78 MOVEMENTS OF CURVATURE 



seems to indicate that the same mechanism is involved as in the pulvini of 

 Mimosa and the stamens of Cynareae. 



SECTION 17 (continued]. 



The mode in which the fall of turgor is produced in the cell-sap 1 is 

 uncertain and need not always be the same. The rapidity with which this 

 occurs affords little evidence as to its character, for a rapid fall of turgor can 

 be produced in various ways. The escape of water is the natural result of 

 the pressure exerted by the stretched cell- wall when allowed to contract, 

 combined with the permeability of the walls of the cells and tissues 

 concerned 2 . 



Hitherto no visible changes in the cells have been observed which 

 might throw light upon the stimulatory movement. Stimulation does not, 

 for instance, affect the protoplasmic streaming of the stamens of Cynareae, 

 whether the movement takes place or is mechanically prevented 3 . In case 

 any visible reactions should be detected, it would still remain to be deter- 

 mined whether they were directly connected with this stimulatory response 

 or were due to some simultaneously awakened activity. The protoplasmic 

 aggregations shown in stimulated cells of Drosera and Dionaea are partly 

 or entirely connected with the induced secretory activity. Changes in the 

 shape of the protoplast and in the position of the chloroplastids may be 

 produced without any change of turgor, and hence can hardly be responsible 

 for its induction 4 . The same is still the case even when stimulation causes 

 the protoplast to retract from the cell- wall 5 . 



. * For details see Pfeffer, I.e., 1890, p. 333. 



3 Cf. Pfeffer, 1. c., 1890, p. 329. Vines (Arb. d. hot. Inst. in Wtirzburg, 1878, Bd. II, p. 146) 

 and Gardiner (Annals of Botany, 1887-8, Vol. I, p. 366) assumed that an active contraction of the 

 protoplasm was responsible for the movement, without bringing any real arguments forward, and 

 without explaining how the high energy of contraction could be developed in this way. Pfeffer 

 has further shown that the fall of turgor is not produced by any active pumping action, and that the 

 escape of water is not the result of a local tearing in the protoplasm, such as occurs in many contract- 

 ing vacuoles. It hardly needs to be mentioned that so long as no exosmosis of dissolved materials 

 occurs, an increase in the permeability of the protoplasm or cell-wall cannot produce any fall of 

 lurgor. 



3 Pfeffer, Physiol. Unters., 1873, p. 138 ; Bot. Ztg., 1875, p. 290, footnote. 



* Borzi (L'apparato di moto delle sensitive, Rivista di Scienze Biologiche, 1899) does not pay 

 sufficient attention to the principles indicated here. The same applies to the studies of Chauveaud 

 (Compt. rend., 1894, T. cxix, p. 103) and Heckel on the stamens of Berberis. Cf. the criticism of 

 this work in the Bot. Ztg., 1875, p. 289, and 1876, p. 9. Heckel has, in fact, in part regarded the 

 appearances produced by plasmolysis or death as being the result of stimulation. 



5 Hitherto the changes in the electrical currents as well as in the production of heat have not 

 been used to throw light upon the phenomena of stimulation and response. Bert (Mem. de 1'Acad. de 

 Bordeaux, 1870, T. vm, p. 43 ; Compt. rend., 1889, T. LXIX, p. 895) observed by means of thermo- 

 electric needles that the pi imary pulvinus of Mimosa pudica is somewhat cooler than the petiole and 

 stem, and remains so in spite of the slight rise of temperature on stimulation. According to Kraus 

 (Wasservertheilung i. d. Pflanze, 1880, n, p. 68) the percentage of sugar increases in continually 



