vi ELECTROMOTIVE ACTION IN VEGETABLE CELLS 15 



Haberlandt localises this process in certain funnel-shaped cells, 

 situated in the leptoma of the vascular bundle (soft bast) ; the 

 structure of these cells is so far remarkable that each transverse 

 wall bears a single large pit, closed by a porous membrane, and 

 traversed by plasma threads from the adjacent cells. Although 

 these " conducting " cells are in juxtaposition with the ring of 

 collenchyma which surrounds the central bundle of the pulvinus 

 (the cells of which are again connected by plasma bridges with 

 those of the conducting parenchyma), Haberlandt refuses to 

 admit any direct connection between the conducting cells and 

 the collenchyma. There must thus be two systems of cells, 

 functionally co-ordinated, but not in direct conducting, i.e. plas- 

 matic, connection. 



Haberlandt's theory assumes a very high hydrostatic pressure 

 of sap in the intact conducting cells of the leptoma, which gives 

 elastic tension to the longitudinal walls of the conducting cells ; 

 the resulting wall tension represents the immediate source of 

 pressure which, on injury to the conducting system, causes a 

 movement of the sap towards the seat of the sudden diminution 

 of pressure. Clearly this can only be possible on the supposition 

 of a filtration of sap from within the cell, through the intact 

 cross-walls of the adjacent cells. And this entails the further 

 and somewhat improbable assumption that the plasmatic layer 

 covering the pit must invariably be permeable, in a high degree ; 

 for thus only is it possible that the conducting cells should 

 act as a system of fused and communicating hollow spaces. 



The next point is the mode in which, under the above 

 presumptions, the flow of water within these funnel-cells can act 

 as a stimulus upon the excitable parenchyma of the pulvinus. 



Haberlandt refers the propagation of the stimulus entirely to 

 alterations of form and volume in the conducting tissue or excitable 

 parenchyma, correlative with variations of pressure. " When, in 

 consequence of injury to stem or petiole, there is a sudden fall of 

 turgor from the adjustment of differences of pressure in the con- 

 ducting cells bordering on the collenchyma ring of a joint, the 

 contracting walls of these cells (which are diminishing in diameter) 

 exert a powerful traction on the adjacent collenchyma. Owing 

 to the pliability of the latter this tug is easily transmitted through 

 the ring (which is 23 layers deep) to the most internal 

 layer of the excitable parenchyma. If the mechanical force of 



