THEORIES OF REGENERATION 267 



If either of these substances combines with the protoplasm of any part, 

 a stem or a root is produced from that part. When a piece of the 

 stem is cut from a plant, these two substances accumulate, one at the 

 distal end and the other at the proximal end of the piece, and their 

 presence in these regions determines that new shoots develop at or 

 near the apex, and new roots at the base. Sachs tried to show that 

 the direction of the flow of these two substances is determined by the 

 action of gravity, the lighter substance flowing to the higher parts, 

 and the heavier to the lower parts. We find here reproduced Bonnet's 

 idea of specific substances flowing in definite directions ; but Sachs 

 goes farther, and gives an explanation of the cause of the different 

 directions taken by the two kinds of substances, viz. that it is due to 

 the action of gravity. Vochting has shown, as we have seen, after a 

 thorough examination of the method of development of pieces of plants, 

 that Sachs's hypothesis fails to account for the results ; and he shows 

 also that an internal factor, which he calls the polarization, has the 

 most important influence on the regeneration. 



It is not difficult to show that there are many other cases to which 

 the stuff hypothesis does not apply. If, as Bonnet attempted to show, 

 the regeneration is due to different stuffs, there is no explanation to 

 account for the flow in animals of head-forming stuffs forward and 

 tail-forming stuffs backward. In animals that regenerate laterally as 

 well as anteriorly and posteriorly, we should be obliged to assume 

 side-forming stuffs as well as head-forming and tail-forming stuffs ; 

 and since the kind of structures that are produced at the side are 

 different at each level, we should be obliged to assume that there are 

 many kinds of lateral stuffs. If regeneration cah take place in a 

 dorsal and in a ventral direction, as, for example, when the dorsal and 

 the anal fins of teleostean fishes regenerate, there must also be stuffs to 

 account for their development. When regeneration takes place from 

 an oblique surface, it must be supposed that two or more of these 

 kinds of stuff are brought into action. The regeneration of just as 

 much of the limb of the salamander as is cut off also offers difficulties for 

 Sachs's view. If we assume a leg-forming substance, it fails to account 

 for the difference in the result at each level. If we assume that 

 different substances come into play according to the amount of the leg 

 that has been cut off, the hypothesis becomes as complicated as the 

 facts that it pretends to explain. 



A special case, to which the stuff hypothesis has been applied by 

 Loeb and by Driesch, is that of tubularia, although the latter writer 

 has used the hypothesis only to a limited extent as involving quanti- 

 tative rather than qualitative results. There is present in the hydranth 

 and stem of tubularia a red pigment in the form of granules in the 

 endodermal cells. There is more of the red pigment in the stem near 



