Regeneration

by Thomas Hunt Morgan

Vöchting has also studied the regeneration of pieces of the liverwort, Lunularia vulgaris. The results have been already partly given in the first chapter. If cross-pieces are taken from the thallus, each produces a new bud at its anterior or apical end ([Fig. 9], A, A¹). The new bud arises from the cut-surface, or very near it, from a group of cells of the midrib that lies nearer the under side ([Fig. 9], A²). The bud gives rise to a new thallus that springs from a narrow base at its origin from the old piece. If a piece is cut longitudinally from the thallus along the old midrib, the new bud arises at the anterior end from the midrib ([Fig. 9], B). It comes either from the anterior cut-surface near the inner edge, or from the anterior end of the inner edge, and in some cases two new buds arise, one at each of these places. If the piece is removed from one side of the midrib it does not regenerate as quickly as when a part of the midrib is present, but when the new bud develops it arises from the anterior part of the inner edge ([Fig. 9], B¹). If the piece is cut far out at one side, it may be a long time before the new bud arises. This difference in the rate of development of these pieces is explained by Vöchting as due to the simpler character of the cells near the midrib.

If oblique pieces are cut off, with an anterior oblique cut-edge, as shown in [Fig. 9], C, C¹, the new bud arises along the anterior surface. If the piece includes a portion of the old midrib at its inner end, the new bud arises from this ([Fig. 9], C), but if the piece lies to one side of the midrib, the new bud arises near the anterior end of the anterior oblique surface ([Fig. 9], C¹, C²).

A number of experiments that were made in order to determine what part gravity and light may take in the regeneration gave nearly negative results. The regeneration appears to result largely from internal factors.

If a piece of the thallus is divided parallel to its surface, the two parts may each produce a new thallus, but this arises much more readily from the lower piece. If a piece of the latter is cut into small pieces no larger than half a cubic millimetre, and even much smaller, each may produce a new thallus.

Vöchting also studied the regeneration of parts having a limited growth. If a gemmiferous capsule is cut off, then split into two or four pieces, and these are placed on moist sand, it is found that new buds arise along the basal cut-edge. In order to show that this is not due to the new part arising on the basal end because there is no other cut-surface, the apical part of some of the pieces was cut off. These pieces, with two free ends, produced new buds only on their basal ends.

The sexual organs of lunularia are borne on the top of erect reproductive branches having a limited growth ([Fig. 9], D), which carry later the sporiferous branches. The branches have a stalk and a terminal disk. If pieces of the stalk are cut off they do not produce any new parts for a long time, but ultimately each produces from the basal cut-surface, or not far from the basal end, a new bud (Fig. E¹). If the disk is left attached to the piece, the result is the same as before (Fig. D¹). If a twisted part of the stalk is used, new buds may develop at the base and also near the twisted region, as shown in Fig. 9, E¹. If pieces of the stalk are stuck into the sand, some with the apical end, others with the basal end in the sand, the former produce new buds at the upper basal end, the latter produce buds on the stalk just above the surface of the sand. Pieces that retain the old disk when stuck into the sand ([Fig. 9], D) produce one or more buds along the stalk above the sand, often some distance above it. The part buried in the sand does not seem able to develop new buds, and as a result they are produced at the first region of the basal part of the stalk, where the conditions make it possible for buds to develop.

If the disk is cut entirely from the stalk and placed on moist sand, it produces adventitious buds in the region at which the stalk was removed. Buds are also produced at the bases of the rays that go off from the disk. They arise from the under side of the rays without regard to the position of the disk, i.e. whether it is turned upward or downward. If the rays are cut off they produce new buds at the base ([Fig. 9], F), and if the outer tip of the ray is also cut off, the new bud still arises at the base, as shown in [Fig. 9], F¹. These results on pieces with limited growth agree in every respect with those that have been obtained in flowering plants. Vöchting thinks that the phenomenon is due in all cases to the limited growth of the parts. Goebel rejects this interpretation, and thinks that the results can be accounted for by the direction of the movement of formative or, at least, of building material. In favor of this view, he points out that in other liverworts the polarity is not shown in the same degree as in lunularia (according to Schostakowitsch), and also that in very old pieces of marchantia, as Vöchting has shown, the polarity disappears. In the latter case the attractive action at the vegetative point, to which the building stuff is supposed to flow, is less strong; and in longer pieces the influence of the apical region may not extend throughout the entire length of the thallus. In favor of this interpretation he points out that in young prothallia of osmunda, adventitious shoots do not appear, but in older plants, that have become longer, these shoots may appear at the base, because this region is no longer influenced by the apex, and consequently it is possible for building material to accumulate at the basal end. It may be granted that Goebel’s idea is possibly correct, viz. that the apex, or the apical end of a piece, may have some influence in preventing the development of shoots at the base, but it does not follow that this influence can be accounted for on the ground of a withdrawal of building stuff from the basal part. As I shall attempt to show in a later chapter, this influence may be of a different nature.

It has been found by Pringsheim and others that pieces of the stem of mosses may also produce new plants, and this holds even for pieces of the stalk of the sporophore and of the wall of the spore capsule ([Fig. 10], A-D). In this case, however, there is not produced a new moss plant directly from the end of the piece, but threads or protonemata grow out, as shown in [Fig. 10], A, B, and from these new moss plants are formed in the same way as on the ordinary protonema. The threads that arise from the piece grow out from single cells in the middle part of the stem. These cells are less differentiated and are richer in protoplasm than are the other cells in the stem.

The prothallia of certain ferns are said by Goebel to regenerate if cut in two; at least this is true for the part that contains the vegetative point. In a piece without the growing point, the cells are very little specialized, and the piece may remain alive; yet it is incapable of producing a new growing point. Comparing this result with the power of regeneration possessed by lower animals, Goebel states[34] that since in a plant new organs may arise without the typical form of the plant being produced, “therefore, the completion of a leaf, for instance, that has been injured, would be of no use to the plant, while in animals that do not have a vegetative point, the loss of an organ is a permanent disadvantage in case the organ removed cannot be regenerated.” The “explanation” of the difference in the two cases is supposed, apparently, by Goebel, to depend on the usefulness, or non-usefulness, of the regenerative act!

Brefeld has described several cases of regeneration in moulds. There is produced from the zygospore of Mucor mucedo a germinating tube that forms at its end a single sporangium. If the tube is destroyed or injured, a second one is formed from the zygospore, and if this is injured a third time, a new tube is produced. Each time the sporangium is smaller than in the preceding case.