ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OF TRANSMISSION OF
THE MOTOR IMPULSE.
Glands and summits of the tentacles alone sensitive—Transmission of the
motor impulse down the pedicels of the tentacles, and across the blade
of the leaf—Aggregation of the protoplasm, a reflex action—First
discharge of the motor impulse sudden—Direction of the movements of the
tentacles—Motor impulse transmitted through the cellular tissue—
Mechanism of the movements—Nature of the motor impulse—Re-expansion of
the tentacles.
We have seen in the previous chapters that many widely different
stimulants, mechanical and chemical, excite the movement of the
tentacles, as well as of the blade of the leaf; and we must now
consider, firstly, what are the points which are irritable or
sensitive, and secondly how the motor impulse is transmitted from one
point to another. The glands are almost exclusively the seat of
irritability, yet this irritability must extend for a very short
distance below them; for when they were cut off with a sharp pair of
scissors without being themselves touched, the tentacles often became
inflected. These headless tentacles frequently re-expanded; and when
afterwards drops of the two most powerful known stimulants were placed
on the cut-off ends, no effect was produced. Nevertheless these
headless tentacles are capable of subsequent inflection if excited by
an impulse sent from the disc. I succeeded on several occasions in
crushing glands between fine pincers, but this did not excite any
movement; nor did raw meat and salts of ammonia, when placed on such
crushed glands. [page 230] It is probable that they were killed so
instantly that they were not able to transmit any motor impulse; for in
six observed cases (in two of which however the gland was quite pinched
off) the protoplasm within the cells of the tentacles did not become
aggregated; whereas in some adjoining tentacles, which were inflected
from having been roughly touched by the pincers, it was well
aggregated. In like manner the protoplasm does not become aggregated
when a leaf is instantly killed by being dipped into boiling water. On
the other hand, in several cases in which tentacles became inflected
after their glands had been cut off with sharp scissors, a distinct
though moderate degree of aggregation supervened.
The pedicels of the tentacles were roughly and repeatedly rubbed; raw
meat or other exciting substances were placed on them, both on the
upper surface near the base and elsewhere, but no distinct movement
ensued. Some bits of meat, after being left for a considerable time on
the pedicels, were pushed upwards, so as just to touch the glands, and
in a minute the tentacles began to bend. I believe that the blade of
the leaf is not sensitive to any stimulant. I drove the point of a
lancet through the blades of several leaves, and a needle three or four
times through nineteen leaves: in the former case no movement ensued;
but about a dozen of the leaves which were repeatedly pricked had a few
tentacles irregularly inflected. As, however, their backs had to be
supported during the operation, some of the outer glands, as well as
those on the disc, may have been touched; and this perhaps sufficed to
cause the slight degree of movement observed. Nitschke*says
* ‘Bot. Zeitung,’ 1860, p. 234. [page 231]
that cutting and pricking the leaf does not excite movement. The
petiole of the leaf is quite insensible.
The backs of the leaves bear numerous minute papillae, which do not
secrete, but have the power of absorption. These papillae are, I
believe, rudiments of formerly existing tentacles together with their
glands. Many experiments were made to ascertain whether the backs of
the leaves could be irritated in any way, thirty-seven leaves being
thus tried. Some were rubbed for a long time with a blunt needle, and
drops of milk and other exciting fluids, raw meat, crushed flies, and
various substances, placed on others. These substances were apt soon to
become dry, showing that no secretion had been excited. Hence I
moistened them with saliva, solutions of ammonia, weak hydrochloric
acid, and frequently with the secretion from the glands of other
leaves. I also kept some leaves, on the backs of which exciting objects
had been placed, under a damp bell-glass; but with all my care I never
saw any true movement. I was led to make so many trials because,
contrary to my previous experience, Nitschke states* that, after
affixing objects to the backs of leaves by the aid of the viscid
secretion, he repeatedly saw the tentacles (and in one instance the
blade) become reflexed. This movement, if a true one, would be most
anomalous; for it implies that the tentacles receive a motor impulse
from an unnatural source, and have the power of bending in a direction
exactly the reverse of that which is habitual to them; this power not
being of the least use to the plant, as insects cannot adhere to the
smooth backs of the leaves.
I have said that no effect was produced in the above
* ‘Bot. Zeitung.’ 1860, p. 437. [page 232]
cases; but this is not strictly true, for in three instances a little
syrup was added to the bits of raw meat on the backs of leaves, in
order to keep them damp for a time; and after 36 hrs. there was a trace
of reflexion in the tentacles of one leaf, and certainly in the blade
of another. After twelve additional hours, the glands began to dry, and
all three leaves seemed much injured. Four leaves were then placed
under a bell-glass, with their footstalks in water, with drops of syrup
on their backs, but without any meat. Two of these leaves, after a day,
had a few tentacles reflexed. The drops had now increased considerably
in size, from having imbibed moisture, so as to trickle down the backs
of the tentacles and footstalks. On the second day, one leaf had its
blade much reflexed; on the third day the tentacles of two were much
reflexed, as well as the blades of all four to a greater or less
degree. The upper side of one leaf, instead of being, as at first,
slightly concave, now presented a strong convexity upwards. Even on the
fifth day the leaves did not appear dead. Now, as sugar does not in the
least excite Drosera, we may safely attribute the reflexion of the
blades and tentacles of the above leaves to exosmose from the cells
which were in contact with the syrup, and their consequent contraction.
