REGENERATION IN PLANTS
The series of experiments that Vöchting has carried out on the
regeneration of the higher plants are so much more complete than all
previous experiments, and his analysis of the problems concerning the
factors that influence regeneration is so much more exact than any other
attempts in this direction, that we may profitably confine our attention
largely to his results. Many of his experiments were made with young
twigs or shoots of the willow (salix), which, after the removal of the
leaves, were suspended in a glass jar containing air saturated with
water. Under these circumstances the pieces produced new shoots from the
buds (leaf-buds) that are present near the point at which the leaves
were attached, and new roots, in part from root-buds, that are also
present on the stem.
If the piece is suspended in a vertical position with its _apex upward_
(Fig. 32, _A_), small swellings appear after three or four days near the
lower, _i.e._ the basal, end of the piece. These break through quickly
and grow out as roots. If a leaf-bud is present near the basal end of
the piece, the first roots appear at the side of or under this; later
others appear around the same region. The first roots to appear under
these conditions come from pre-formed root rudiments, the others are, in
part at least, new, adventitious roots. If the lower end of the cut is
made through the lower part of a long internode, _i.e._ just _above_ a
bud, the roots appear as a rule only near the cut-end, and few if any of
the roots develop at the first bud above this region. In many cases
there is formed over the basal cut-surface, in the region of the
cambium, a thickening, or callus, and not infrequently from this also
one or more roots may develop. The direction taken by the new roots is
variable, being sometimes downward, sometimes more or less nearly at
right angles to the stem.
While these changes have been taking place at the base, the leaf-buds at
the apical end have begun to develop. One, two, three, four, or even
five of the higher buds begin to elongate, the number and extent of
development depending on the length of the piece. The topmost or apical
bud grows fastest, and the others grow in the order of their position.
In the region below the lowest bud that develops there may be one or
more buds that do not grow; but if the piece is cut in two just above
these buds, they will then grow out.
[Illustration: 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.
[Illustration: 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.
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.
[Illustration: FIG. 34.--After Vöchting. _A._ End of a piece of
_Heterocentron diversifolium_. Apex downward. _B._ Piece of same bent
and suspended “with concave-side upward.” _C._ Piece of a stem of _Salix
viminalis_. Apex upward. A piece of the side has been lifted up and two
wedges inserted.]
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.
[Illustration: FIG. 35.--After Vöchting. _A._ Leaf-stalk of _Begonia
rex_ with a portion of the lamina. Suspended with base upward. _B._
Piece of lamina of leaf of same. _C._ Leaf of _Heterocentron
diversifolium_. _D._ Leaf-stalk of _Begonia discolor_.]
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, C_²).
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.
[Illustration: 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.
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.