ON PLASMOLYSIS.
151
On Plasmolysis and its bearing upon the Relations between Cell Wall and Protoplasm.
By
F. O. B o w e r , M.A.,
Lecturer on Botany at the Normal School of Science, South Kensington.
(From the Jodrell Laboratory, Royal Gardens, Kew.)
With Plate VIII.
IT is not surprising that, after devoting their efforts for so
long to the study of the nucleus, botanists should again turn
their attention to the cell wall and its relation to the protoplasm. It was only to be expected that by the application of
those accurate methods of study, elaborated during investigations of the nucleus, to the formation and origin of the cell
wall, new results would be obtained. Such expectation has
been amply justified by the works of Dippel, Schmitz, and
Strasburger. The mode of increase of substance of cell walls
by apposition, and more especially the mode of formation of
walls in the first instance in cell division by the lateral coalescence of " microsomata,"1 leads naturally to the supposition
that if there be such a genetic connection between the cell
wall and the protoplasmic body, it would also be possible to
demonstrate that the physical connection between them is very
close. Further, the idea that cells may be connected with one
another by delicate threads of protoplasm, which keep up a
protoplasmic continuity through their cell walls, also presents
itself as a natural corollary on these observations.3 Such con1
Strasburger, ' Ueber den Bau uud das Wachsthum der Zellhaiite,' p. 174.
3
Cf., Strasburger, 1. c, p. 246.
152
F. 0. BOWER.
tinuity has actually been observed by Tangl • in the endosperm
of certain seeds, and by Gardiner 3 in the pitted cells of the
pulvinus of Mimosa, Robinia, and Amicia. Again, Fromman 3
states that he has been able to observe, in various cases, a
continuous network extending from the protoplasm into the
cell wall.
Such observations as these do not harmonise with the view at
present held of plasmolysis, which is derived mainly from the writings of H. de Vries.4 According to his descriptions of the process
of plasmolysis (1. c , pp. 37—39, and esp. pp. 47,48), the protoplasmic body would appear to separate with a " smooth surface"
from the cell wall on treatment with the plasmolysing solution,
and, when the solution is strong enough, to be completely isolated from the cell wall. Hence is derived the idea, which is,
it is true, more often tacitly understood than directly expressed
in words, that the smooth surface of the protoplasmic body is
merely in apposition with the cell wall, and not more closely
connected.
The observations detailed below will tend to show that results
obtained by plasmolysis do not disagree with those obtained by
the direct observations of the above-named authors, i.e. that
the connection of the protoplasmic body with the cell wall is
very close.
Before entering upon the description of my own observations
it would be well shortly to review the chief sources from which
our present information on the subject is derived.
V. Mohl, in his treatise on the vegetable cell, speaking of
the "primordial utricle," remarks that " i t usually adheres
firmly to the cell wall."5 His results were, however, obtained
for the most part by treatment with acids, &c.
1
Pringsh., ' Jahrb.,' vol. xii, p. 170.
'Quart. Journ. Micr. Sci.,' Oct., 1882 ; 'Roy. Soc. Proc./ Nov. 11,1882.
''Beob. fiber Strnctur und Beweg. d. Protoplasma der Pflanzenzellen.'
Jena, 18 SO.
4
'Unters. iiber die Mechanischen Ursachen der Zellstreckung.' Leipzig,
1877.
8
V. Mohl, ' Vegetable Cell,' English translation, p. 37.
:
3
ON PLASMOLYSIS.
158
1
In an early work by Pringsheim a description is given of
the process of plasmolysis, in which (pp. 12, 13) he compares
the separation of the protoplasm from the cell wall with the
separation of a sticky substance from a membrane to which it
had hitherto adhered. He further notices the way in which
the protoplasm remains here and there adherent to the
cell wall, while sometimes, though separated almost entirely
from the wall, it remains connected with it by isolated threads
of protoplasm. He goes on to describe how these strings, afterundergoing various changes of form, finally break off (cf. his
Taf. iii, figs. 16 — 21) . 3 Naegeli (' Pflanzenphysiologische
Untersuchungen,' 1855, Heft 1) also observed and described
strings of protoplasm which connect the contracted proto-.
plasmic body with the cell wall in plasmolysed cells. He
observed them in various instances (epidermis of petals, Spirogyra, & c ) , but did not recognise their appearance as of general
occurrence (cf. his, Taf. i, 2 3 ; Taf. ii, 2 - 6 ; Taf. iii, 4, 5, 12).
