§2] 



GEOTROPISM 



401 



Ions are markedly negatively geotropic. As the following 

 figure shows (Fig. 113), it is only the newly growing part 

 which exhibits this geotropism. These geotropic movements 



Fig. 113. -^ Diagrams of a Sertularella stock, showing how the direction of growth 

 is determined by gravity. The arrow points towards the earth's centre. The stock 

 originally placed in the position a is subsequently reversed as at 6, and again as 

 at d. The asterisk indicates a hydranth. (From Dkiesch, '92.) 



in animals, which occur near the growing apex, are clearly due 

 to unequal growth on the two sides of the inclined stem or 

 stolon. 



Many untouched questions on geotropism in animals arise 

 for solution — questions about the location of the perceiving 

 and the responding portions of the stem, latent period and 

 after effect, and others. Especially is it desirable to find how 

 wide-spread the geotropic phenomena are in sessile animals. 



/. After-effect in Geotropism. — When a root or stem is 

 placed horizontally, and retained in that position for a part of 

 its latent period, and then, before the curving has appeared, is 

 rotated on its long axis through 180°, the turning takes place 

 at about the time and towards the same side as it would had 

 the organ been left undisturbed ; in its new position the root 

 turns up, and the stem, for the moment, down. The response, 

 once set in motion, works itself out, until finally annihilated 

 by an opposing response (Hofmeistbr, '63 ; Ciesielski, '72). 



This experiment has been variously modified in ways which 

 throw light on the meaning of the after-effect. It has repeat- 

 edly been observed that if the root tip be decapitated before 

 2d 



