126 STUDIES IN GENERAL PHYSIOLOGY 



as do geotropic or heliotropic curvatures in many growing 

 plants. To bring about this stereotropic curvature it is 

 necessary that the / ><>!///> itxelf should come in contact with 

 the solid body. If any part of the stem alone comes in con- 

 tact with the solid, 110 bending occurs, even though the 



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growing part of the stem, close to the polyp, touches the 

 solid. The contact-irritability of the polyp is opposite in 

 kind to that of the stem ; the stem is positively, the polyp is 

 negatively, stereotropic. 



The negative stereotropism of the polyp may be clearly 

 demonstrated iu the following simple manner: Beheaded 

 Tubularians \vrre fixed in a beaker half-filled with sand in 

 such a way that one end was fixed in the sand, while the 

 other end just touched the side of the vessel. As soon as 

 tin- new polyps were formed and the Hydroids began to 

 grow in length, the tips of all the stems bent away from the 

 glass sides of the vessel. The direction of the rays of light 

 had no effect upon this process. 



In all these e.i-()erhnents the polyps formed at tJie aboral 

 end helm ml < -.racily like- those formed <it lite oral end. 



2. I have not succeeded in bringing about either helio- 

 tropic or geotropic curvatures in Tubularia mesembryanthe- 

 iimrn. When I fastened the stem in the middle, and when 

 both ends were surrounded by sea-water on all sides, the 

 stem of the bioral Tubularian continued to grow in the 

 direction of the old piece; it mattered not whether it lay in 

 a vertical or in a horizontal position, or in which direction 

 the light struck it. This is a remarkable fact, for, in looking 

 at a colony of Tubularians, one might easily be led to 

 think that they possess heliotropic or geotropic irritability, 

 as the stems of such a colony upon the surface of a solid are 

 all arranged in the same way. Yet the similarity in the 

 orientation might be determined in the main by their 

 contact-irritability. The oral ends of the young stems 



