372 



THE MAINTENANCE OF THE INDIVIDUAL 



which centrifugal force was substituted for gravity. In this experi- 

 ment the roots grew outward while the stems grew toward the center 

 of the revolving disk, instead of assuming the normal geotropic 

 positions. 



Roots of Vicia faba with tips in glass slippers: at left, a, b, c, three stages 

 in the curvature of the same root, to 20 hours; at right, a, b, two stages of the 

 same root; h, 18 hours after being placed in position a. (After Czapek.) 



Experiments by Czapek, in which the tips of growing roots were 

 placed in glass slippers smaller than those used by Cinderella, show 

 that the region sensitive to the pull of gravity, "at least in certain 

 plants," is located in the last two millimeters of the root-tip. Recent 

 investigators have tried to account for this location of the response. 

 In animals, definite organs which "perceive " gravity are found. Such 

 are the otocysts of the crustaceans and the balancing organs (semi- 

 circular canals) of higher animals. In the crustaceans small but 

 relatively heavy particles, known as otoliths, give the animal its sense 



.y.doc;>3C 



sensory 

 iMY' "hairxs 



part of an 

 antsnnule 



sand 



otolit'bs— .„ ^, >, 



enlarged view 

 of otocysts 



Balancing organs of a crustacean. How do they function.^ 



