CHAPTER XLVII 



RESPONSES TO GRAVITY 



287. Geotropism. We have already learned that gravity is a 

 constantly acting force, and that plant organs respond to the 

 direction of this force (see 



p. 37). Animals also have 

 occasion to adjust them- 

 selves to gravity, and they 

 do this in a variety of 

 ways. Some of the simple 

 marine animals are posi- 

 tively geotropic (or down- 

 ward swimming) under the 

 influence of light ; when 

 light is withdrawn (as at 

 night), they become nega- 

 tively geotropic, and swarm 

 to the surface of the water. 

 This agrees with the com- 

 mon observation of those 

 who are a great deal on 

 the water, that certain 

 animals can be found 

 near the surface only at 

 night. This is also an 

 example of a tropism reversed by changes in the protoplasm. 



288. Insects and gravity. The common house fly seems to 

 be indifferent to the direction of gravity ; it will crawl upon 

 a surface in any plane and in any direction, and will come to 

 rest in any possible position. 



241 



FIG. 102. Statolith of a snail 



A hollow structure filled with a fluid and con- 

 taining an unattached solid that is heavier than 

 the fluid ; as the position of the body changes, 

 the solid touches on different portions of the 

 sensitive lining, which is connected with nerves. 

 In some animals the lining of the statolith bears 

 delicate hairs 