H e also notes in Spirogyra that strings are often attached at
corresponding points on opposite sides of the wall, but leaves
it an open question whether this is significant or not.
Hofmeister3 describes the appearance of the contracted protoplasm of cells with large vacuoles (p. 8, &c.) as lying free in
the cell cavity, but makes no mention of any connecting protoplasmic strings as of general occurrence, though (p. 15) he
notices the occurrence of such strings connecting the contracted protoplasm of cells of certain Algse with the terminal
walls.
It is to H . de Vries 4 that we owe the most extended treatment
of the subject of the action of dehydrating reagents upon the
1
'Ban und Bildung der Piianzenzelle,' 1854.
Erom his description and figures, I conclude that Pringslieim has onljr
seen.the coarser strings to be described below. As 1 there point out, however, the difference between these and the finer strings, which appear to have
escaped his observation, is only one of degree.
3
' Die Pflanzenzelle,' 1867.
4
' Unters. iiber die Mechanischen Ursachen der Zellstreckung.' Leipzigj
1S77.
2
154
F. 0. BOWER.
living cell. We may leave on one side the very valuable
conclusions as to the connection between turgescence and
growth, which he obtained by the use of plasmolysis, since
these fall outside my present subject. It is unfortunate that
the importance of these conclusions made him lose sight of the
structural details, which had already been partially observed
by Naegeli and Pringsheim. He even quotes (p. 38) the description of the latter word for word, though in the text he
repeatedly ignores his results, speaking of the contracted protoplasm as free on all sides (" allseitig frei," pp. 9, 38, &c).
His figures (p. 35) also represent the contracted protoplasm as
completely disconnected from the wall, with which it was
originally in contact.
The results of these earlier observations being thus but little
taken into account in what is certainly the most important of
the more recent works on this subject, it was only natural that
for a time no further advance should be made. The description
of de Vries and his figures were adopted in text-books subsequently written, and, as far as I know, there has been no
further statement on this subject1 till Gardiner, in a notice
communicated to the Royal Society (Nov. 11, 1882), described
observations on plasmolysis of cells of the pulvinus of
R o b i n i a pseud acacia, which were made in connection with
his work " On the Continuity of Protoplasm in the Motile
Organs of Leaves." 3 He also extended his observations to
pulvini of a number of other plants, and also to stems and
roots. He found that in a very great number of cases strings
of protoplasm connect the contracted protoplasmic body with
the cell wall. His attention was naturally attracted to the
relation of these strings to the pits, and he found that " i n
several well-defined instances many threads do go to pits, and
also that in two adjoining cells many threads on different sides
of the common cell wall are exactly opposite one another."
Before these observations of Gardiner were published, and
1
The matter seems to have been entirely overlooked by Pfeffer in his
• Osmotische Untersuchungen,' and in his ' Pflanzenphysiologie.'
s
' Quart. Journ. Micr. Sci.,' 1882.
ON PLASMOLYSIS.
155
quite independently of them, I had already arrived at conclusions
in the main similar to them, as the result of observations on
the plasmolysis of the prothalli of ferns, which were instituted
with a very different object, viz. that of finding whether plasmolytic contraction of the protoplasmic body would be a good
method for preparing the apical region of the prothallus, so as
to show the form and arrangement of the individual cells. I t
was impossible to overlook the fact that strings of protoplasm
are very universally to be seen connecting the contracted
protoplasm with the cell wall in cells of prothalli thus
prepared.
For the reasons which determined the choice of De Vries
(1. c , pp. 7—13) I have adopted as the dehydrating agent solutions
of common salt of varying strength, from I per cent, to 10 per
cent., according to the requirements of the object under treatment. It has been my practice to use as weak a solution as
will suffice to bring about the desired result, and it will be seen
that in the majority of cases solutions varying from 2 per cent,
to 5 per cent, have proved strong enough. As changes in the
appearance of the protoplasm follow slowly upon its contraction
the time at which certain appearances are presented is usually
given. The following are the details of the experiments :
PKOTHALI/US OF NEPHRODIUM VILLOSDM AND ASPIDIUM
FlLIX-MAS.J
On treating a prothallus of either of the above species (others
have not been examined) with a 2 per-cent. solution of common
salt, the protoplasmic body in each cell is seen to separate itself
gradually from the cell wall, the process beginning as a rule
at the corners of the cells. The contraction goes on slowly for
a considerable time, and usually results in the protoplasmic
body assuming a more or less regular spherical form, as has
been frequently described by former writers. When stronger
solutions are used the contraction is more rapid but usually
less regular.
