74 MOVEMENTS OF CURVATURE 



walls are not stretched to this extent, and hence a chloroformed filament 

 undergoes a considerable elastic elongation when weights are attached 

 to it. On the other hand, when a contracted filament is suddenly killed 

 by dropping it into boiling water, it undergoes an additional shortening 

 of 10 to 40 per cent, of its length, owing to the fact that the previous 

 stimulation caused a fall of turgor but not its entire removal. This 

 naturally applies only when the filament is highly irritable and before 

 the fall of turgor which precedes death has begun. A shortening corre- 

 sponding to that produced by excitation results from the action of an 

 injected solution of 0-5 to I per cent, potassium nitrate, which diminishes 

 the osmotic pressure in the cell by 1-7 to 3-5 atmospheres. 



These general considerations are not affected by the fact that the 

 realized movement of the filament results from the interaction of dissimilar 

 cells, for the association of the active cells with passive ones merely acts 

 like an increase in the thickness of the cell-wall and not only diminishes 

 the extent of the contraction produced by the available energy, but also 

 lessens the elastic stretching produced when turgor is restored. Presumably 

 not only the parenchyma, but also the epidermal cells and possibly also 

 the living cells of the vascular bundles are all active agents in producing 

 the contraction l . If this were not the case and only a limited number 

 of cells were active, we could hardly have so pronounced an energy of 

 contraction per unit area as is actually shown. The fact that the epidermis 

 and vascular bundles are under tension both in the contracted and uncon- 

 tracted conditions is the direct result of the fact that only a diminution 

 and not a removal of turgor is involved. In fact a further fall or an entire 

 removal of turgor causes a longitudinal compression of the vascular bundle, 

 and allows the walls of the parenchyma cells to show wavy bulgings 2 . 



The fall of turgor allows the stretched cell-wall to contract until the 

 decreasing tension of the wall is balanced by the internal osmotic pressure, 

 which rises somewhat as water escapes and the sap becomes consequently 

 more concentrated. A renewed production of osmotic materials causes 

 the extruded water to be again absorbed and the cell to be once more 

 distended and ready to respond to excitation. The mechanism can there- 

 fore be compared to an india-rubber tube in whose walls a spiral wire 

 is imbedded, so that on forcing in water under pressure the tube is 

 distended longitudinally but not transversely, and shortens when some 

 of the water is allowed to escape. The cell-walls do actually permit of 

 the rapid filtration through them of water under pressure, required to allow 

 sudden contraction. 



Since a perceptible diminution in the size of a cell can only be 



1 Pfeffer, 1. c., pp. 102, 1 12. An excitation is produced not only by touching the hairs, but also 

 the epidermal cells free from hairs. See also Haberlandt, Sinnesorgane im Pflanzenreich, 1901, p. 35. 



2 Pfeffer, I.e., p. 114. 



