334 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



SECTION 71. The Action of Gravitational and Centrifugal Forces. 



Mechanical Actions. 



Small particles only respond slowly to the action of gravity in water, 

 and still less in more viscous liquids or ones of nearly the same density. 

 Hence very slight agitation suffices to keep particles suspended in a 

 liquid. Strong centrifugal forces may, however, effect rapid separation 

 such as gravity is unable to perform. Crystals and particles of precipitated 

 methyl blue 1 sink rapidly in the cell-sap, and only slowly, or not at all, 

 in the protoplasm according to its viscosity. The nuclei and chloroplastids, 

 although denser than the cytoplasm, as well as the vacuoles and oil-drops 

 which are less dense, do not appear to have their position in the cell 

 influenced by gravity so long as they are embedded in cytoplasm. The 

 latter, even when thin, does not collect on the under side of the cell, although 

 denser than the cell-sap. In streaming endoplasm of low viscosity, however, 

 a feeble gravitational accelerating or retarding action upon floating particles 

 of less and greater density can frequently be observed 2 , and the denser 

 particles come perceptibly nearer to the periphery on the under side of 

 the cells of Chara and Nitella 3 . When large crystals of calcium oxalate 

 or large starch-grains occur in the protoplasm they are commonly found 

 on the under side 4 , and the starch-grains carry with them the attached 

 chloroplastids or leucoplastids. The movements of the starch-grains are 

 shown very well in the starch-bearing endodermal cells, as well as in the cells 

 of the root-cap, and when inverted the change of position may begin at 

 favourable temperatures in a few minutes and be completed in from ten 

 to twenty minutes. At low temperatures the movement is slower 5 owing 

 to the higher viscosity of the protoplasm 6 . The starch-grains in the cells 

 of such an organ when rotated once every half-hour or hour on a klinostat 

 in a warm room will, as Dehnecke found, be in continual movement, and 

 the changes of position of the starch-grains are now considered to aid 

 largely in the perception of geotropic stimuli 7 . 



In cells subjected to strong centrifugal action starchless chloroplastids, 



1 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1886, Bd. n, p. 189 ; Zimmermann, Beitrage z. 

 Morphol. u. Physiol. d. Pflanzenzelle, 1893, p. 68. 



2 Ewart, Protoplasmic Streaming in Plants, 1903, p. 23. 



3 Nageli, Beitr. z. wiss. Bot., 1860, Heft ii, pp. 67-74. 



4 Dehnecke, Ueber nicht assimilirende Chlorophyllkorper, 1880, p. 10 ; Heine, Ber. d. bot. Ges., 

 1885, p. 189, Landw. Versuchsst., 1888, Bd. XXXV, p. 170. 



5 Nmec, 1. c., 1901, p. 129. 



6 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 16-20 ; W. Ostwald, Zool. Jahrb., 1903, 

 Bd. xvm, p. 3. 



Nemec, Jahrb. f. wiss. Bot, 1901, Bd. xxxvi, pp. 108, 127; Ber. d. bot. Ges., 1902, p. 342 ; 

 Haberlandt, Ber. d. bot. Ges., 1902, p. 190 ; Jahrb. f. wiss. Bot., 1903, Bd. xxxvin, p. 487. 



