FURTHER INVESTIGATIONS ON HELIOTROPISM 103 



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1. I have endeavored to find other animals in which helio- 

 tropic curvatures are formed only in the growing parts. 

 These efforts have been successful in the Hydroids. Stems of 

 Sertularia (polyzonias ?) were cut off near the root and fixed 

 in the sand in an inverted position, so that the cut end was 

 directed upward. The stems were placed near a window 

 through which the light fell obliquely and from above. The 

 animals began to regenerate ; new polyp-bearing stems grew 

 from the cut end as well as new roots; 1 but while the new 

 stems grew upward and toward the window, the roots grew 

 downward and toward the room side. The polyp-bearing 

 shoots are positively, the roots negatively, heliotropic. That 

 the negatively heliotropic elements were true roots was 

 proved by the fact that when brought in contact with a solid 

 body they attached themselves to it and continued to grow 

 over its surface in close contact with it. They could be 

 loosened from their attachments only by force. The polyp- 

 bearing stems do not possess this kind of contact-irritability. 

 The heliotropic phenomena will be readily understood by the 

 aid of Figs. 13, 14, and 15 : ab is the old stem, b the cut 

 end ; the stem is fixed in the sand to the point ac. From the 

 cut end b arise newly formed roots W iy which bend down- 

 ward away from the light and toward the room side of the 

 aquarium. The new polyp-bearing shoots S grow upward 

 and toward the window. The arrow marks the direction of 

 the rays of diffuse daylight in this experiment. 



2. In these experiments new growths occasionally sprang 

 from the middle of the old stem, which, so far as their con- 

 tact irritability was concerned, reacted as roots. Those 

 tendrils which attached themselves to solid bodies were 

 always negatively heliotropic. They grew downward and 



1 Which is of importance in the theory of organization. 



