Regeneration

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.

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.

It is not necessary to separate completely portions of the stem in order to produce roots near one end and shoots near the other. If a ring, including the cambium layer, is cut from the piece, as indicated in Fig. 32, C, the part above and the part below act independently of each other, and each behaves as a separate piece. In various other ways the same result may be obtained, as by simply making an incision in the stem at one side, or by partially splitting off parts of the stem (Fig. 34, C).

If instead of a piece of the stem, a piece of a root is removed, the results are as follows.[31] It should be remembered that the basal end of a root is the part nearer the stem, the apex is the part nearer the apex of the root. If pieces of the root of the poplar, Populus dilatata, are suspended vertically (Fig. 32, D) in a moist chamber, a covering of new cells, a callus, appears over the cut-ends. From the basal callus numerous leaf-shoots may develop. Pieces of large roots may produce over a hundred of these shoots from a single basal callus. In some cases adventitious shoots may also arise from the side of the root near the basal end. Roots develop from the callus over the apical end; less often from the sides near the end. If a similar piece of root is suspended with its apical end upward, the new shoots arise as before over the basal end, that is now turned downwards.

The leaves of some plants, as has long been known, are able to produce new plants. The begonias are especially well suited for experiments of this kind. A piece of the stalk of a leaf suspended in a moist atmosphere produces roots near its base. In most cases the opposite end of the stalk, i.e. the end nearest the leaf, putrefies and slowly dies toward the base. Near the base there may arise, before the breaking down of the piece has reached this point, leaf-buds that arise just above the first-formed roots. When these new shoots have reached a certain size they may produce their own roots at or near the base. If, however, a portion of the leaf is left attached to the leaf-stalk (Fig. 35, A), new roots arise near the basal end of the stalk, and later shoots grow out near the point of union of the leaf and its stalk at the point where the veins of the leaf come off. These shoots produce roots of their own near the base, and roots may also appear on the part of the leaf-stalk near its union with the lamina. If a part of the mid-vein, or of any large vein of the leaf, is cut out, leaving 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. Vöchting 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.[32]

Later, Vöchting 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 Vöchting on the regeneration of pieces of the higher plants show that a piece possesses an innate polarity, or “force,” as Vöchting 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 factors 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 studied. This analysis that Vöchting has made of the problem of regeneration is in the highest degree instructive, since it shows how several factors,—some internal, others external,—take a hand in the result; and it is only possible to unravel the problem by combining different experiments carried out in such a manner that one by one the different factors at work are separated.

If a piece of a young stem of Salix viminalis is suspended vertically in a moist atmosphere, with the lower end in water (for ¾ of a centimetre), and the piece kept in the dark, the result is, in the main, the same as when similar pieces are suspended in moist air without coming into contact with water. Roots arise near the base, and shoots near the apex, without regard to which end is in the water.

If the same experiment is repeated in ordinary air, i.e. air not saturated with water, the result is somewhat different. If the twig is suspended vertically, with its apex upward, roots soon appear on the basal end that is in the water, but no roots develop above the water. Small protuberances may appear above the water in the places at which roots would develop if the piece were surrounded by a moist atmosphere, but they do not break through the bark. If the piece is then covered by a jar containing air saturated with moisture, these protuberances may become roots. It is clear, therefore, that the dryness of the air has prevented their development.

If a similar twig is suspended (in the air) with its apex downward, and the lower end in water, root protuberances appear, at first, only around the base, i.e. at the upper end. Under the water, at the apical end, small and weak roots may develop, or may even not appear at all.

