REGENERATION. 1 95 



change is to be sought in the organ itself and in the external 

 conditions surrounding that organ. They are not content to 

 rest their "explanations" on "the phyletic origins" of the 

 changes. It is not necessary to deny the theory of descent, 

 but it is unsafe and in many cases unscientific to base " causal 

 explanations " on an imaginary line of ancestors. It is cer- 

 tainly unprofitable to shift our difficulties back to these historic 

 forms, and most unfortunate to find our " explanations " also 

 resting on the same shadowy past. 



In a masterly essay, entitled " Stoff und Form der Pflanzen- 

 organe," Julius Sachs has considered the question of regenera- 

 tion in plants, and has outlined an hypothesis to account for 

 the phenomena. Sachs bases his view on the conception that 

 the form of a plant is the outcome of its chemical structure ; 

 that whenever the form changes there has been an antecedent 

 change in the material (Stoff). Sachs vigorously combats Voch- 

 ting's idea that there exists in the organism a polarization of 

 every part, and that this polarization is a directive agent that 

 determines the kind of regeneration that takes place. No less 

 earnestly does Sachs protest against the metaphysical concep- 

 tion of many morphologists, expressed or implied, viz., that for 

 each species of organism there is a form that tends to express 

 itself and controls the development of each part. According 

 to Sachs there are no such formative forces in the organism, 

 but all changes are brought about by differences in the chem- 

 ical composition of the " Stoff," and this leads to the develop- 

 ment of the form peculiar to that material. For example, the 

 flower-buds of plants are produced, not because of some innate, 

 mystical force that causes the plant to complete its typical form, 

 but because some substance is made in the leaves that, flowing 

 into the undifferentiated growing point, there acts on the ma- 

 terial of the growing point and changes it into that sort of stuff 

 from which a flower develops. 



Applying this idea to regeneration, Sachs supposes that in 

 the plant two substances are being produced ; one of these is 

 a leaf-forming substance, the other produces roots. When a 

 piece of the stem is cut from a plant these two substances con- 

 tained in it flow in their respective directions, and bring about 



