126 SruDIES IN GENERAL PHYSIOLOGY 
as do geotropic or heliotropic curvatures in many growing 
plants. To bring about this stereotropic curvature it is 
necessary that the polyp itself should come in contact with 
the solid body. If any part of the stem alone comes in con- 
tact with the solid, no bending occurs, even though the 
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 in the following simple manner: Beheaded 
Tubularians were 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 
the 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 experiments the polyps formed at the aboral 
end behaved exactly like those formed at the oral end. 
2. I have not succeeded in bringing about either helio- 
tropic or geotropic curvatures in Tubularia mesembryanthe- 
mum. 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 
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