These results agree, in the main, with those in which the piece is surrounded by moist air, and give evidence of an inner polarity that is an important factor in the regeneration. The results show that in a piece with the basal end in water and the rest of the piece in the air the tendency to produce roots above the water is suppressed by the dryness of the air. In an inverted piece, however, with the apex in water, the innate tendency to produce roots at the basal end is strong enough to overcome the effect of the dryness of the air to suppress their development. The abundance of water absorbed by the apex of the piece makes the development of the roots possible under these conditions despite the dryness of the air.[33]

There is another factor connected with the submergence of the end of the stem in water that can be shown by putting a longer part of the end under the water. Neither roots, if it is a basal end, nor leaf-buds, if it is an apical end, appear on the deeper parts of the submerged end. This is due, in all probability, to the insufficiency of oxygen in the water, and as a result the buds are prevented from developing.

It can be shown that light has also an influence on the regeneration of pieces, and that it has a stronger influence on some plants than on others. In some plants roots develop only on that side of the stem that is less illuminated. In Lepismium radicans, for instance, adventitious roots are produced by the plant even in dry air. Pieces of the stem can produce roots on either the upper or the lower surface, according to which side is less illuminated. A piece of the stem of this plant that had been kept in the dark produced two roots, one above and one below,—one, therefore, opposed to the direction of the action of gravity, and the other in the direction of that action. Even in pieces of the willow, suspended in a moist atmosphere, roots develop better and over a greater length of the stem on the less illuminated side.

Although the experiments with pieces of young willow-twigs may seem to show that gravity is not a factor in regulating the development of the new parts, the results show in reality only that internal factors have a preponderating influence. By means of another series of experiments it can be shown that gravity does have an influence on the production of the new parts. It is evident that in order to test the action of gravity, pieces must be placed in different positions in relation to the vertical. It will be found, if this is done, that different results are obtained according to the angle that the piece makes with the vertical. If a piece is suspended in a moist atmosphere, with its apical end upward, the smaller the angle that the piece makes with the vertical so much the more are the leaf-buds that develop confined to the upper part of the piece, and so much the more do they develop from all sides of the upper end; conversely, the greater the angle with the vertical, i.e. the more nearly horizontal the position of the piece, so much the more are the leaf-buds that develop found along the upper side of the apical end (as well as around the end). If the piece is placed in a horizontal position, the leaf-buds develop not only around the apex, but they develop along the entire length of the upper surface, best, however, near the apical end.

If similar pieces are suspended in oblique positions, with the basal end upward, different results are obtained. In the preceding experiment the polarity of the piece and gravity act together, while in this experiment their action is opposed. Although there is a great amount of variability in the results, yet the action of gravity is found to have less influence on the result than has the inner polarity, and the influence of the latter is so much greater that the action of gravity is hardly noticeable.

The roots do not show as markedly the influence of gravity as do the leaf-buds, yet Vöchting has found that the position in which they appear varies with the position of the piece with respect to the vertical.

In the preceding cases the rudiments of the leaf-buds and of the roots were probably present in most cases, so that gravity only awakens them into activity. In other forms, as, for instance, in heterocentron, it is possible to show that gravity may even determine the production of new buds. If pieces of the end of a branch, including the growing point, are suspended vertically, some with the apical end upward, others with the basal end upward (Fig. 34, A), the former produce roots only around the base, but in the latter roots appear frequently, not only at the base, but even extending along the stem. They appear not only at the nodes, where pre-formed rudiments may be present, but also in the internodes, where there are no rudiments of roots.

Stems of heterocentron placed in a horizontal position produce a circle of roots around the base, and later, in several cases, roots from the under surface of the stem, both from the nodes and the internodes; but these roots are smaller than those at the base. Those around the base are often longer on the lower side than on the upper side.

Vöchting has also studied the regeneration of pieces of roots of the poplar and of the elm suspended horizontally in a moist chamber. A callus develops from the cambium region of the basal end, and from this a thick bunch of adventitious sprouts grows out. A weak callus may develop on the apical end also, from which a few roots develop. In other cases adventitious shoots are produced also from the apical callus, especially from the upper edge of the callus. The results are variable, but show that at times leaf-shoots may develop from the apical end of the root. It is also singular to find that, while pieces of the root produce new leaf-shoots very readily, yet they often fail to produce new roots, or produce only a few that arise from the apical callus or from the sides near that region. It is difficult to show that gravity has any influence on the result.

