FURTHER INVESTIGATIONS ON HELIOTROPISM 105 



unable to determine accurately. The Sertularia cultivated 

 in a dark room ceased to grow, though I question whether 

 this was entirely due to lack of light. 



4. Light (and perhaps gravity) influences not only the 

 orientation, but also the position of the newly formed organs. 

 I have observed, and not in the case of Sertularia only, that 

 the new polyp-bearing branches always arise from the upper 

 surface of the stem. In Fig. 15, a new stem $ springs from 

 the upper side (the side directed toward the source of light) 

 of the stolon W^ I do not desire to discuss these points 

 more minutely here, as they will form the basis of a paper 

 which is to appear soon, on the form of animals. 



The experiments on Sertularia described here serve only 

 to complete the general consideration of animal heliotropism 

 and to show more fully the identity of animal and plant 

 heliotropism. The special investigation of the heliotropic 

 behavior of Hydroids is to be the subject of future study. 

 That this is both an interesting and a fruitful field 

 is shown by the beautiful work of Hans Driesch, which has 

 just appeared, on the "Heliotropism of Hydroids." 1 Driesch 

 arrives at the following result : 



The stolons which are produced instead of polyps under 

 unfavorable conditions in Sertularella polyzonias, are with the 

 exception of the first, which is turned away from the light from the 

 very beginning, all positively heliotropic at first, becoming nega- 

 tively heliotropic after the growth of the daughter-stolons. They 

 arise from the side of the mother-stolon, which is turned toward 

 the light. (P. 152.) 



This observation of Driesch agrees very well with mine. I 

 shall return to them in my "Physiological Morphology of 

 Animals." 



The results of this study may be summarized as follows : 



1. Certain sessile animals (Serpulidse, Hydroids) which 



are compelled to react to light and gravity as radial organ- 



1 Zoologische Jahrbiicher, Vol. V. 