When the protoplasm first contracts in this way there is
156
F. 0. BOWER.
often little or no visible connection remaining between it and
the cell wall. Frequently, however, there is to be seen from
the first a faint silky striation in the space between the protoplasmic body and the cell wall, running in a radiating manner
between them. This is in most cases extremely delicate, and
even with Zeiss, obj. F, it is sometimes impossible to define the
appearance as any distinct system of lines. Again, in other
instances coarser threads, the outlines of which can readily be
made out with high powers, are seen from the first to maintain^
a connection between the cell wall and the contracted protoplasm. On these coarser threads are often to be seen nodal
thickenings, similar to those described by Gardiner. Though
the above difference is easily recognised under the microscope,
there can be little doubt that the appearances are merely
phases of one and the same phenomenon.
In those cases where there is at first no visible trace of a
connection between the protoplasm and the cell-wall, (here
usually appears, after the lapse of a short time, a striation of
the intervening space similar to that which may often be
observed from the first; while in the latter case the striation
becomes more obvious, and after a short time (e. g. quarter or
half an hour) it may be seen that it is due to the existence of
numerous very delicate threads, which extend from the protoplasmic body to the cell wall. Some idea of the fineness of
these threads in the first instance may be gained from the fact
that they cannot be individually defined even with high power
(F., Zeiss). Fig. 1 represents cells as they appear about a
quarter of an hour after plasmolysis j 1 the threads, being tense
at first, appear quite straight. Some time after plasmolysis
has taken place, and the strings have become more obvious,
they may be seen to be executing rapid and more or less irregular vibratory movements; these show that they are not then
very tightly stretched. The strings run not only to those
walls which separate contiguous cells, but also to the free
marginal walls as represented in the figure, and further, as
may be ascertained by careful focussing, to the walls which
1
Compare Pringsheim's Og. 16, Tuf. iii, 1. c.
ON PLASMOLYSIS.
157
fdrm the upper and lower surfaces of the prothallus. As far as
I was able to judge, they run as a rule in just as large
numbers to the free walls as to the walls separating contiguous cells.
I t may often be seen that strings appear to cross one
another, as in the lower cell in fig. 1. This appearance may
be explained by reference to the protoplasmic body, which will
in such cases be found to have contracted irregularly. I t may
also be seen that strings, which thus cross one another, are
not in the same plane, a conclusion which might easily be
drawn from fig. 1. W h e r e , as in other cells of fig. 1, the contraction goes on more regularly, such crossing of the strings is
not seen.
Remembering Strasburger's obse'rvations on the formation of
the walls in cell division, as well as the results obtained by
Tangl and Gardiner, it was of course a matter of interest to
observe whether these strings in two contiguous cells are
opposite to one another, and thus point to a direct continuity
of protoplasm through the walls, or whether this is not the
case. In many instances it does appear that the strings on
opposite sides of a wall are attached at corresponding opposite
points ; in a much greater proportion of cases, however, they
appear to have no relation to one another, but to be distributed
quite independently over the walls. I t should be remembered
in connection with this that the strings run with equal frequency to the free walls, and to those separating contiguous
cells.
. Such connection of the contracted protoplasm with the cell
wall, as that above described, is found to exist in the cells
throughout the prothallus. I t has been observed in the cells
at the extreme growing point in young prothalli, and also in
the root hairs at points close to their apex. I n such cells,
however, the phenomenon is not so well marked as in cells of
medium age, the threads being of finer texture.
