72 MOVEMENTS OF CURVATURE 



respond to mechanical stimuli. Actual experiment is required, however, 

 in each case before any final conclusion can be made, since similar move- 

 ments may be produced in various ways. Although the movement of 

 the leaf of Dionaea appears to be accompanied by growth, it is nevertheless 

 possible that the cell-mechanism may be the same as in the irritable stamens 

 of Cynareae and in the pulvini of Mimosa pudica. It is indeed possible 

 that every movement of the young pulvinus may be accompanied by 

 growth-changes, whereas when adult pure movements of variation may 

 take place. As was shown by Pfeffer \ the movements both of the leaves 

 of Mimosa pudica and of the stamens of Cynareae result from the fact 

 that stimulation induces a sudden fall of turgor, and hence a sudden 

 equilibration of the elastic stresses in the motile organ, which are gradually 

 reproduced as the original turgor is restored. The phenomenon can best 

 be followed in the stamens of Cynareae, of which those of Ccntaurea jacea 

 shorten by 10 to 30 per cent., and those of Cynara scolymus by 8 to 20 

 per cent, of their length when stimulated by a touch. The whole length 

 of the filament takes an equal part in this contraction, with the exception 

 of the two extremities where less shortening is shown. An isolated stamen 

 remains capable of contraction, and when stimulated performs lateral curva- 

 tures or convolutions. 



The construction of the filament from longitudinal rows of cylindrical 

 cells symmetrically disposed around the central vascular bundles results 

 in a close correspondence between the degree of contraction of the individual 

 cells and of the whole filament. Direct measurements have established 

 the fact that the epidermal and neighbouring parenchyma cells do actually 

 shorten, but retain their original transverse diameter and experience no 

 lateral curvature. The fall of turgor in the cells by lessening the tangential 

 stretching compensates for and prevents the broadening which would other- 

 wise result from the shortening of the cell 2 . Hence when the filament 

 shortens by 20 per cent, of its length, the individual cells also become one- 

 fifth shorter and hence correspondingly decrease in volume. This involves 

 an escape of water from the cells into the intercellular spaces, the displaced 

 air streaming away through the communicating intercellular spaces so 

 that its compression is avoided. If the filament is injected with water, 

 a drop of liquid exudes from the cut end when the stamen shortens on 

 excitation, although the shortening is less than before. This water appears 

 to escape from the intercellular spaces of the parenchyma, and hence it 

 is easy to understand how the stamens of Centanrea jacea and of Cynara 

 scolymus are able to shorten without increasing in diameter 3 . It is, 



1 Pfeffer, Physiol. Unters., 1873; Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 325. 



2 Pfeffer, 1. c., 1873, p. 96. 



3 Pfeffer, 1. c., 1873, p. 89. The methods of measurement of other authors and a criticism 

 of them is given in this work. The matter is in no wise altered by the apparently somewhat 



