be given to this result is, I believe, that the new material assumes a definite structure, or what we may call an organization, 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, metaphysical 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. 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. Vöchting 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.