STEEEOTEOPISM 



137 



to solid bodies, while the polyps bend and continue to grow 

 away at right angles from solid bodies with which they 

 come in contact. Thus the stem of Tubularia mesem- 

 bryanthemum, a marine hydroid, grows in a straight line. 

 When such stems, after their polyp is cut off, are put 

 with one end in sand, the free end forms a new polyp 

 and the stem continues to grow in a vertical direction 

 upward. When, however, the stem is put near the glass 

 wall as soon as the polyp grows out it bends away from 



FIG. 40. The regenerating polyp of Tubularia when in contact with the glass wall of an 

 aquarium bends at right angles to the glass wall. 



the solid wall, and the stem will now continue to grow 

 at right angles to the vertical wall (Fig. 40). 



This phenomenon raises the question whether or not 

 the law of chemical mass action underlies phenomena 

 of stereotropism. We have seen that this law dominates 

 the phenomena of heliotropism, inasmuch as the Bunsen- 

 Eoscoe law is the expression of the influence of light on 

 the mass of the photochemical reaction product. We 

 have also been able to show that in the case of the geo- 

 tropic curvature of Bryopliyllum the mass of the apical 



