MECHANICAL ACTIONS 357 



SECTION 77. Mechanical Actions. 



Pressure exercises in the first place a purely mechanical action, but 

 if suddenly applied produces a certain shock-effect. As might be expected, 

 the movement of swarm -spores is much retarded in viscous media such 

 as solutions of gum-arabic or gelatine 1 , and ceases like the movements 

 of plasmodia 2 in solidified 2 to 5 per cent, gelatine, although Oscillaria 

 is still able to move slowly in this medium. 



Gravity and still more powerful centrifugal forces are able to produce 

 accumulations of the denser constituents at one end of the cell in a purely 

 mechanical manner. The protoplast, indeed, is able in virtue of its 

 plasticity to undergo very pronounced deformation or may even be broken 

 up into pieces without death ensuing. Deformations may result from 

 rapid changes of temperature, from the action of certain chemicals, from 

 severe pressure, as well as from the action of weak induction-shocks, which 

 are especially well adapted to produce localized effects 3 (Fig. 63). 



Streaming may continue in the internodal cells of Nitella^ and in 

 root-hairs of Hydrocharis 4 even when these are sharply bent, and similarly 

 plants may be strongly shaken without any pronounced effect on streaming. 

 In some cases, however, a transitory slowing or cessation of streaming 5 

 may result, and this may be followed by a temporary acceleration. Shaking 

 and all mechanical shocks produce a distinct effect if sufficiently intense 

 and suddenly applied, although all cells are not equally sensitive. Cells 

 in which the streaming is permanent or has been a long time aroused 

 (Chara, Nitella, Eloded) are always more sensitive than ones in which 

 the streaming is only temporary and has been recently excited by stimuli 

 (Elodea, Vallisneria) 6 . The plasmodia of Myxomycetes exhibit a tendency 

 to assume a rounded shape when subjected to mechanical stimuli, and 

 strong shaking as well as the action of electrical discharges causes a 



1 See Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, pp. 390, 420. 



3 Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 277. 



8 Kiihne, Unters. ii. das Protoplasma, 1864, pp. 74, 94; Klemm, Jahrb. f. wiss. Bot., 1895, 

 Bd. xxvni, p. 647, and the literature here given. On methods see these works and also Nageli u. 

 Schwendener, Mikroskop, 2. Aufl., 1877, p. 462 ; Zimmermann, Mikroskop, 1895, p. 231. 



4 Dutrochet, Ann. d. sci. nat, 1838, 2 e s^r., T. ix, p. 32 ; Meyen, Pflanzenphysiologie, 1838, 

 Bd. n, p. 210; Hofmeister, Pflanzenzelle, 1867, p. 50. On the influence of injuries on the direction 

 of streaming in Caulerpa, cf. Janse, Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 206, and in other plants, 

 Ewart, Protoplasmic Streaming in Plants, 1903, p. 34 seq. 



5 Dutrochet, 1. c., p. 32 ; Hofmeister, I.e., p. 50; Borscow, Bull, de l'Acad.de St. Petersbourg, 

 1868, T. xn, p. 213 ; Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, p. 217 ; Hermann, Studien 

 ii. d. Protoplasmastromung b. d. Characeen, 1898, p. 39; Rhumbler, Zeitschr. f. allgem. Physiol., 

 1902, Bd. I, p. 305. 



6 Ewart, 1. c., 1903, p. 72. The detailed action of momentum, impact, and of pressure-waves 

 is given here. 



