V4 STUDIES IN GENERAL PHYSIOLOGY 
1. Contact-irritability (stereotropism). 
a) In a series of Hydroids the root attaches itself to the 
surface of a solid body as soon as it comes in contact with it, 
and continues to grow over its surface, adhering to it as 
closely as possible. 
The stems of these Hydroids do not possess this irrita- 
bility. In Tubularia, in which the polyps are large enough 
to permit one to experiment upon them, it can be shown that 
they possess the opposite kind of contact-irritability. When 
brought in contact with the surface of solid bodies they bend 
away from it. By taking advantage of this irritability of 
the polyps, it is possible to bring about permanent (stereo- 
tropic) curvature in the Tubularian stem. 
b) Only the tip of a root which is growing attaches itself 
to the surface of a solid body. 
c) In order that the root may attach itself it is necessary 
that the contact stimulus should act for some time. 
d) It has already been mentioned that contact-irritability 
can cause the growth of roots in the middle of a stem—for 
example, in Eudendrium. 
e) When in a number of Hydroids stems arise from the 
roots which have become attached to a solid body, these new 
stems originate on that side of the roots which lies diametri- 
cally opposite the solid body. 
2. Heliotropism. 
a) In Sertularia (polyzonias?) the branches are positively, 
the roots negatively, heliotropic. Only the growing parts 
show any heliotropic curvatures, 
3. Geotropism (?). 
a) When possible, Cerianthus membranaceus always 
assumes a position in which its long axis is vertical, its oral 
pole upward, and its aboral pole downward. If the animal 
is placed in a different position, the foot tries to gain its nor- 
mal orientation by bending vertically downward. 
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