HELIOTROPISM 



121 



behind the polyp in which the hehotropic curvatures occur possesses 

 contractility, and the forces underlying protoplasmic contraction are 

 responsible for the hehotropic curvature. The hehotropic curvature 

 consists here in the stem undergoing a stronger contraction or short- 

 ening on the more strongly illuminated side of the polyp than on the 

 opposite side. When the aquarium is turned by an angle of i8o° soon 



Figs. 20, 21. — Positive heliotropism of the polyps of Eudendrium. The new polyp-bearing stems 

 all grow in the direction of the rays of light which is indicated by an arrow in each figure. 

 (From nature.) 



after the curvature occurs, the stem turns and bends in the opposite 

 direction.* Sachs mentions that in the stems of plants also the heho- 

 tropic curvature can be again reversed, provided the experimenter does 

 not wait until the bent region of the stem has become too hard. The 

 hehotropic curvature in Eudendrium is therefore a phenomenon of 

 contractihty and not a phenomenon of growth, although growth may 

 accidentally occur at the same time. 



* These observations were made in 1895 at Woods Hole, and were mentioned briefly in 

 Pfiuger's Archiv, Vol. 63, p. 273, 1895. 



