HETEROMORPHOSIS 143 
tropic root arises from the main stem, and from this (wpon 
the side directed toward the light) spring new positively 
heliotropic stems. This arrangement corresponds with that 
so often found in Plumularia pinnata. 
The roots are positively stereotropic as in the other 
Hydrozoa. When brought in contact with a solid body they 
attach themselves to its surface. As soon as the roots have 
attached themselves, the position of the new stems forming 
upon the roots is determined by the contact stimulus; the 
new stems arise from those points on the surface of the root 
which are diametrically opposite the substratum to which the 
root is attached. 
The fact that roots and stems may arise simultaneously 
and side by side from the basal end of an organ has also 
been observed in certain organs of plants; e. g., in fragments 
of leaves which form both roots and stems at their bases. 
The protoplasm retracted from that piece of the Sertularia 
stem which was covered by sand. 
2. When the tips were cut off of stems which were fixed 
in the sand in a vertical and upright position (with the tip 
upward), simple regeneration of the tip followed in the great 
majority of cases. Only once or twice did I see a root arise 
from the tip of a vertical and upright stem. 
3. In inverted stems occasionally new stems arose from 
the middle of the old stem upon the side directed toward the 
source of light. These grew in a direction determined by 
their positive heliotropism; when the light came from above, 
they grew upward toward the basal cut end. 
Roots which were formed in the middle of inverted stems 
(Fig. 22a) grew downward and toward the room side of the 
aquarium, when the light fell upon them from above. 
4. Driesch has observed a phenomenon of growth in 
specimens of Sertularella polyzonias which were cultivated 
under “unfavorable”’ conditions that I have never found in 
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