Vöchting recognizes another sort of influence that determines the position of new organs on a piece. If a young, growing end of a stem of Heterocentron diversifolium is suspended by two threads in a horizontal position, the ends bend upward as a result of the negative geotropism of the piece. The new roots appear at the base of the piece, and also on the convex side of the bent part of the stem, as shown in Fig. 34, B. The same result can be obtained by forcibly bending a twig, and then tying the ends together, so that it remains in its bent position. If a piece of this sort is suspended in a moist atmosphere, with the bent inner concave side turned upward, the roots appear on the base and at the bend, especially on the under side, both from the nodes and internodes. If now in order to see if gravity takes any part in the result the next piece is suspended with the outer convex side of the bent part turned upward, it is found that many of the pieces produce roots only at the base, but others produce roots also at the bent portion of the stem, but they are fewer than in the last experiment. The roots arise for the most part on the under side of the arch, and only a few arise from the upper part. It is clear that gravity is also one of the factors in the result. Leaf-buds arise in these pieces with the concave side turned upward only near the apex; rarely one may develop on the lower part of the basal end. In pieces with the concave side turned downward the leaf-buds arise for the most part at the apex, but sometimes they appear on the upper part of the basal arm. The results are due to two factors, gravity and an inner “force” that is supposed to be the resultant of a growth phenomenon taking place in the bent portion. Vöchting supposes that a process of growth takes place as a result of the bending; “the plasma streams to this region, and a new development takes place here more easily.” Vöchting adds that this view will not explain the morphological character of the new organs, and that this must be due to quite other causes. The results may, I venture to suggest, find a simpler explanation as the result of the bending, disturbing the tensions of the protoplasm, causing the two arms of the piece to act as if they had been separated from each other. This idea is more fully developed in a later chapter.

Sachs has criticised Vöchting’s general conclusion in regard to the internal factors that determine the regeneration in a piece of the stem of a plant. He gives very little weight to the innate polarity of the piece, and attempts to explain the results as due to certain substances in the stem of such a sort that, accumulating in any region, they determine the kind of regeneration that takes place. Sachs also assumes that gravity acts on these substances in such a way that the root-forming substances flow downward and the shoot-forming substances flow upward. In a piece of a stem, the two formative substances contained in it accumulate at the two ends, and determine the kind of regeneration that takes place. It is evident that Sachs’ hypothesis fails to explain the method of regeneration of an inverted piece suspended in a vertical position, since the roots appear at the upper end and the shoots at the lower end. Sachs explains this as the result of the previous action of gravity on the piece, while the piece was a part of the tree and stood in a vertical direction. He supposes the longer time that gravity has acted on the piece has determined its basi-apical directions, so that this influence is shown in the inverted piece, rather than the action of gravity on it in its new position. This conception involves quite a different idea from the original one of formative substances flowing in definite directions. Moreover, Vöchting has met this interpretation by using the twigs of the weeping willow, that hang downward on the tree. If gravity has acted on these drooping twigs in the way that Sachs supposes it can act, then we should expect to find, if Sachs’ view is correct, that roots would develop at the apical end of a piece of the twig, and leaves at the basal end, if the piece is hung vertically with its basal end (i.e. the end originally nearer the trunk of the tree) upward. The regeneration of these pieces shows, however, that they behave in the same way as do pieces of twigs that have always stood vertically on the tree. There can be, therefore, no doubt that the distinction between base and apex is an expression of some innate quality of the plant itself. That an external factor, gravity, is also a factor in the regeneration of the pieces, is abundantly shown by the experiments of Vöchting and others, but that innate factors are also at work cannot be doubted. We find evidence in many animals of a similar difference between the two ends of a piece, and we speak of this difference between the anterior and posterior ends of a piece as its polarity. What this polarity may be we do not know, and it is even doubtful whether we should be justified in speaking of it as a force in the sense that the difference in the ends of a magnet is the result of a magnetic force. The kind of polarity shown by animals and plants does not seem to correspond to any of the so-called forces with which the physicist has to deal, but a further discussion of this question will be deferred to a later chapter.

