Licence: Public Domain Mark
Credit: Regeneration / by Thomas Hunt Morgan. Source: Wellcome Collection.
Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.
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![part of the mid-vein, or of any large vein of the leaf, is cut out, leav- ing 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. Vochting 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.^ Later, Vochting 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 Vochting on the regeneration of pieces of the higher plants show that a piece possesses an innate polarity, or force, as Vochting 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 fac- tors 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 1 Vochting points out that a parallel case is found in certain conifers. In these there arise from a vertical many-sided main stem whorls of side branches that are symmetrical in one plane. These lateral branches, if cut off and planted, produce new roots and new branches, but the latter are always side-branches, like the parts from which they arise. They never produce a normal main axis. Nevertheless, although these branches cannot them- selves produce a main shoot, a callus may be formed at the base of the piece, and from this a new main stem may arise.](https://iiif.wellcomecollection.org/image/b21170277_0092.jp2/full/800%2C/0/default.jpg)
