266 REGENERA TION 



that causes the plant to complete its typical form, but because some 

 substance is made in the leaves which, being carried into the growing 

 region, becomes there a part of the material of that region, and from 

 this new material a flower is formed. Simple and clear as this 

 hypothesis appears to be at first sight, it will be found on more care- 

 ful examination that it fails to account for some of the most charac- 

 teristic phenomena of development and of regeneration. It may be 

 granted at the outset that the presence of certain substances may 

 undoubtedly influence the kind of growth of a new part ; but, on the 

 other hand, one of the most characteristic things of the organism is 

 that it asserts its specific nature within quite a wide range of change, 

 and, on the whole, resists the influence of other kinds of substances 

 than those connected with its ordinary life. While Sachs looked no 

 farther than the material substratum, and supposed that any change 

 in this altered the form, there is, at present, sufficient evidence to 

 show that it is the structure of the material that determines the most 

 important changes that take place in it. This means, if we attempt 

 to divest the statement of its somewhat metaphysical appearance, 

 that the material of the organism is not simply a mixture of different 

 kinds of materials, but a special kind of substance that has a definite 

 structure of its own. This structure may, of course, be changed, but 

 only by the addition of materials that the structure can take up as a 

 part of itself. If the material does not become a part of the struc- 

 ture or organization, it is without effect on the form. 1 My meaning 

 can, perhaps, best be illustrated by the method of regeneration of 

 the tail of the fish from an oblique cut-surface. The growth of the 

 new part is not determined by the kind or by the amount of the new 

 material that is brought to the growing part, for, if it were, the new 

 part would grow at an equal rate at every point ; but the growth of 

 the new part is regulated by the form of the tail of each particular 

 kind of fish. The structure of the new part controls the growth of 

 the material of the new part, and not the reverse. The only inter- 

 pretation that can be given to this result is, I believe, that the new 

 material assumes a definite structure, or what we may call an organi- 

 zation, and the subsequent changes are controlled by the kind of 

 structure that is present ; and since this structure has, as a whole, a 

 definite form, we can state that the form controls the material, 

 although the substitution of the word "form" for that of "the structure 

 of the new material " may give the statement an unfortunate, meta- 

 physical appearance. 



In order to explain the regeneration of a piece of a plant, Sachs 

 supposes that two substances are produced by the plant, one a stem- 

 (or leaf-) forming substance and the other a root-forming substance. 

 1 Unless it produces a physical change in the structure. 