The preceding account of regeneration in some of the higher plants has shown that their usual method of regeneration is by means of latent buds that are present along the sides of the stem, or by means of adventitious buds that develop anew along the sides of the stem. In a few cases new buds may develop from the new tissue of the callus that forms over the cut-ends, but in such cases the new shoots, or the new roots, are much smaller in diameter than the end from which they arise, and usually several or many new shoots develop on the same callus. In these respects the regeneration of the higher plants is different from that of the higher animals, for, in the latter, the new part arises from the entire cut-surface. This difference is no doubt connected with differences in the normal method of growth in plants and in animals, and an explanation of the growth would, perhaps, also give an explanation of the mode of regeneration. The normal method of growth in higher plants takes place largely by the formation of lateral buds, as well as by terminal growth, and we find that regeneration takes place in most cases from the same lateral buds or from others of a similar kind that develop after the piece has been separated.

It is sometimes stated that the higher plants do not regenerate at the cut-ends, because they produce buds at the sides. The statement implies that there is some sort of antagonism between the regeneration of a bud at the end, and the development of buds at the side. It may be true that the development of a latent bud at the side might suppress the tendency to produce a bud at the end, if such a tendency exists; but if we remove the lateral, pre-formed buds, new ones develop at the sides, and not at the end. That there need not be an antagonism between the formation of a bud, or of buds, at the end, and also at the sides, is shown in Vöchting’s experiments with the roots of the poplar. In these, leaf-shoots and root-shoots developed both from the callus over the cut-end, and at the side of the piece also. It has further been shown that, although a piece of the internode does not produce new leaf-buds at the sides, neither does it regenerate a new apical bud at the end.

A most interesting fact connected with the regeneration of the higher plants is, as has been pointed out, that even when a callus is formed over the cut-end, and new growth takes place from this callus, there is produced, not a single terminal bud, but a number of separate buds. The piece does not complete itself, but produces new buds, that make new branches. The explanation of this mode of regeneration in plants is not known. It appears to be connected with the production, by means of buds, of all the new structures. Why this should occur we do not know, and the only suggestion that offers itself is that the result may be in some way connected with the hard cell walls in plants that make difficult the organization of large areas into a new whole. As a result, the new development takes place in a small group of similar cells, that are sufficiently near together to organize themselves into a whole despite the interference met with in the cell walls.

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.

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.

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.

In this same connection another question needs to be discussed. It is assumed by several botanists that in a normal plant the latent shoots or buds along the stem do not develop so long as the terminal shoots are growing, because the latter use up all the food material that is carried to that region. If the terminal bud is destroyed the lateral shoots then burst forth, in consequence, it is assumed, of the excess of food stuff that now comes to them. I do not believe that the phenomena can be so easily explained. If a piece of a plant is cut off, the leaves removed, and the piece suspended in a moist chamber and kept in the dark, the lateral buds at the apex will begin to develop. If we assume that the piece cannot develop any new food substance in the dark, then it contains just the same amount as it did while a part of the plant, and yet that amount is ample for the development of the lateral buds. Moreover, only the more apical buds develop; but if the piece is then cut in two, the apical buds of the basal piece, that had remained undeveloped, will now develop. How can this be explained by the amount of food substances in the piece? If it is assumed that in the normal plant the food substances flow only to the growing points, and the buds are out of the main current and fail in consequence to develop, it can be shown that this idea also fails to explain certain results. Vöchting has found, for example, that if an incision is made below a bud and the piece containing the bud be lifted up somewhat from the rest of the piece, remaining attached only at its anterior end, the bud will begin to develop. In this case the conditions preclude an accumulation of food substances in the piece, and the bud is even farther removed than at first from the main current, yet it begins to develop.

