Regeneration

by Thomas Hunt Morgan

It is not necessary to separate completely portions of the stem in order to produce roots near one end and shoots near the other. If a ring, including the cambium layer, is cut from the piece, as indicated in Fig. 32, C, the part above and the part below act independently of each other, and each behaves as a separate piece. In various other ways the same result may be obtained, as by simply making an incision in the stem at one side, or by partially splitting off parts of the stem ([Fig. 34], C).

If instead of a piece of the stem, a piece of a root is removed, the results are as follows.[31] It should be remembered that the basal end of a root is the part nearer the stem, the apex is the part nearer the apex of the root. If pieces of the root of the poplar, Populus dilatata, are suspended vertically ([Fig. 32], D) in a moist chamber, a covering of new cells, a callus, appears over the cut-ends. From the basal callus numerous leaf-shoots may develop. Pieces of large roots may produce over a hundred of these shoots from a single basal callus. In some cases adventitious shoots may also arise from the side of the root near the basal end. Roots develop from the callus over the apical end; less often from the sides near the end. If a similar piece of root is suspended with its apical end upward, the new shoots arise as before over the basal end, that is now turned downwards.

The leaves of some plants, as has long been known, are able to produce new plants. The begonias are especially well suited for experiments of this kind. A piece of the stalk of a leaf suspended in a moist atmosphere produces roots near its base. In most cases the opposite end of the stalk, i.e. the end nearest the leaf, putrefies and slowly dies toward the base. Near the base there may arise, before the breaking down of the piece has reached this point, leaf-buds that arise just above the first-formed roots. When these new shoots have reached a certain size they may produce their own roots at or near the base. If, however, a portion of the leaf is left attached to the leaf-stalk ([Fig. 35], A), new roots arise near the basal end of the stalk, and later shoots grow out near the point of union of the leaf and its stalk at the point where the veins of the leaf come off. These shoots produce roots of their own near the base, and roots may also appear on the part of the leaf-stalk near its union with the lamina. If a part of the mid-vein, or of any large vein of the leaf, is cut out, leaving a part of the lamina on each side ([Fig. 35], B), and the piece is suspended vertically, roots appear on the basal end of the vein, and in the same region one or more shoots arise.

Leaves of heterocentron with the stalk attached, if kept in diffuse light, produce roots along the stalk, especially near the basal end, but shoots do not appear, even after five months ([Fig. 35], C).

These experiments show that the leaves do not exhibit the same polar relations that are shown by pieces of the stem and root. Vöchting points out that the results may be explained in either of two ways. The stem and the root have in general an unlimited growth with a vegetative point at the apex. The leaf has only a limited growth. Its cells form permanent tissue, hence the leaf does not produce a new plant from its outer part. The second possibility is this: the phenomenon is connected with the symmetrical relations that different structures possess. Stem and root are symmetrical in two or more directions, the leaf on the other hand is a flat structure with one plane of symmetry, and even symmetry in one plane may be absent. If the leaf could produce shoots at its apex and roots at its base, from the semilunar fibrovascular bundle of the leaf, then an individual (the leaf) with its single plane of symmetry would produce shoots and roots that are symmetrical in two planes. Such a result would be so anomalous that one may well doubt the possibility of its coming into existence.[32]

Later, Vöchting attempted to see if the same relation found in the leaf would hold for other organs that have a limited growth. He found that such structures, as spines, for example, produce both shoots and roots near the base, as do leaves.

These experiments of Vöchting on the regeneration of pieces of the higher plants show that a piece possesses an innate polarity, or “force,” as Vöchting sometimes calls it (although he explicitly states that he does not use the word “force” in its strict, physical sense). It does not follow, of course, that external conditions may not also influence the regeneration, but in those experiments in which the pieces were freely suspended in a moist atmosphere, the external factors are as far as possible excluded, so that the effect of the innate tendencies are most clearly seen. In another series of experiments the influence of external conditions on the regeneration was especially studied. This analysis that Vöchting has made of the problem of regeneration is in the highest degree instructive, since it shows how several factors,—some internal, others external,—take a hand in the result; and it is only possible to unravel the problem by combining different experiments carried out in such a manner that one by one the different factors at work are separated.

If a piece of a young stem of Salix viminalis is suspended vertically in a moist atmosphere, with the lower end in water (for ¾ of a centimetre), and the piece kept in the dark, the result is, in the main, the same as when similar pieces are suspended in moist air without coming into contact with water. Roots arise near the base, and shoots near the apex, without regard to which end is in the water.

If the same experiment is repeated in ordinary air, i.e. air not saturated with water, the result is somewhat different. If the twig is suspended vertically, with its apex upward, roots soon appear on the basal end that is in the water, but no roots develop above the water. Small protuberances may appear above the water in the places at which roots would develop if the piece were surrounded by a moist atmosphere, but they do not break through the bark. If the piece is then covered by a jar containing air saturated with moisture, these protuberances may become roots. It is clear, therefore, that the dryness of the air has prevented their development.

If a similar twig is suspended (in the air) with its apex downward, and the lower end in water, root protuberances appear, at first, only around the base, i.e. at the upper end. Under the water, at the apical end, small and weak roots may develop, or may even not appear at all.