T h a t these connecting strings consist of protoplasmic substance can hardly be doubted from their mode of origin and
their properties to be detailed b e l o w ; the application of r e r
158
F. 0. BOWER.
agents to them is, however a matter of difficulty, as, under the
action of reagents which injure living protoplasm, they assume
a ropy appearance, and often break away, while they refuse to
take up neutral colouring matters. It has been ascertained
that they stain slightly brown with iodine solution, while they
give a characteristic reaction with gold chloride.1
It has been stated above that the strings, which are at first
as a rule extremely thin, become more obvious a short time
after plasmolysis, there being usually a marked change in the
first quarter of an hour (fig. 11). This is due to an increase
in thickness of the strings, which might be produced by either
of two processes, or by both simultaneously—(1) by the supply
of fresh substance from the main protoplasmic body; (2) by
the lateral coalescence of two or more originally separate
strings.
Exact observation shows that the first process does take part
in the change. It has been noted above that nodal swellings
are sometimes to be found on the threads. By fixing the
pointer of an indicating eyepiece upon one of these swellings,
on a thread of a recently plasmolysed cell, and watching it for
a period of a quarter of au hour or more, it has been seen and
verified in a number of instances that the nodal swelling moves
slowly from the main mass of protoplasm. Since the motion
is, as far as my observations go, always from the main mass of
protoplasm, we have thus an indication of the supply of fresh
substance from it to the threads, which may account for the
increasing prominence of the latter. The lateral vibratory
motion, which is seen in the strings some time after plasmolysis, but is not so marked or is absent immediately after the
contraction, has been alluded to above. From these movements it is inferred that the strings, though apparently
tightly stretched at first, become gradually slacker as time
goes on, a conclusion which harmonises with the observations
1
The method adopted was as follows : after plasmolysis with 3 per-cent
salt solution treat with a solution containing 3 per cent, salt and 1 per-cent.
gold chloride, then wash with water and expose to the light in very dilute
acetic acid.
ON PLASMOLYS1S.
159
on the movement of the nodal swellings from the main mass
of protoplasm. It may then be inferred that fresh substance
is derived from the main mass of protoplasm after the original
plasmolytic contraction.
The question still remains, whether the increase in prominence of the strings may not in part be due to lateral coalescence of originally separate strings. I have no direct
evidence that such coalescence does occur. Branching strings,
such as those represented in figs. 2, B, and 3, A, B, are often to
be found, which might appear to give colour to the idea that
the branches had originally been separate, and had subsequently
coalesced. It is, however, as far as my observations go, a
universal rule that the branching is in the direction of the
cell wall. This being the case, and taking into account the
process of drawing out of fresh material from the main mass
of protoplasm as above described, the following is a more probable explanation of such branchings. That two strings (or
more), originally separate but attached to the main body of
protoplasm at points very close to one another, had drawn out
from that body a common string on which they appear as
branches. Direct evidence that such a process does take place
is afforded by such objects as are represented in fig. 3, A and B.
In A are seen numerous strings, branched and unbranched, as
they appeared twenty minutes after plasmolysis. B represents
the same cell half an hour later; only one of the most prominent branched strings is drawn; on comparing it with the
corresponding string in A it will readily be seen that the
change of appearance points to a process such as that above
suggested. The instances of branching, which are represented
in the figures, are only the last and roughest examples of the
process above described. On examining cells of prothalli
soon after plasmolysis with a high power (Hartnack, 13),
it was seen that not uncommonly strings, which appeared
single throughout the greater part of their length, branched
close to the cell wall, and were thus attached at a number
of points.
It would appear, then, that the change, which gradually
160
F. 0. BOWER.
comes over the strings after plasmolysis, is due, at least in a
great measure, to a drawing out of fresh substance from the
main protoplastic body, and a consequent thickening of the
individual strings, which at the same time become less tightly
stretched. It is not, however, asserted that lateral coalescence
of strings never occurs; it is only to be expected that in their
rapid vibratory movements strings should come into contact
with one another and remain coherent, but this has not been
directly observed.
It has been stated' that before reagents, which are liable to
injure living protoplasm, the strings alter their appearance,
become ropy and slack, and often break away.
Similar
changes occur after plasmolysis has been continued for a long
time, and death supervenes in the plasniolvsed cells (cf.
de Vries, I.e., p. 66, &c). In dead cells the contracted protoplasm is completely isolated, or only connected with the walls
by a few ropy strings, which differ in general appearance from
those of living cells, and do not show the vibratory movements.
When the strings break away their free ends often execute
irregular movements, while they contract gradually, as described
by Pringsheim and Gardiner, on the one hand to the protoplasm, on the other to the cell wall.