We shall find, I think, that the idea of food stuffs fails to explain some of the simplest phenomena, and while it need not be denied that under certain conditions the presence or accumulation of food materials may produce certain definite results, yet such food stuffs seem to play a very subordinate part as compared with certain other internal or innate factors.

CHAPTER V

REGENERATION AND LIABILITY TO INJURY

Réaumur in 1742 pointed out that regeneration is especially characteristic of those animals whose body is liable to be broken, or, as in the earthworm, subject to the attacks of enemies. Bonnet (1745) thought that such a connection exists as has just been stated, and that the animals that possess the power of regeneration have been endowed with germs set aside for this very purpose. He further believed that there would be in each animal that regenerates as many of these germs as the number of times that it is liable to be injured during its natural life. Darwin in his book on Animals and Plants under Domestication says: “In the case of those animals that may be bisected, or chopped into pieces, and of which every fragment will reproduce the whole, the power of regrowth must be diffused throughout the whole body. Nevertheless, there seems to be much truth in the view maintained by Professor Lessona[36] that this capacity is generally a localized and special one serving to replace parts which are eminently liable to be lost in each particular animal. The most striking case in favor of this view is that the terrestrial salamander, according to Lessona, cannot reproduce lost parts, whilst another species of the same genus, the aquatic salamander, has extraordinary powers of regrowth, as we have just seen; and this animal is eminently liable to have its limbs, tail, eyes, and jaws bitten off by other tritons.”

Lang, referring to the brittleness of the tails of lizards, points out that this is a very useful character, since the bird of prey that has struck at a lizard gets hold of only the last part of the animal to disappear under cover; the lizard escapes by breaking off its tail. The brittleness of the tail is, therefore, an adaptive character that has become fixed by long inheritance.

To this example may be added that of certain land snails in the Philippine Islands. The individuals of the genus helicarion live on trees in damp forests, often in great droves. They are very active, and creep with unusual swiftness over the stems and leaves of the trees. Semper has recorded that all the species observed by him have the remarkable power of breaking off the tail (foot) close behind the shell, if the tail is roughly grasped. A convulsive movement is made until the tail comes off, and the snail drops to the ground, where it is concealed by the leaves. Semper adds that in this way the snails often escaped from him and from his collectors, leaving nothing behind but their tails. The tail is said to be the most obvious part of the animal, and it is assumed that this is, therefore, the part that a reptile or bird would first attack.[37] Lang states that in this case external influences have produced an extraordinarily well-developed sensitiveness in the animal, so that it reacts to the external stimulus by voluntarily throwing off the tail. It would be, of course, of small advantage to be able to throw off the tail unless the power of regenerating the lost organ existed, or was acquired at the same time as the extreme sensitiveness that brings about the reaction. Lang does not state, however, explicitly that he believes the regenerative power to have arisen through the exposure of the tail of the lizard and the tail of the snail to injury, although he thinks that the mechanism by means of which these parts are thrown off has been acquired in this way. Several other writers have, however, used these same cases to illustrate the supposed principle of liability to injury and power of regeneration.

