212 



PROTOPLASM 



strands attached to the cell wall. This suggests that the con- 

 cavity and the numerous and persistent threads are more likely 

 due to greater tensile strength and greater adhesive qualities 

 of the protoplasm rather than to higher consistency. 



The Gravity Method. — A common commercial method for 



determining the viscosity of liquids is 

 that of allowing lead shot to fall through 

 a long column of the liquid. The time 

 of fall is taken and compared with that 

 obtained in a standard liquid such as 

 water. Timing the rise of lighter bodies 

 such as air bubbles does just as well. 

 The viscosity value obtained in this way 

 may be relative to water or absolute in 

 poises when it is calculated with the aid 

 of Stokes' law of falling bodies, which 

 is ^pressed by the formula 



V = 



2rKD - d)g 



97? 



where V is the velocity of fall; r, the 

 radius of the particle; D, the specific 

 gravity of the particle; d, the specific 



FXG.107.-Statoliths ^'^""'^^ «^ *^f ^^^^j^'"^' ^' *^^ g^^^i^^ 



(starch grains) resting at constant; and 7], the viscosity of the 



the bottom of plant cells. medium 



Nemec and Haberlandt used a similar method for measuring 

 the consistency of protoplasm. The living cell is itself made to 

 serve as a viscometer. Plants sometimes contain freely sus- 

 pended starch grains (statoliths), which normally lie at the 

 bottom of the cell and presumably serve as gravitational sense 

 organs (Fig. 107). If the cell is turned upside down, the starch 

 grains slowly fall. By exposing a living cell to view, repeatedly 

 reversing the plant, and noting the time of fall of the grains, an 

 indication of the consistency of the protoplasm can be obtained. 

 The viscosity value is calculated with the aid of Stokes' law. 



The Centrifuge Method.— Particles suspended in a liquid 

 can be thrown out by centrifuging if they are of a density differ- 

 ent from that of the medium. The centrifugal force required 