It has already been noted by several observers in various
plants that the protoplasm does not always contract as a single
mass. This is sometimes the case in cells of the prothallus,
the protoplasm dividing into two (or more?) rounded portions;
when this occurs the masses are usually seen to be connected
by strings of protoplasm of rather coarse texture; these are,
doubtless, of a similar nature to those which connect the contracted protoplasm with the cell wall.
In conclusion, it may be noted that the walls separating
contiguous cells of the prothallus of the above species have
not a perfectly smooth surface, but show, after the protoplasm
has receded, slight inequalities in thickness when observed
with a high power.
Such being the results obtained by the study of plasmolysis
of cells of prothalli of ferns, the next step was to see whether
ON PLASMOLYSIS.
161
these phenomena are of general occurrence in vegetable cells,
and more especially whether they are to be observed in those
of which the plasmolysed condition has already been described
by other writers.
It being already late in the year, young flower stalks of
C e p h a l a r i a l e u c a n t h a (the plant used by de Vries) were
not to be had : experiments were, however, made with a 5 percent, salt solution upon sections of young leafy stems of this
plant, with the result that, though the material was not very
favorable, strings of protoplasm, similar to those seen in the
prothallus, were found connecting the contracted protoplasm
with the cell wall in a large number of cells of the cortical,
parenchyma.
Young flower stalks of an allied species ( C e p h a l a r i a
r i g i d a ) were also used: sections were cut through the cortical
parenchyma and treated with 5 per-cent. salt solution. The
same phenomena, as seen in the prothallus, were again reproduced here in their chief features : the strings of protoplasm,
at first not well seen, were quite obvious in the cells after the
lapse of one hour (fig. 4).
The observations of Gardiner on the beet were also verified,
it being found that here, on plasmolysis with 10 per-cent. salt
solution, strings of protoplasm remain connecting the contracted protoplasm with the cell wall. They have frequent
nodal swellings, but the strings are not so numerous nor so
regular as in the prothallus.
Sections of the flesh of a ripe apple were also subjected to
the same treatment with results similar to those obtained in the
beet.
In leaves of V a l l i s n e r i a s p i r a l i s strings, forming a fine
radiating system, are seen some time after plasmolysis with a
5 per-cent. salt solution.
The diaphragms of the intercellular spaces of water plants
supply very good material for the study of the phenomena of
plasmolysis in parenchymatous cells. Those of the petioles
of L i m n o c h a r i s , sp. A p o n o g e t o n d i s t a c h y o n , Alisma
Plantago,
VOL. XXIII.
and
Pontederia (Eichornia)
NJEW SER.
ccerulea>
L
162
F. 0. BOWER.
were used; in all of these the process of plasmolysis was
observed, its main features being the same here as above
described for the prothallus.
Special attention was given to the diaphragms of the petiole
of P o n t c d e r i a (Eichornia) coerulea, which consist of
flattened) polygonal, thin-walled cells, in close contact with one
another, except at the angles where three or more cells meet;
at these points are intercellular spaces, which act as channels
of communication between the cavities above and below the
diaphragm (cf. figs. 5, 6). On treating a transverse section,
including a diaphragm, with 1 per-cent. salt solution, a slight
contraction of the protoplasm takes place, tn a very large
number of cases it is found that the protoplasm first leaves the
wall at those points where two cells are separated from one
another by a thin septum, while it still remains in contact with
the parts of the wall adjoining the intercellular spaces. This
would not be the case, if the connection of the protoplasm
with the septa were more close than with the walls adjoining
intercellular spaces; hence it may be inferred that it is not so.
In fig. 5, which illustrates this, and which was drawn immediately after plasmolysis, there are no strings to be seen running to the cell walls; but when plasmolysis is more complete,
and, after the lapse of a short time, numerous strings may here
be seen, as in other cases (fig. 6). It is found that strings
run both to the septa and to the walls adjoining intercellular
spaces, and no distinction in the numbers which run to these
different parts of the wall has been observed. Comparing this
observation with the fact that the strings run in as large numbers to the free walls as to the septa in the prothallus, it is
seen that the same inference may be drawn from both cases,
viz. that as far as evidence from plasmolysis goes, the connection of the protoplasm is just as close with the free walls
as with walls separating contiguous cells.
