174 STUDIES IN GENERAL PHYSIOLOGY 



1. Contact-irritability (stereotropism). 



) In a series of Hydroids the root attaches itself to the 

 surface of a solid body as soon as it conies 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. 



h) 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. 



<l) It has already been mentioned that contact-irritability 

 can cause the growth of roots in the middle of a stem for 

 example, in Eudendrium. 



c) 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. 



