§2] EFFECT ON STRUCTURE 113 



fastened, radially, the vessels containing the objects of experi- 

 mentation. When the rod revolves, all sides of the object 

 are brought successively and equally under the influence of 

 gravity's pull. By this means the directive action of gravity 

 is eliminated. 



In the case of organisms living in water, the effect of gravity 

 may be overcome by the buoyancy of the medium. It is clear 

 that an organism floating in a medium of its own weight can- 

 not be affected by gravity. This condition can be brought 

 about by increasing the specific gravity of water by adding 

 soluble substances such as gelatine and gum arable. Since 

 the specific gravity of the organism tends gradually to change 

 with that of the medium, this method does for rapid experi- 

 ments only. 



In dealing with larger organisms, which, like slugs, can keep 

 affixed to glass or other smooth surfaces, the inclination of the 

 surface may be varied from a vertical position to a horizontal 

 one, thus varying the active component of gravity. Finally, 

 gravity may be replaced by centrifugal force by rapidly rotating 

 either about a horizontal or a vertical axis. By varying the 

 rate of rotation the centrifugal force will vary, in accordance 



with the formula, /= — —, in which r is the rotating radius 



(in meters) and fi the square of the time of a rotation (in 

 seconds). This varying centrifugal force will act exactly in 

 the same way as gravity, only from the centre of rotation. 



§ 2. Effect of Gravity upon the Steuctuee of 

 Peotoplasm 



Very few observations have been made upon this subject, 

 and yet indications are not wanting that the field would well 

 repay working. Thus, where the cell contains specifically 

 heavier and lighter substances the two will be separated by 

 the action of gravity. This occurs in plant cells in which, 

 according to Dehnecke ('80), various contained bodies, e.g. 

 chlorophyll granules and starch grains, tend to sink to the 

 lower side of the cell. This result is produced in from a few 

 minutes to several hours. This effect is likewise seen in many 