It should be noted that also in P o n t e d e r i a the septa
dividing contiguous cells have not a perfectly smooth surface, though there are no obvious pits in the usual sense of
the term.
ON PLASMOLTSIS.
163
Observations were also made on the cells of the amphigastria
of L u n u l a r i a and M a r c h a n t i a , a 2 per-cent. solution of salt
being found strong enough to induce plasmolysis. The protoplasm in these cells is very meagre; when contracted it was
seen to be connected with the cell wall by a few long, fine
strings of protoplasm.
Filaments of S p i r o g y r a were also treated with salt solutions of various strengths (2, 5, and 10 per cent). The protoplasm of each cell contracts into a rounded mass, usually
leaving the septa entirely, but often remaining in contact with
the lateral walls.. Here also fine strings of protoplasm run
from the contracted mass to the walls, more especially to the
septa. They often have nodal thickenings, and execute
obvious vibratory movements. The phenomenon is better seen
on plasmolysis with 10 per cent, than with weaker solutions,
and even then it is seen only with difficulty.
The above observations having been made upon cells with
approximately smooth walls, the question suggests itself, what
will be the relation of these strings of protoplasm to the
pits in walls where these are present? Peculiar interest is
attached to this question since the publication of the observations of Gardiner on plusmolysis of pitted parenchymatous
cells of the pulvinus of R o b in i a p s e u d a c a c i a , and other
plants, in which he had previously demonstrated the continuity of the protoplasm through the pits.
The leaves of species of T r i c h o m a n e s serve as excellent
material for the study of this point, since the lateral portions
of the lamina consist of a single layer of cells, of which the
. walls separating contiguous cells are thick and have numerous
pits (fig. 7, A, B) ; the walls in these figures are represented as
rather thicker in proportion than they appear in nature. A
10 per-cent. solution of salt was found to give good results.
Here, as in other cases described, there is usually no very
obvious system of strings to be seen immediately after the
contraction of the protoplasm connecting it with the cell wall;
but, as before, the intervening space soon assumes the silky
164
F. 0. BOWER.
striated appearance noted in other objects (fig. 7, A). AS time
goes on the striae become more plain, and resolve themselves
into protoplasmic strings. These were observed to run not
only to the lateral walls separating contiguous cells, but also,
and apparently in equal numbers, to the free walls of the cells,
which are not pitted. As in the prothallus, so here fresh
substance is drawn out from the main mass of protoplasm/ in
this case as thick conical processes (fig. 7, B), which give a
very striking appearance to the whole protoplasmic body about
two hours after plasmolysis. The observations made in the
prothallus as to apparent branching of strings were confirmed
in the behaviour of the strings of protoplasm in these cells.
It being possible after their thickening to trace the individual strings, it could be seen whether they run as a rule or
chiefly to the pits, or whether there is any constant relation
between them and the pits. On examining a large number of
cases I have found that strings of protoplasm often do run to
pits, and that strings from the contracted protoplasm of contiguous cells are often opposite to one another; but that a
much larger proportion of the strings are not opposite to one
another, and run to points on the cell wall where there are no
pits. In other words, I conclude that, in T r i c h o m a n e s
pyxidiferum my observations on plasmolysis give no clue
to there being any special relation of the protoplasm to the
pits. This is, however, no proof that some special relation
does not exist.
Concluding Remarks.
From the above observations it is seen that the connection
between the protoplasm and the cell wall, as shown by plasmolysis in those cases which have been observed, is closer
than is usually described, or at least implied in current
botanical writings. The objects were selected from very different systematic groups ; it is true their number is small, and
it must be admitted that the effect is not visibly produced in
every cell; nevertheless, though it cannot be asserted that the
ON PLASMOLYSIS.
165
phenomenon described is universal, it must at least be admitted that it is very general. I may here suggest that some
difference may be found between the relation of the protoplasm to the cell wall in young and in old cells ; no such difference has been uniformly observed by me, though it has
been alluded to by Naegeli.
