HETEROMORPHOSIS 143 



tropic root arises from the main stem, and from this (iifioii 

 1/ie $idc directed toirard tin', 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 tlu-v 

 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 ; c . 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 heliotropisui ; 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. 22o.) 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 



