OA^ PHYSIOLOGICAL MORPHOLOGY. 



39 



the aquarium, while the stems never show any reaction of this 

 kind. These new parts growing out from the branches of 

 the under side of the stem attach themselves to solid bodies, 

 if we bring them in 

 contact with the same. 

 Moreover, they are pos- 

 itively geotropic (that 

 is, they grow toward 

 the centre of the earth), 

 while the branches 

 never show any pos- 

 itive geotropism. The 

 branches on the upper 

 side are not trans- 

 formed into roots. 

 They either perish or give rise to very long, slender, perfectly 

 straight stems {b, Fig. 4), which grow vertically upward. 

 These stems, as a rule, are too slender to bear branches, 

 but at parts of the upper surface of the main stem there 

 originate new stems (r. Fig. 4) which grow vertically upward 

 and produce the typical little branches with polyps. 



If we bring the stem into an oblique position (Fig 5), with 

 the apex b upward, from every element of the main stem new 



Fig. 4. 



Fig. 5. 



Fig. 6. 



Stems and roots may originate, but with this difference, stems 

 always originate from the upper side of an element and roots 

 from its lower side. But if we place the stem in an oblique 



