FuRTHER INVESTIGATIONS ON Hexriorropism 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 S 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.”’ 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 (Serpulide, Hydroids) which 
are compelled to react to light and gravity as radial organ- 
1 Zoologische Jahrbticher, Vol. V. 
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