66 



TROPISMS 



Heliotropic curvature of sessile animals can be shown 

 equally well in a hydroid, Eudendrimn. It is necessary to 

 cut oft' the old polyps at once when the animal is brought 

 into the laboratory and to put the stem into fresh, clear, 



Fig. 26. — Polyps of Eudendrium, all growing toward source of light. The arrow indi- 

 cates the direction of the rays of light, which in one case fall in from above, in the other 

 from the left side. 



sea water. In a day or two new polyps are formed by 

 regeneration and these polyps will bend toward the light 

 until their axis of symmetry is in the direction of the rays 

 of light (Fig. 26). The region at the base of the polyps 

 is contractile and when light strikes the polyps from one 



