2] 



UPON THE DIRECTION OF GROWTH 



443 



the organism. Since the tube is tough and elastic, its bending 

 towards the light must be due, at first, to muscular action of 

 the animal inhabiting it. Additional secretions are, however, 

 constantly poured forth so that the new position soon becomes 

 in turn the permanent one. Serpula, which has a tube con- 

 taining lime, likewise turns its head end towards the light ; 

 but, since its tube is firm and inelastic, the bending which it 

 finally exhibits must be ascribed alone 

 to growth of the shell by additions to 

 its free upper margin. 



b. Hydroids. - These have been 

 made the object of study by DRIESCH 

 ('90) and LOEB ('90 and '91, p. 36). 

 In stocks of Sertularella polyzonias, 

 reared in an aquarium, one often finds 

 a stolon (primary stolon) growing out 

 from the distal end, at first straight, 

 so as to prolong the axis of the stock, 

 then turning and growing from the 

 source of light. From the convexity 

 of this primary stolon a secondary one a 



buds forth. It grows towards the FIG. 128. Phototropism in re- 

 light for a time, until it in turn buds 

 off a (tertiary) stolon; then it becomes 

 negatively phototropic. Tertiary and 

 succeeding generations of stolons fol- 

 low the same law. We have here the 

 remarkable phenomenon of change in 

 the sense of response depending upon 

 the condition of development of the sto- 

 lon (DRIESCH). In Eudendrium the 

 hydranths, in contradistinction to the 

 stolons, grow towards the light they 

 are positively phototropic (Fig. 128). 



The effective rays in animal phototropism have not been 

 determined. It is highly probable that, as in plants, they are 

 the more highly refractive ones. The responding region in 

 hydroids is the growing region. The fully formed stolon does 

 not turn in response to light. The perceiving region is still 



generation of Sertularia (poly- 

 zonias?). The stock was cut 

 near the stolon at b and in- 

 serted reversed in the sand, 

 being buried from a to c. 

 From the upper end b, both a 

 stolon, Wi, and Lydranths, S, 

 regenerated, and both grew 

 in the axis of the infalling 

 ray of light, indicated by the 

 arrow. The hydranths are di- 

 rected toward the source of 

 light ; the stolon tip in the 

 opposite direction. Magnified 

 2 diameters. (From LOEB, 

 '90.) 



