96 MOVEMENTS OF CURVATURE 



(sudden pressure, or the impact of a falling body) by a temporary stoppage of stream- 

 ing, and the stimulus may be transferred to a neighbouring cell by a hydrostatic 

 pulsation in the cell-sap. The pulsation must, however, be a sharp one, and changes 

 of pressure produced in the same way as in Macdougal's experiment are ineffective 

 as stimuli even when high pressures are used. A hydrostatic impulse produced by 

 a blow upon a piston-rod does, however, produce a sufficiently intense wave to act as 

 a stimulus to the cell, and to be capable of propagation to the next one l . It is 

 evident, therefore, that this question needs further investigation before a definite con- 

 clusion can be made. It is in any case by no means improbable that other changes 

 besides the hydrostatic pulsation may co-operate in the transmission of stimuli in 

 Mimosa, and it hardly needs to be mentioned that the structure of the pulvinus affords 

 no evidence as to the means by which the stimulus is transferred to the motile cells. 

 The mere existence of inter-protoplasmic communications 2 does not indicate whether 

 these are of fundamental importance in a particular case, and the manner in which 

 stimuli travel from one part to another is dependent upon the course and connexions 

 of the vascular bundles, whether the stimuli travel in the phloem or in the xylem 3 .] 



Biophytum sensitivum also responds, according to Macdougal 4 , to stimuli travel- 

 ling through a dead portion of the leaf axis, although, according to Haberlandt 5 , this 

 is not the case. A peculiarity of the latter plant 6 -lies in the fact that the removal of 

 a leaflet acts as a sub-maximal stimulus to the pulvini of the remaining leaflets, and 

 this incomplete movement is repeated several times without any further stimulus being 

 applied. Since this periodicity might be produced in various ways, further research 

 is necessary to reveal its mode of origin. Under appropriate periodic stimulation 

 a periodic movement may be induced in the leaves of Mimosa pudtca, owing to the 

 gradual recovery or increase of excitability, but it does not follow that the periodic 

 movements of the leaflets of Biophytum are produced in a similar way. If we are 

 actually dealing in this case with a prolonged stimulatory action, it can hardly be due 

 to a temporary hydrostatic pulsation or movement of water. 



1 Ewart, Protoplasmic Streaming in Plants, 1903, p. 72. 



2 Haberlandt's statement (1890, 1. c., p. 25) that no inter protoplasmic communications exist 

 between the tannin-sacs, and between the collenchyma cells is incorrect according to Kienitz-Gerloff 

 (Bot. Ztg., 1891, p. 25), but the positive statement of this author may be accepted with some caution. 



3 Cf. Pfeffer, 1. c., p. 318 ; Haberlandt, 1. c. 



4 1. c., p. 296. 



5 Haberlandt, Ann. du Jard. bot. de Buitenzorg, 1898, Suppl. II, p. 38. On Oxalis dendroides 

 cf. Macfarlane, Biological Lectures, 1894, p. 194. 



6 Haberlandt, 1. c., p. 35. 



