Regeneration

by Thomas Hunt Morgan

If the spore-bearing stalk of Coprinus stercorarius is cut off, the end grows out and produces a new sporangium. If pieces of the stem are cut off and placed in a nourishing medium, they produce from the ends a new mycelium, and from this new erect hyphæ may develop. In the former case, the cut-end regenerates the part removed in somewhat the same way that an animal regenerates at the cut-end; in the latter, there is a return to the mycelium stage, as in the piece of the moss that produces a new protonema. If the mycelium and the protonema are looked upon as an embryonic stage in the formation of the sexual form, there is a return in these cases to an embryonic form or mode of development.

Fig. 36.—After Goebel. Achimenes Haageana. A leaf-cutting of a plant in flower. The new plant, regenerating at base of leaf-stalk, proceeded at once to produce a flower.

One of the most remarkable and important discoveries in connection with the regeneration of plants is that the new individuals that develop from leaves cut off from certain plants differ according to the region of the old plant from which the leaf has been taken. Sachs discovered in 1893 that when the leaves of the begonia are taken from a plant in bloom, the adventitious buds that develop from the leaves very quickly produce new flowers. If the leaves are taken from a plant that has not yet produced flowers, the new plant that develops from the leaf does not produce flowers until after a much longer time. Goebel repeated the experiment with achimenes, and found that the new plants that develop from leaves from the flowering part of the stem ([Fig. 36]) produce flowers sooner than do the plants that develop from leaves from the base of the same plant. The former produce, as a rule, only one or two leaves and the flower stalk; the latter, a large number of leaves.

Sachs explains these results as due to a flower-forming stuff that is supposed to be present in the leaves when the plant is about to blossom. This material is supposed to act on the new plant that develops from the leaves, and to bring it sooner to maturity. Goebel points out that the result may also be explained by the fact that the leaves in the flowering region may be poorer in food materials and, in consequence, the adventitious buds that they produce are weaker, and, as experience has shown in other cases, a weakening of the tissues brings about more quickly the formation of flowers. Nevertheless, Goebel inclines to Sachs’ hypothesis of specific or formative stuffs, without, however, denying that there is also an inner polarity or “disposition” that also appears in the phenomena of regeneration. But Goebel seems to think that the phenomena of polarity “can most easily be brought under a common point of view by means of Sachs’ assumption that there are different kinds of stuffs that go to make the different organs. In the normal life of the plant shoot-forming stuffs are carried to the vegetative points, while root-forming materials go to the growing ends of the roots. In consequence, when a piece is cut off and the flow of the formative stuffs is interrupted, the root-forming stuff will accumulate at the base of the piece and the shoot-forming stuffs at the apex. In the leaf the flow of all formative substances is toward the base of the leaf, and it is in this region that the new plants arise after the removal of the leaf.” A confirmation of this point of view, Goebel believes, is furnished by the following cases. Some monocotyledonous plants seldom set seed because the vegetative organs, the bulbs, tubers, etc., that reproduce the plant, exert a stronger attraction upon the building stuff than do the young seeds.[35] Lindenmuth has shown in some of these forms that pieces of the stem produce, near the base, numerous bulblets, because the building stuff moves toward the base. In Hyacinthus orientalis, on the other hand, bulblets are produced at the apical part of a piece of the flowering plant. In this plant the seeds ripen normally, presumably because of the migration of stuffs toward the developing seeds. The results in all these cases are due, Goebel thinks, to the direction of the flow of formative stuffs, and cannot be explained as connected in any way with the limited growth of the part.

These cases, cited by Goebel, are not in my opinion altogether to the point; and they fail also to establish convincingly the conclusion that Goebel draws from them. It may be granted that starch is stored up in certain parts of the plant, and if these parts are removed the starch may be stored up in other parts, as Vöchting (’87) has shown; but that the movement of this starch to the base can account for the lack of development of the seeds in certain cases seems to me improbable, or, at least, far from being established by the cases cited. It may be granted that the presence of starch in a region may act on the organs there present and determine their fate. Vöchting has shown in the potato that by removing the tubers the axial buds, especially in the basal leaves, become tuber-like bodies, but it should not be overlooked that the tubers themselves are formed from underground stolons, that arise in the same way as do those in the axils of the leaves. It would be erroneous, I think, to conclude from these cases of the effect of food stuffs on certain regions that there are formative stuffs for all the organs of the plant, and that these stuffs migrate in different directions and determine the nature of the part. Even the migration of such substances in definite directions in the tissue is itself in need of explanation, since it has been made highly probable by Vöchting’s experiments that this is not produced by agents outside of the plant. Furthermore, Vöchting has shown that the tendency of starch to accumulate in the tubers and the formation of the tuber are separate phenomena.

This hypothesis of formative stuffs held by such able botanists as Sachs and Goebel demands nevertheless serious consideration, if for no other reason than that if it is true it offers quite a simple explanation of many phenomena of growth and of regeneration. We should, I think, distinguish between specific or formative stuffs and building or food stuffs. By specific stuffs is meant a special kind of substance which, being present in a part, determines the nature of the part. Sachs supposes, for instance, that a specific substance is made by the leaves of a plant which is transported to the vegetative, growing region (which has so far produced only leaves), and changes its growth so that flowers are produced. Goebel does not commit himself altogether to specific stuffs of this sort, but speaks also of building stuffs. By building stuff we may understand food material that is necessary for growth, and from which any part of the plant may be made. Its presence in larger or smaller quantities may determine what a particular part shall become, but further than this it exerts no specific action. This means that the presence of a certain amount of food substance may determine what a given region shall produce, but it is not supposed that there are different kinds of food materials that correspond to each kind of structure. If there were such, they would not differ from specific substances, unless we wish to make subtle distinctions without any basis of fact to go upon.

Goebel points out that there is evidence to show that the greater or less quantity of food substance contained in a plant often determines the nature of its growth, as for instance the production of flowers when the food supply runs low and the production of foliage when the food supply is abundant. This difference may explain Sachs’ experiment with begonia leaves; and if so, there is no need for supposing specific flower stuffs to be made in the plant.

There is another point of view which has been, I think, too much neglected, viz. that the production of food stuffs is itself an expression of changes taking place in the living tissues, and if the structure is changed so that it no longer produces the same substances it may then lead to the development of different kinds of organs. The difference in the regeneration of an apical and a basal leaf of begonia may be due to some difference in the structure of the protoplasm. The greater or smaller amount of starch produced in these leaves may be only a measure of, and not a factor in, the result.