MOVEMENTS PRODUCED BY MECHANICAL STIMULI 



73 



however, always possible that the contraction of other stamens may involve 

 a decrease or increase in diameter. These facts, together with the absence 

 of any active contraction of the protoplasts, and of any transitory increase 

 in the elasticity of the stretched cell-walls, suffice to show that the shortening 

 is due to a fall of turgor, and the subsequent re-expansion to its gradual 

 restoration. The energy of contraction as determined by comparing the 

 maximal load with the area of cross-section of the filament amounts to 

 as much as i or 3 atmospheres. Hence it cannot possibly be produced 

 by an active contraction of a viscous fluid like the protoplasm l , and the 

 diameters of the cells are too 

 great to enable changes in the 

 peripheral surface tension to have 

 much effect. 



The filament when con- 

 tracted possesses the same elas- 

 ticity as when expanded and 

 rendered non-irritable by chloro- 

 form. Further, the same weight 

 which is required to stretch a 

 contracted filament to its original 

 length also suffices to prevent 

 any contraction. Hence it is 

 obvious that no increase in the 

 elasticity of the cell-walls occurs 

 during contraction, although by 

 raising the pressure exerted by 

 the cell-wall against the internal 

 osmotic pressure this might pro- 

 duce a contraction of the cell ac- 

 companied by an outward filtration of water under pressure 2 . It is evident 

 therefore that changes in the osmotic pressure are solely responsible for 

 the contraction, although, since these are only temporary in character, they 

 cannot be detected by plasmolytic methods 3 .' 



The reason for the pronounced contraction resulting from a fall of 

 turgor lies in the fact that the cell-walls are as extensible as india-rubber, 

 and when not under any permanent tension can be stretched to double 

 their length without their limit of elasticity being passed, that is, without 

 undergoing any permanent stretching. Even when fully turgid the cell- 



FIG. 26. A portion of the longitudinal half of a filament of 

 Centaurca montaiia (magnified), g = vascular bundle, p = 

 parenchyma, e = epidermis, i = intercellular spaces, h = hairs. 



careless experiments of Schenkemeyer, Ueber die Contraction der Filamente von Centaurea, Breslauer 

 Dissertation, 1877. 



1 Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, pp. 326, 329. 



2 Pfeffer, Physiol. Unters., 1873, pp. no, 117 ; also 1. c., 1890, p. 327. 



3 Pfeffer, 1. c., 1890, p 327. 