Weismann in his book on The Germ Plasm has adopted the principle of a connection between regeneration and liability to injury and has carried it much farther than other writers. We can, therefore, most profitably make a careful examination of Weismann’s position. His general idea may be gathered from the following quotation:[38] “The dissimilarity, moreover, as regards the power of regeneration in various members of the same species, also indicates that adaptation is an important factor in the process. In proteus, which in other respects possesses so slight a capacity for regeneration, the gills grow again rapidly when they have been cut off. In lizards again this power is confined to the tail, and the limbs cannot become restored. In these animals, however, the tail is obviously far more likely to become mutilated than are the limbs, which, as a matter of fact, are seldom lost, although individuals with stumps of legs are occasionally met with. The physiological importance of the tail of a lizard consists in the fact that it preserves the animal from total destruction, for pursuers will generally aim at the long trailing tail,[39] and thus the animal often escapes, as the tail breaks off when it is firmly seized. It is, in fact, as Leydig was the first to point out, specially adapted for breaking off, the bodies of the caudal vertebræ from the seventh onward being provided with a special plane of fracture so that they easily break into two transversely. Now if this capability of fracture is provided for by a special arrangement and modification of the parts of the tail, we shall not be making too daring an inference if we regard the regenerative power of the tail as a special adaptation, produced by selection, of this particular part of the body, the frequent loss of which is in a certain measure provided for, and not as the outcome of an unknown ‘regenerative power’ possessed by the entire animal. This arrangement would not have been provided if the part had been of no, or of only slight, physiological importance, as is the case in snakes and chelonians, although these animals are as highly organized as lizards. The reason that the limbs of lizards are not replaced is, I believe, due to the fact that these animals are seldom seized by the leg, owing to their extremely rapid movements.” Overlooking the numerous cases of the regeneration of internal organs that have been known for several years, and basing his conclusion on a small, unconvincing experiment of his own on the lungs of a few salamanders, Weismann concludes: “Hence there is no such thing as a general power of regeneration; in each kind of animal this power is graduated according to the need of regeneration in the part under consideration; that is to say, the degree in which it is present is mainly in proportion to the liability of the part to injury.”

After arriving at this conclusion the following admission is a decided anticlimax: “The question, however, arises as to whether the capacity of each part for regeneration results from special process of adaptation, or whether regeneration occurs as the mere outcomewhich is to some extent unforeseen—of the physical nature of an animal. Some statements which have been made on this subject seem hardly to admit of any but the latter explanation.” After showing that some newts confined in aquaria attacked each other, “and several times one of them seized another by the lower jaw, and tugged and bit at it so violently that it would have been torn off had I not separated the animals,”[40] and after referring to the regeneration of the stork’s beak, Weismann concludes: “Such cases, the accuracy of which can scarcely be doubted, indicate that the capacity for regeneration does not depend only on the special adaptation of a particular organ, but that a general power also exists which belongs to the whole organism, and to a certain extent affects many and perhaps even all parts. By virtue of this power, moreover, simple organs can be replaced when they are not specially adapted for regeneration.” The perplexity of the reader, as a result of this temporary vacillation on Weismann’s part, is hardly set straight by the general conclusion that follows on the same page: “We are, therefore, led to infer that the general capacity of all parts for regeneration may have been acquired by selection in the lower and simpler forms, and that it gradually decreased in the course of phylogeny in correspondence with the increase in complexity of organization; but that it may, on the other hand, be increased by special selective processes in each stage of its degeneration, in the case of certain parts which are physiologically important and are at the same time frequently exposed to loss.”

There are certain statements of facts in the same chapter that are incorrect, and the argument is so loose and vague that it is difficult to tell just what is really meant. As a misstatement of fact I may select the following case: It is stated that lumbriculus does not have the power of regenerating laterally if cut in two, and it is argued that a small animal of this form could rarely be injured at the side without cutting the animal completely in two. As a matter of fact, lumbriculus can regenerate laterally, and very perfectly, as any one can verify if he takes the trouble to perform the experiment; but, of course, if the whole animal is split in two lengthwise the pieces die, or if a very long piece is split from one side the remaining piece usually disintegrates. If, however, the anterior end is split in two for a short distance, or if a piece is partially split in two, the half remaining in contact with the rest of the piece completes itself laterally. The same result follows also in the earthworm.

As an example of looseness of expression I may quote the following from Weismann: “A useless or almost useless rudimentary part may often be injured or torn off without causing processes of selection to occur which would produce in it a capacity for regeneration. The tail of a lizard again, which is very liable to injury, becomes regenerated because, as we have seen, it is of great importance to the individual and if lost its owner is placed at a disadvantage.” And as an example of vagueness, the following statement commends itself: “Finally the complexity of the individual parts constitutes the third factor which is concerned in regulating the regenerative power of the part in question; for the more complex the structure is, the longer and the more energetically the process of selection must act in order to provide the mechanism of regeneration, which consists in the equipment of a large number of different kinds of cells with the supplementary determinants which are accurately graduated and regulated as regards their power of multiplication.”

