Physiological Morphology 91 



in which such forms of heteromorphosis can be produced. 

 Another form, Pennaria, is just as favorable. In Pennaria I 

 succeeded repeatedly in producing roots at both ends of a 

 small stem that bore no polyps (Fig. 28).' 



In these experiments on Margelis and Pennaria organs 

 brought into contact with solid bodies continue to grow as 

 roots, if they grow at all. Organs surroimded on all sides by 

 water continue to grow in the form of polyps, if they grow at 

 all. In Margelis, contact with a solid body plays the same role 

 as did gravitation in the case of Antennularia. In what way 

 the contact may have an influence shall be mentioned later on, 

 but here one more point may be mentioned. In Antennularia, 

 gravitation not only determines the place of origin of the various 

 organs, but also the direction of their growth; the stem, 

 growing upward, is negatively geotropic, the root, growing 

 downward, is positively geotropic. In Pennaria, the nature 

 of the contact not only determines the place of origin of the 

 various organs, but also the direction of their growth. If we 

 bring an outgrowing polyp of Pennaria into contact with a 

 solid body, the polyp begins to grow away from the body, and 

 the new stem is very soon nearly perpendicular to the part of 

 the surface with which it came into contact. 



I have called this form of irritability stereotropism. We 



' In a Tubularian X was able to produce the opposite result, namely, to get 

 an animal that ended at both ends in a polyp and had no root. Weismann seems 

 to assume, in his Germ Plasm, that the latter result is to be explained by the 

 principle of natural selection, inasmuch as an animal without polyps could not 

 continue to live, and hence it would be impossible to produce roots at both ends. 

 In Pennaria this supposed impossibility was realized; one may say that these 

 roots in Pennaria may give rise later on to polyps. In the special case that I 

 observed they did not, although as a rule they do: but the same is the case in 

 Tubularia, in which polyps also arise from the roots. It might be said, perhaps, 

 that the formation of roots in Pennaria is, for some reason, absolutely necessary; 

 but it is just as easy to produce polyps at both ends. Even if it were possible to 

 reconcile these facts with the principles of natural selection, causal or physiological 

 morphology would not gain thereby, as the circumstances that determine the 

 forms of animals and plants are only the different forms of energy in the sense in 

 which this word is used by the physicist, and have nothing to do with natural 

 selection. 



