Sachs has criticised Vöchting’s general conclusion in regard to the internal factors that determine the regeneration in a piece of the stem of a plant. He gives very little weight to the innate polarity of the piece, and attempts to explain the results as due to certain substances in the stem of such a sort that, accumulating in any region, they determine the kind of regeneration that takes place. Sachs also assumes that gravity acts on these substances in such a way that the root-forming substances flow downward and the shoot-forming substances flow upward. In a piece of a stem, the two formative substances contained in it accumulate at the two ends, and determine the kind of regeneration that takes place. It is evident that Sachs’ hypothesis fails to explain the method of regeneration of an inverted piece suspended in a vertical position, since the roots appear at the upper end and the shoots at the lower end. Sachs explains this as the result of the previous action of gravity on the piece, while the piece was a part of the tree and stood in a vertical direction. He supposes the longer time that gravity has acted on the piece has determined its basi-apical directions, so that this influence is shown in the inverted piece, rather than the action of gravity on it in its new position. This conception involves quite a different idea from the original one of formative substances flowing in definite directions. Moreover, Vöchting has met this interpretation by using the twigs of the weeping willow, that hang downward on the tree. If gravity has acted on these drooping twigs in the way that Sachs supposes it can act, then we should expect to find, if Sachs’ view is correct, that roots would develop at the apical end of a piece of the twig, and leaves at the basal end, if the piece is hung vertically with its basal end (_i.e._ the end originally nearer the trunk of the tree) upward. The regeneration of these pieces shows, however, that they behave in the same way as do pieces of twigs that have always stood vertically on the tree. There can be, therefore, no doubt that the distinction between base and apex is an expression of some innate quality of the plant itself. That an external factor, gravity, is also a factor in the regeneration of the pieces, is abundantly shown by the experiments of Vöchting and others, but that innate factors are also at work cannot be doubted. We find evidence in many animals of a similar difference between the two ends of a piece, and we speak of this difference between the anterior and posterior ends of a piece as its polarity. What this polarity may be we do not know, and it is even doubtful whether we should be justified in speaking of it as a force in the sense that the difference in the ends of a magnet is the result of a magnetic force. The kind of polarity shown by animals and plants does not seem to correspond to any of the so-called forces with which the physicist has to deal, but a further discussion of this