66 BIOLOGY: GENERAL AND MEDICAL 



plants whose stems twine about upright supports. The 

 lateral unequal growth is supposed to depend upon 

 geotropic stimulation of the cells in one lateral plane 

 with resulting horizontal curvatures. 



In all of these examples the geotropic influences are 

 manifested through the combined effects of minute 

 changes in many cells, the changes in the individual 

 cells being too slight to be recognized. 



Among animals, as has been pointed out, geotropic 

 reactions are less easy to define. With few exceptions, 

 however, all animals tend to maintain a definite position 

 with reference to the earth's surface and axis, which, of 

 course, is a form of geotropism. Plant-like animals are 

 best adapted to the purpose of demonstration and an 

 examination of the members of Ccelenterata, especially 

 the Hydrozoa and Actinozoa, reveal a number of forms 

 whose geotropic reactions are pronounced. 



These animals are characterized by a cylindrical body 

 with a stolon or foot at one end and a circle of tentacles 

 at the other. In general they assume a position with 

 the stolon down and the tentacles up. One of these 

 hydroids, Tubularia mesembryantheum, was experi- 

 mented upon by Allman who found that if the polyp 

 was cut from the stem, a new polyp regenerated; if the 

 stolon was cut from the other end a new stolon developed. 

 If both were amputated, a new polyp was reproduced 

 and a new stolon reproduced, but always at those ends 

 from which they had respectively been removed. 



Loeb endeavored to find out whether this was due to 

 the animal being, as Allman expressed it, polarized, and 

 discovered that when both extremities were amputated 

 and the oral or polyp end embedded in sand, while the 

 upward directed aboral or stolon end was surrounded 

 on all sides by water, a polyp instead of a stolon in- 

 variably developed at the aboral end. 



While not attributed to geotropic influences by Loeb, 

 it would seem as if this peculiarity might have some 

 reference to them. 



