutting across the 

 motile organ at the bomidary of the petiole, where the axial strand is still undivided, 

 the organ is at first in the highest degree stimulated, and thus curves downwards. 

 If the plant is now left in an atmosphere saturated with moisture, under a large 

 glass bell-jar for instance, the motile organ thus freed from its leaf-stalk erects 

 itself again, and after a short time becomes again irritable : on now attentively 

 regarding the cut surface, and irritating the lower side of the organ by means of 

 a somewhat rough touch with a blunt needle, the organ curves downwards, and 

 a drop of water escapes immediately from the section. This water, as Pfeffer 

 showed, comes from the parenchyma itself, and almost exclusively out of that 

 which surrounds the axial strand and contains large intercellular spaces ; occa- 

 sionally, however, I have seen the transverse section of the strand itself become 

 moist. If the parenchyma of the upper side is removed from a motile organ, and 

 the rest of the organ makes a vigorous movement of irritation, it is occasionally 

 possible, according to the above observer, to see water escape from the horizontal 

 longitudinally cut surface of the motile organ. 



It is thus certainly established that in the movement of irritation water escapes 

 from the lower parenchyma. The small increment of volume of the upper paren- 

 chymatous half during the curvature above mentioned, shows moreover, that part of 

 the water enters into this tissue ; the decrease in volume of the whole organ, as 

 well as its becoming flaccid during the movement, as already mentioned, show just 

 as definitely that a portion of the water expelled from the lower parenchyma must 

 flow somewhere else, probably at the same time into the rigid tissue of the petiole 

 and into that of the shoot-axis ; probably a very small quantity also passes into 

 the axial strand of the organ. 



The importance of the subject may justify further reference to Fig. 369, 

 and to the changes in turgescence in the motile organs, especially the large 

 ones at the base of the petiole in the Mimosse. 



On cutting away the parenchyma of the upper side of the large motile 

 organ, as far as the axial strand, the petiole not only erects itself again subse- 

 quently, but it becomes in fact more erect than usual, and the organ operated upon 

 retains a certain degree of irritability. If, on the contrary, the parenchyma of the 

 lower side is removed, the petiole falls sharply downwards, and the organ operated 

 upon exhibits no further irritability. Thus it is the lower side only which is 

 irritable, the parenchyma of the upper side being only accessory in the movement. 



If one of the large motile organs is cut away close to the shoot-axis, 

 without being separated from its petiole, it becomes curved as usual, a drop of 

 water escaping from it at the same time. If it is then split by a longitudinal section 

 which divides the axial strand into an upper and a lower half, the former curves 

 still more strongly downwards, but the lower one becomes almost straight or 

 only a little bent downwards. If, further, the upper and lower parenchyma are 

 separated from the axial strand by means of two longitudinal sections, the former 

 becomes curved strongly downwards and the latter a little upwards, both 

 elongating at the same time, so that they project considerably beyond the axial 

 strand. 



These and other experiments show that considerable tension exists between 