It has been repeatedly observed in various instances that
according to the evidence of plasmolysis the connection between the protoplasm and free cell walls is as close as between
the protoplasm and walls separating contiguous cells; also it
has been seen, in the one example investigated in connection
with that point, that there was no evidence to show any
special relation between the protoplasm and pits of an ordinary
parenchymatous tissue [this will of course require confirmation
in other cases]. From these results it may be inferred that
the phenomena observed are due to a close mode of connection between protoplasm and cell wall, which is uniform
wherever they are in contact with one another in the living
cell. In the light of recent observations on the mode of formation and growth of cell walls by apposition and coalescence
of microsomata, the connection thus demonstrated by plasmolysis acquires a special interest. Unfortunately, the kernel of
the whole matter, viz. the ultimate mode of application of the
protoplasm to the cell wall, cannot be arrived at with certainty by plasmolysis, owing to the obvious difficulties of
observation of minute details with high powers in uninjured
cells. Still, collateral evidence may be gained, and as such I
regard the observations above described.
I would suggest two possible explanations of the phenomena
observed in plasmolysis, and their bearing upon the ultimate
mode of application of protoplasm to cell wall—(1) that the
main mass of protoplasm on retreating may leave the cell wall
still completely lined with a thin film of protoplasm; (2) that
the peripheral part of the protoplasm being entangled, as a
network, among the deposited microsomata may, on the con
traction of the main mass, be drawn out at the points of
entanglement, into fine strings like those observed, while the
166
.
F. 0. BOWER.
surface of the wall is for the most part left free, and not
covered by a film of protoplasm.
In the former case the phenomena observed would be
entirely intra-protoplasmic. The process might in fact be
compared with what is seen when two surfaces, having a layer
of a semi-fluid plastic substance, such as canada balsam,
between them, are suddenly separated. Both surfaces remain
covered with a film of the balsam, while between them run
strings of balsam of varying thickness, which are occasionally
branched, and sometimes have nodal thickenings. If the formation of strings io plasmolysis be thus intra-protoplasmic, their
position would, as in the case of the balsam, be mainly determined by the conformation of the surface of the wall, and by
internal determining causes in the plastic substance itself, and
would not throw light on the present question of the mode of
connection between cell wall and protoplasm. I have repeatedly
examined the cell walls of plasmolysed cells, both in surface
views and when seen edgeways, and have not been able to
observe any continuous film of protoplasm covering their surface. Having, however, learned from the experiments above
detailed that protoplasm may be drawn out into strings so thin
as to remain undefined with very high powers, the failure to
observe such films does not prove their absence, as they might
also be exceedingly thin.
Taking the second possible explanation of trie phenomena
into consideration, we have a strong presumption in its favour
from recent observations. In the first place, those of Strasburger on the deposition of microsomata on the cell wall, would
suggest that the protoplasm might be, so to speak, entangled
between these microsomata, and thus be continuous into and
held fast by the cell walls. Thus the attachment would not be
equally close over the whole surface of the wall, but would be
most strong at a number of points where the processes of protoplasm are continued into the body of the wall. Secondly,
Fromman asserts that he has seen a continuous network extending from the protoplasm into the cell wall. Further, we
have evidence that where the protoplasm certainly does pene-
ON PLASMOLYSIS.
167
trate the cell walls (i. e. in sieve plates) it still may retain its
connection with the cell wall after contraction by means of a
number of strings, which run severally to the pores of the
sieve (De Bary, 'Vergl. Anat.,' figs. 12, 75). These, if the
second explanation of the above phenomena were true, would
differ in degree but not in kind from the strings of protophism
observed on plasmolysis. It is probable, from Gardiner's
account (1. a ) , that the same may be the case in the perforated
pits in the cells of the pulvini on which he has worked.
I have already stated that oji careful observation of the
terminal parts of the strings soon after plasmolysis in the protliallus they are often seen to split up close to the cell wall into
fine branches, and that they are thus attached to the cell wall
at a number of points. This observation gives still further
support to the second mode of explanation of these phenomena
of plasmolysis.
Though it is impossible at present to decide with certainty
which of these interpretations of the phenomena is nearer the
truth, the latter seems to me to coincide best with the facts.
It is unfortunately hardly to be anticipated that the phenomena of plasmolysis will yield us any very certain conclusions
as to the ultimate structural relations between cell wall and
protoplasm, since the difficulties are so great in using high
powers on objects at least as thick as one whole cell; and it is
only by the use of high powers that this point can be decided.
We must, therefore, look to the study of fine sections for
further and secure information on this most important
question.