SOME PROBLEMS OF REGENERATION. 197 



difference is found between the higher animals and plants ; for 

 instance, in vertebrates and flowering plants. In the former, 

 regeneration takes place by the formation of a knob of indiffer- 

 ent cells and subsequent differentiation. In the higher plants, 

 on the contrary, regeneration of this kind is extremely rare 

 (root-tips). The new part does not appear at the cut end, but 

 the injury acts as a stimulus to the development of buds more 

 or less distant from the exposed surface. The buds that, develop 

 may be in some cases already formed, but in other cases new 

 buds appear after the injury, and in places where there were 

 no buds before the injury. Why do we find this difference in 

 the renewal of organs between the higher animals and plants ? 

 The usual reply is that plants do not regenerate at the cut end, 

 because they have acquired a new method of replacing lost 

 parts. The answer is not only teleological, but a quibble, so 

 long as it cannot be shown that there is a connection between 

 the development of buds and regeneration at the cut ends. If, 

 for instance, the new buds be continually destroyed as soon as 

 they appear, will the plant then regenerate at the cut surface ? 

 Can we imagine that, were it possible to destroy or injure the 

 buds for many generations, the plant would then " acquire " a 

 power to regenerate at the cut end ? If, on the other hand, it 

 is assumed that the plant has lost its power of regeneration at 

 the cut end, because it acquired a new method of renewal of 

 lost parts, the argument may be met by the fact that certain 

 hydroids regenerate both at the cut end of a stem, as Loeb has 

 shown, and also along the stem itself. There is, so far as we 

 can see, no contradiction between development from the cut 

 surface and development by means of buds at other points. 

 Our immediate problem is to examine the conditions of regener- 

 ation in the plant itself, to see if we can discover why regeneration 

 takes place at some distance from the exposed surface. 



III. 



It is not unusual to find that development will take place 

 in one direction and not in another. A newt that has had a 

 leg cut off will regenerate the lost limb (Spallanzani, Bonnet, 



