Regeneration

by Thomas Hunt Morgan

Stems of heterocentron placed in a horizontal position produce a circle of roots around the base, and later, in several cases, roots from the under surface of the stem, both from the nodes and the internodes; but these roots are smaller than those at the base. Those around the base are often longer on the lower side than on the upper side.

Vöchting has also studied the regeneration of pieces of roots of the poplar and of the elm suspended horizontally in a moist chamber. A callus develops from the cambium region of the basal end, and from this a thick bunch of adventitious sprouts grows out. A weak callus may develop on the apical end also, from which a few roots develop. In other cases adventitious shoots are produced also from the apical callus, especially from the upper edge of the callus. The results are variable, but show that at times leaf-shoots may develop from the apical end of the root. It is also singular to find that, while pieces of the root produce new leaf-shoots very readily, yet they often fail to produce new roots, or produce only a few that arise from the apical callus or from the sides near that region. It is difficult to show that gravity has any influence on the result.

Vöchting recognizes another sort of influence that determines the position of new organs on a piece. If a young, growing end of a stem of Heterocentron diversifolium is suspended by two threads in a horizontal position, the ends bend upward as a result of the negative geotropism of the piece. The new roots appear at the base of the piece, and also on the convex side of the bent part of the stem, as shown in Fig. 34, B. The same result can be obtained by forcibly bending a twig, and then tying the ends together, so that it remains in its bent position. If a piece of this sort is suspended in a moist atmosphere, with the bent inner concave side turned upward, the roots appear on the base and at the bend, especially on the under side, both from the nodes and internodes. If now in order to see if gravity takes any part in the result the next piece is suspended with the outer convex side of the bent part turned upward, it is found that many of the pieces produce roots only at the base, but others produce roots also at the bent portion of the stem, but they are fewer than in the last experiment. The roots arise for the most part on the under side of the arch, and only a few arise from the upper part. It is clear that gravity is also one of the factors in the result. Leaf-buds arise in these pieces with the concave side turned upward only near the apex; rarely one may develop on the lower part of the basal end. In pieces with the concave side turned downward the leaf-buds arise for the most part at the apex, but sometimes they appear on the upper part of the basal arm. The results are due to two factors, gravity and an inner “force” that is supposed to be the resultant of a growth phenomenon taking place in the bent portion. Vöchting supposes that a process of growth takes place as a result of the bending; “the plasma streams to this region, and a new development takes place here more easily.” Vöchting adds that this view will not explain the morphological character of the new organs, and that this must be due to quite other causes. The results may, I venture to suggest, find a simpler explanation as the result of the bending, disturbing the tensions of the protoplasm, causing the two arms of the piece to act as if they had been separated from each other. This idea is more fully developed in a later chapter.

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 question will be deferred to a later chapter.

The preceding account of regeneration in some of the higher plants has shown that their usual method of regeneration is by means of latent buds that are present along the sides of the stem, or by means of adventitious buds that develop anew along the sides of the stem. In a few cases new buds may develop from the new tissue of the callus that forms over the cut-ends, but in such cases the new shoots, or the new roots, are much smaller in diameter than the end from which they arise, and usually several or many new shoots develop on the same callus. In these respects the regeneration of the higher plants is different from that of the higher animals, for, in the latter, the new part arises from the entire cut-surface. This difference is no doubt connected with differences in the normal method of growth in plants and in animals, and an explanation of the growth would, perhaps, also give an explanation of the mode of regeneration. The normal method of growth in higher plants takes place largely by the formation of lateral buds, as well as by terminal growth, and we find that regeneration takes place in most cases from the same lateral buds or from others of a similar kind that develop after the piece has been separated.

It is sometimes stated that the higher plants do not regenerate at the cut-ends, because they produce buds at the sides. The statement implies that there is some sort of antagonism between the regeneration of a bud at the end, and the development of buds at the side. It may be true that the development of a latent bud at the side might suppress the tendency to produce a bud at the end, if such a tendency exists; but if we remove the lateral, pre-formed buds, new ones develop at the sides, and not at the end. That there need not be an antagonism between the formation of a bud, or of buds, at the end, and also at the sides, is shown in Vöchting’s experiments with the roots of the poplar. In these, leaf-shoots and root-shoots developed both from the callus over the cut-end, and at the side of the piece also. It has further been shown that, although a piece of the internode does not produce new leaf-buds at the sides, neither does it regenerate a new apical bud at the end.

Fig. 35.—After Vöchting. A. Leaf-stalk of Begonia rex with a portion of the lamina. Suspended with base upward. B. Piece of lamina of leaf of same. C. Leaf of Heterocentron diversifolium. D. Leaf-stalk of Begonia discolor.

A most interesting fact connected with the regeneration of the higher plants is, as has been pointed out, that even when a callus is formed over the cut-end, and new growth takes place from this callus, there is produced, not a single terminal bud, but a number of separate buds. The piece does not complete itself, but produces new buds, that make new branches. The explanation of this mode of regeneration in plants is not known. It appears to be connected with the production, by means of buds, of all the new structures. Why this should occur we do not know, and the only suggestion that offers itself is that the result may be in some way connected with the hard cell walls in plants that make difficult the organization of large areas into a new whole. As a result, the new development takes place in a small group of similar cells, that are sufficiently near together to organize themselves into a whole despite the interference met with in the cell walls.