Regeneration

by Thomas Hunt Morgan

Fig. 32.—After Vöchting. A. Piece of willow cut off in July, suspended in moist atmosphere with apex upward. B. Older piece of willow (cut off in March) suspended in moist atmosphere with apex downward. C. Piece of willow with a ring removed from middle. Apex upward. D. Piece of root of Populus dilatata. Basal end upward. Shoots from basal callus. E. Piece of root of same with two rings removed. New shoots develop from basal callus, and from basal end of each ring.

The results show that at the base of the piece the same factors that bring about the development of the rudiments of preëxisting roots also cause the development of new roots, if the lower end is in a region in which there are no rudiments of roots present. The influence that produces the new roots is confined to the basal part of the piece. In the apical part of the piece there are no adventitious structures produced, but a longer region is active, and several pre-formed leaf-buds begin to elongate. The topmost shoot grows faster than the others, showing that the influence that produces the growth is stronger near the apical end than at points further removed.

If another piece of a willow stem be placed under the same conditions, but suspended with the basal end uppermost, results that are in many respects similar to the last are obtained. Roots appear around the base of the piece, i.e. around the upper end, and the leaf-buds that develop are those that stand nearest to the apical, at present the lower, end of the piece.

These results seem to indicate that, in the main, the chief factors that determine the growth of the new part are internal ones; and although internal factors do appear to be the dominating ones, since roots appear in both cases at the base and shoots at the apex, yet it would be wrong to conclude that gravity has no influence at all on the result. In fact, other experiments show that it does have an influence.

If an older branch (8-12 mm. in diameter) is cut off and hung up with its base upward, the result is somewhat different from that with younger branches. The roots appear along the entire length of the piece, as shown in [Fig. 32], B; the largest are those near the base, and they decrease in size toward the apex of the piece. It is also noticeable that all the roots come from preëxisting root-buds, and no adventitious roots are formed, even at the base. The leaf-buds that develop are those arising near the apex, as in the last experiments. They bend upward as they grow longer. A comparison of the results obtained from younger and older pieces may, at first, seem to show that the difference in their development is due to the greater amount of reserve food stuff in the older piece, and Vöchting thinks it probable that this influence may account for the strength, length, and even for the number of roots that develop, but he believes that it is improbable that their mode of origin and their location can be so determined. Furthermore, the development of new roots around the base of the younger piece can hardly be explained as due to the absence of food stuff. The explanation of the production of a smaller number of roots in a young piece is that its tissues are less highly specialized, its buds less advanced, and the piece itself is in a lower stage of development. Another explanation must be found for the greater number of roots that develop in the older piece. This is due, as Vöchting tries to show, in part to the influence of gravity on the piece.

Vöchting’s general conclusion is that “the force or forces that determine the polar differences in the piece are most evident and most energetic in very young twigs; that this difference decreases with the age of the twig whose leaf-buds and root-buds become further developed. It is clear that the new roots of young twigs could appear in corresponding number and strength in exactly the same regions in which they grow out from pre-formed buds of a year-old twig. Since this does not occur, and since the roots appear only near the base of young twigs, the explanation must be that the innate polar forces act more energetically in young twigs, and the buds that develop in the older twigs must arise in antagonism to the action of this force.” The polar difference between apex and base is present, nevertheless, as Vöchting’s experiments show, even in quite old pieces.

Fig. 33.—After Vöchting. A. Internodal piece of Begonia discolor. Apex upward. B. Same with apex downward. C. Internodal piece of Heterocentron diversifolium. Apex upward. D, E. Pieces of leaf of Heterocentron diversifolium. Apex downward. F. Same with apex upward. D, E, F. Same planted in earth.

A series of experiments was carried out with the internodes of several plants in order to see if, in the absence of pre-formed buds, new buds would develop. The experiments were undertaken in order to ascertain whether the same polarity, exhibited by longer pieces, would be also found in internodal pieces. In most plants pieces of this kind do not produce new structures, but in Heterocentron diversifolium an internode produces roots at its basal end without regard to the position of the piece ([Fig. 33], C). Leaves do not appear on these pieces. On the other hand internodes of Begonia discolor give the opposite result, as shown in [Fig. 33], A, B. In this case leaf-buds appear at the apex of the internodal piece ([Fig. 33], A), even when the apical end is downward ([Fig. 33], B). From the bases of the new shoots roots may then develop, as also shown in the figure ([Fig. 33], B). Vöchting concludes that the same polarity that is a characteristic feature of longer pieces is also present in internodal pieces.