Without attempting to disentangle the ideas that are involved in these sentences, let us rather attempt to get a general conception of Weismann’s views. In a later paper (1900), in reply to certain criticisms, he has stated his position somewhat more lucidly. In the following statement I have tried to give the essential part of his hypotheses: Weismann believes the process of regeneration to be regulated by “natural selection”; in fact, he states that it has arisen through such a process in the lower animals—since they are more subject to injury—and that it has been lost in the higher forms except where, on account of injury, it has been retained in certain parts. Thus when Weismann speaks of regeneration as being an adaptation of the organism to its environment, we must understand him to mean that this adaptation is the result of the action of natural selection. We should be on our guard not to be misled by the statement that because regeneration is useful to the animal, it has been acquired by natural selection, since it is possible that regeneration might be more or less useful without in any way involving the idea that natural selection is the originator of this or of any other adaptation. It will be seen, therefore, that in order to meet Weismann on his own ground it will be necessary to have a clear understanding in regard to the relation of regeneration to Darwin’s principle of natural selection. With Weismann’s special hypothesis of the “mechanism,” so-called, by which regeneration is made possible we have here nothing to do, but may consider it on its own merits in another chapter.

In order to have before us the material for a discussion of the possible influence of natural selection on regeneration, let us first examine the facts that bear on the question of the liability of the parts to injury and their power to regenerate, and in this connection the questions concerning the renewal of parts that are thrown off by the animals themselves in response to an external stimulus are worthy of careful consideration. A comparison between the regeneration of these parts with that of other parts of the same animal gives also important data. Furthermore, a comparison may be made between different parts of the same animal, or between the same parts of different animals living under similar or dissimilar conditions.

There are only a few cases known in which a systematic examination has been carried out of the power of regeneration of the different parts of the body of the same animal. Spallanzani’s results show that those salamanders that can regenerate their fore legs can regenerate their hind legs also. Towle, who has examined in my laboratory the regeneration of a number of American newts and salamanders, finds also that both the fore and hind legs regenerate in the same forms. The tail and the external gills, in those newts with gills, also regenerate. It has also been shown in triton that the eye regenerates if a portion of the bulb is left. Broussonet first showed (1786) that the fins of fish have the power to regenerate, although, strangely enough, Fraisse and Weismann state that very little power of regeneration is present in the fins of fish. I have found that the fins of several kinds of fish regenerate, belonging to widely different families.[41] In Fundulus heteroclitus I have found that the pectoral, pelvic, caudal, anal, and dorsal fins have the power of regeneration. In reptiles the feet do not regenerate,—at least no cases are known,—but the tail of lizards has this power well developed. In birds neither the wings nor the feet regenerate, but Fraisse has described the case of a stork in which, the lower jaw being broken off, and the upper being cut off at the same level, both regenerated. Bordage has recorded the regeneration of the beak of the domesticated fighting cocks (of the Malay breed) of Mauritius. In the mammals neither the legs, nor the tail, nor the jaws regenerate, although several of the internal organs, as described in the next chapter, have extensive powers of regeneration.

The best opportunity to examine the regenerative power in similar organs of the same animal is found in forms like the crustacea, myriapods, and insects, in which external appendages are repeated in each or many segments of the body. In decapod crustacea, including shrimps, lobsters, crayfish, crabs, hermit-crabs, etc., regeneration takes place in the walking legs of all the forms that have been examined, and this includes members of many genera and families. I have made an examination of the regeneration of the appendages (Fig. 37) of the hermit-crab. In this animal, which lives in an appropriated snail’s shell, only the anterior part of the body projects from the shell. The part that protrudes is covered by a hard cuticle, while the part of the body covered by the shell is quite soft. Three pairs of legs are protruded from the shell. The first pair with large claws