ACTION OF GRAVITATIONAL AND CENTRIFUGAL FORCES 335 



the nucleus and the greater part of the cytoplasm when a vacuole is 

 present, are driven to the outer end of the cell, only a very thin peripheral 

 film and fine threads remaining at the other end l (Fig. 56). The oil- 

 drops in chloroplastids of Vaucheria move with the chloroplastids, whereas 

 free drops or masses of oil move in the opposite direction. Displacements 

 of this kind were found by Mottier to be produced in most plant-cells by 

 centrifugal forces 1,900 times more powerful than that of gravity after expo- 

 sures of from half an hour to several hours. In a few thin filamentous Algae, 

 however, only slight changes of configuration were shown, probably owing 

 to the small diameter of the cell coupled with the properties and mode of 

 arrangement of the cell-contents 2 . Similarly, owing to the packing of the 

 cell with grains of starch and aleurone, Andrews only observed slight dis- 

 placements in the cells of turgid cotyledons of Vicia 

 sativa and Pisum when exposed to a centrifugal force 

 of 4,400 g., whereas a pronounced displacement was 

 shown as soon as a portion of the reserve- materials had 

 been consumed. Usually, no apparent effect is pro- 

 duced by centrifugal forces of 100 g. strength, but in 

 a few cells relatively feeble gravitational forces may 

 produce pronounced displacement. The fact that a thin 

 film always remained adherent to the cell- wall is ex- 

 plained by the increased degree of cohesion as the 

 membrane becomes thinner, and by the fact that in 

 short cells the centrifugal action is unable to overcome c %y%/a 

 the osmotic pressure which keeps the plasmatic mem- ^S?32io?S 

 brane pressed against the cell-wall. Similarly, in thin jS[^ > dKio?fiS 

 threads the surface-tension pressure becomes so great n^st^the'^opltm 

 as to render them relatively rigid. ftSS^S'SZZS. 



The tearing away of the chloroplastids acts in- ofe) 700 ' (After An - 

 juriously and even fatally upon Characeae, but Mottier 

 found that other plants remained living. This applies even to the cells 

 of Spirogyra in which the chlorophyll-band had been driven to one end 

 by centrifugal action. The original positions are restored in less than half 

 an hour in cells showing active streaming, whereas the original configuration 

 is not entirely resumed until after a few days in the case of Spirogyra^ 

 and after one or more weeks in the case of Cladophora. The restoration 

 was found to be still slower by Andrews, in cells of the cotyledons of 

 Helianthus and Cucurbita, filled with reserve-materials, if growth was pre- 



1 Mottier, Annals of Botany, 1899, v l- XIII > P 3 2 5J Andrews, Jahrb. f. wiss. Bot., 1902, Bd. 

 xxxvui, p. i ; Miehe, Flora, 1901, p. 109. 



8 On the influence of these factors upon the resistance to movement within cells cf. Ewart, 

 Protoplasmic Streaming in Plants, 1903, pp. 16-33. 



