IRRITABILITY. 583 



stimuli (p. 474) in the case of roots. There is no doubt what- 

 ever that the stimulus of contact is transmitted in tendrils 

 (p. 487), and in irritable leaves, such as those of Mimosa 

 (p. 547), and Drosera (p. 548). We have now to enquire into 

 the means by which the transmission is effected. 



This subject has been more especially investigated in the 

 case of Mimosa pitdica. Dutrochet came to the conclusion 

 that the stimulus travels along the fibrovascular bundles. He 

 found, namely, that when he had removed either the cortex 

 or the pith, or both, of a portion of the stem, a stimulus was 

 transmitted from a leaf above the wound to one below it, or 

 from one below the wound to one above it, but that no such 

 transmission took place when the fibrovascular tissue was 

 cut through. In another experiment he removed the whole 

 of the parenchymatous tissue of the pulvinus of a leaf, 

 leaving only the fibrovascular bundle ; stimulation of this 

 leaf caused the other leaves on the stem to move. Clearly, 

 then, the fibrovascular tissue transmits the stimulus, but 

 we have yet to learn what the mode of transmission is. 

 Sachs considers that it is due to a disturbance of the 

 hydrostatic equilibrium in the plant. He regards the state 

 of equilibrium to be of this nature, that the parenchyma- 

 tous cells of the pulvinus are in a state of high tension so 

 that water would escape from them were it not prevented by 

 the counter hydrostatic pressure in the fibrovascular tissue. 

 In support of this view he cites the fact that when an incision 

 is made into the stem of a Mimosa, which extends to the 

 fibrovascular tissue, it is immediately followed by a fall of the 

 petiole of the nearest leaf. The incision has diminished the 

 hydrostatic pressure in the fibrovascular tissue ; consequently 

 the cells of the pulvinus of the nearest leaf give up water and 

 a movement ensues. When a movement of a leaf is induced 

 by stimulation, the converse of this takes place. The irritated 

 pulvinus forces water into the fibrovascular tissue, thereby 

 causing a disturbance of hydrostatic equilibrium which is pro- 

 pagated to adjacent pulvini. Though we cannot accept this 

 view of the intimate mechanism of the movement of a pulvi- 

 nus, yet it is possible to imagine that if two pulvini were 



