218 



DYNAMICS OF LIVING MATTER 



(Fig. 60). The parts which grow down (Fig. 59) have no polyps, but at- 

 tach themselves like true roots to solid bodies. Thus the changed posi- 

 tion of the stem alone, without any operation, suffices to 

 transform the lateral branches, whose growth is limited, 

 into roots with unlimited growth. The lateral branches 

 on the upper side of the stem do not undergo such a 

 transformation except in the immediate surroundings 

 of the place where a new stem arises. It seems that the 

 formation of a new stem also causes an excessive growth 

 of roots, possibly because the formation of new branches 

 causes the removal of substances which naturally inhibit 

 the formation of roots. If a piece from the stem be put 

 vertically into the water with top downward, the upper- 

 most point may continue to grow as a stem, while the 

 lowest point may give rise to roots. In this case, therefore, 

 a change in the orientation of organs has the effect of 

 changing the character of organs. 



We have already mentioned the fact that in Eudendrium 

 the formation or regeneration of polyps is only possible in 

 the light. 



In many hydroids, contact with solid bodies seems to 

 favor the formation of stolons, although such contact 

 is not the only 

 condition that 

 brings about this result. Fig- 

 ure 6 1 shows a piece of a stem 

 of Pennaria, a hydroid, which 

 was lying on the bottom of an FIG. 61. 



aquarium, and which formed 



stolons at both ends. In Margelis, another hydroid, I observed 

 that, without any operation, the apical ends of the branches which were 

 in contact with solid bodies continued to grow as roots, while those 

 surrounded by sea water continued to grow as stems. 



FIG. 60. 



7. THE ROLE OF REVERSIBLE PROCESSES IN PHENOMENA OF 



REGENERATION 



In all the cases mentioned thus far one point has not yet been dis- 

 cussed ; namely, where the material of which the new organs consist 

 comes from. In the case of plants, where the green leaves assimilate, 

 and the salts are taken up from the soil or water, this source is evident, 

 as it is also in the case of the animals which take up food; but in the 



