Regeneration

by Thomas Hunt Morgan

If pieces are cut from the stem of antennularia and suspended in

Fig. 14.—After Loeb. Normal stalk of Antennularia antennina. B. Piece regenerating in vertical, normal position. C. Piece regenerating in inverted position. D. Piece regenerating in inclined, vertical position. E. Piece regenerating in inclined, inverted position. F. Piece regenerating in horizontal position.

the water, regeneration takes place at the cut-ends. If a piece is suspended with its apical end upwards ([Fig. 14], B), a new stem develops at the upper cut-end, and new roots from the lower cut-end. If a piece is suspended with its basal end upwards ([Fig. 14], C), there is formed at its upper (basal) end a new stem with its branches also slanting upwards as shown in the figure. Roots appear at the lower (apical) end. Since gravity is the only force that acts in a vertical direction under the conditions of the experiment, Loeb concluded that it plays an important rôle in determining the kind of regeneration that takes place. Its action is of such a nature that a new stem develops from the upper cut-end, and roots from the lower end, regardless of whether the upper end is the basal or the apical end of the piece. Similar results are also obtained, according to Loeb, if the pieces are suspended obliquely. In a piece of this sort, it is found that new stems arise along the upper surface of the old stem, and roots from the lower surface as well as from the lower cut-end ([Fig. 14], D, E). If a piece of the stem is placed horizontally on the bottom of an aquarium, the branches that come off from the under surface of the stem begin to grow downwards at their ends, and where they come in contact with a solid body they fasten themselves to it, thus showing that they are true roots ([Fig. 14], F). One or more stems may arise from the upper side of the main stem. These stems grow vertically upwards, and produce lateral branches. Only in one case did a new stem, or stem-like structure, arise from one of the vertical branches, as shown to the left in [Fig. 14], F.

Loeb found it also possible to change the character of the growth of the apex of the normal stem and to transform it into a root. A long piece of the hydroid was cut off and suspended vertically with the basal end upwards. From the upper end a new stem began to grow, and then the entire piece was reversed, so that the new stem pointed downwards. Under these circumstances the young stem did not bend around and begin to grow upwards, as a young plant might have done, but it ceased to grow as a stem, and at its apex one or more roots developed. Loeb concludes: “I cannot imagine by what means the place of the formation of organs in antennularia is determined in connection with the orientation of the animal except by means of gravity.”

The response of antennularia to the action of gravity is, I think, conclusively demonstrated by Loeb’s results, but that the phenomenon may be complicated by other factors is shown, I think, by the following experiments. Driesch found that if pieces of antennularia are cut off and placed between horizontal plates, so that both ends are free, roots are produced by the basal end.[23] If the basal end with its new roots is cut off, new roots may appear, but sometimes a thin stem also. If the end is again cut off, a larger stem, and also one or two roots, may appear, and if the operation is repeated again only a stem is formed. The factor that brings about this change is not shown by the experiment. The piece had been kept in a horizontal position throughout the whole time. The apical end died in most cases without producing roots, but it is not stated whether or not roots appear on the stem between the plates of glass. If they develop they may affect the result, as certain experiments that I have made seem to show.

In my experiments, made at a different time of year from that at which Loeb’s experiments were made, pieces of the stem were suspended vertically,—some with the apical end upwards, others with the basal end upwards. In nearly all cases roots were formed by both the upper and lower ends. In a few cases, in which the apical end was upwards, a new stem developed at that end. Pieces suspended in a horizontal position also produced roots at both ends. After removing the ends with their new roots from the pieces suspended vertically, I found that roots again appeared at both ends in nearly every case. The difference between these results and those of Loeb may be due to the time of the year at which the experiments were made, or possibly to some other difference, but the results show that the response to gravity is not always so constant as Loeb’s results indicate.

In a few cases in my experiments the basal end of the hydroid was left attached to the stem on which it had grown, and the piece was put into the same aquarium used for the preceding experiments. In those pieces that lay on the bottom of the aquarium, with the stem standing vertically, a new shoot, and not new roots, appeared on the upper end. Other pieces were hung at the top of the water of the aquarium with the stem turned downwards, and the basal, attached end of the piece upwards. These pieces produced neither a stem nor roots from the apical end. The results show that the presence of roots at one end has an influence on the regeneration at the other end. The same thing was shown in one case in which a short piece sank to the bottom of the dish and, developing roots at its basal end, became fixed: a stem grew out of the apical end.

A number of other experiments that I made, in which pieces of antennularia were fixed to a rotating wheel, gave negative results, since neither roots nor stems appeared on the pieces. The rubbing of the ends of the piece against the water as the wheel turned round, or else the agitation of the water, prevented, most probably, the regeneration from taking place.