When drops of syrup are placed on the leaves of plants with their roots
still in damp earth, no inflection ensues, for the roots, no doubt,
pump up water as quickly as it is lost by exosmose. But if cut-off
leaves are immersed in syrup, or in any dense fluid, the tentacles are
greatly, though irregularly, inflected, some of them assuming the shape
of corkscrews; and the leaves soon become flaccid. If they are now
immersed in a fluid of low specific gravity, the tentacles re-expand.
From these [page 233] facts we may conclude that drops of syrup placed
on the backs of leaves do not act by exciting a motor impulse which is
transmitted to the tentacles; but that they cause reflexion by inducing
exosmose. Dr. Nitschke used the secretion for sticking insects to the
backs of the leaves; and I suppose that he used a large quantity, which
from being dense probably caused exosmose. Perhaps he experimented on
cut-off leaves, or on plants with their roots not supplied with enough
water.
As far, therefore, as our present knowledge serves, we may conclude
that the glands, together with the immediately underlying cells of the
tentacles, are the exclusive seats of that irritability or
sensitiveness with which the leaves are endowed. The degree to which a
gland is excited can be measured only by the number of the surrounding
tentacles which are inflected, and by the amount and rate of their
movement. Equally vigorous leaves, exposed to the same temperature (and
this is an important condition), are excited in different degrees under
the following circumstances. A minute quantity of a weak solution
produces no effect; add more, or give a rather stronger solution, and
the tentacles bend. Touch a gland once or twice, and no movement
follows; touch it three or four times, and the tentacle becomes
inflected. But the nature of the substance which is given is a very
important element: if equal-sized particles of glass (which acts only
mechanically), of gelatine, and raw meat, are placed on the discs of
several leaves, the meat causes far more rapid, energetic, and widely
extended movement than the two former substances. The number of glands
which are excited also makes a great difference in the result: place a
bit of meat on one or two of the discal [page 234] glands, and only a
few of the immediately surrounding short tentacles are inflected; place
it on several glands, and many more are acted on; place it on thirty or
forty, and all the tentacles, including the extreme marginal ones,
become closely inflected. We thus see that the impulses proceeding from
a number of glands strengthen one another, spread farther, and act on a
larger number of tentacles, than the impulse from any single gland.
Transmission of the Motor Impulse.—In every case the impulse from a
gland has to travel for at least a short distance to the basal part of
the tentacle, the upper part and the gland itself being merely carried
by the inflection of the lower part. The impulse is thus always
transmitted down nearly the whole length of the pedicel. When the
central glands are stimulated, and the extreme marginal tentacles
become inflected, the impulse is transmitted across half the diameter
of the disc; and when the glands on one side of the disc are
stimulated, the impulse is transmitted across nearly the whole width of
the disc. A gland transmits its motor impulse far more easily and
quickly down its own tentacle to the bending place than across the disc
to neighbouring tentacles. Thus a minute dose of a very weak solution
of ammonia, if given to one of the glands of the exterior tentacles,
causes it to bend and reach the centre; whereas a large drop of the
same solution, given to a score of glands on the disc, will not cause
through their combined influence the least inflection of the exterior
tentacles. Again, when a bit of meat is placed on the gland of an
exterior tentacle, I have seen movement in ten seconds, and repeatedly
within a minute; but a much larger bit placed on several glands on the
disc does not cause [page 235] the exterior tentacles to bend until
half an hour or even several hours have elapsed.
The motor impulse spreads gradually on all sides from one or more
excited glands, so that the tentacles which stand nearest are always
first affected. Hence, when the glands in the centre of the disc are
excited, the extreme marginal tentacles are the last inflected. But the
glands on different parts of the leaf transmit their motor power in a
somewhat different manner. If a bit of meat be placed on the
long-headed gland of a marginal tentacle, it quickly transmits an
impulse to its own bending portion; but never, as far as I have
observed, to the adjoining tentacles; for these are not affected until
the meat has been carried to the central glands, which then radiate
forth their conjoint impulse on all sides. On four occasions leaves
were prepared by removing some days previously all the glands from the
centre, so that these could not be excited by the bits of meat brought
to them by the inflection of the marginal tentacles; and now these
marginal tentacles re-expanded after a time without any other tentacle
being affected. Other leaves were similarly prepared, and bits of meat
were placed on the glands of two tentacles in the third row from the
outside, and on the glands of two tentacles in the fifth row. In these
four cases the impulse was sent in the first place laterally, that is,
in the same concentric row of tentacles, and then towards the centre;
but not centrifugally, or towards the exterior tentacles. In one of
these cases only a single tentacle on each side of the one with meat
was affected. In the three other cases, from half a dozen to a dozen
tentacles, both laterally and towards the centre, were well inflected
or sub-inflected. Lastly, in [page 236] ten other experiments, minute
bits of meat were placed on a single gland or on two glands in the
centre of the disc. In order that no other glands should touch the
meat, through the inflection of the closely adjoining short tentacles,
about half a dozen glands had been previously removed round the
selected ones. On eight of these leaves from sixteen to twenty-five of
the short surrounding tentacles were inflected in the course of one or
two days; so that the motor impulse radiating from one or two of the
discal glands is able to produce this much effect. The tentacles which
had been removed are included in the above numbers; for, from standing
so close, they would certainly have been affected. On the two remaining
leaves, almost all the short tentacles on the disc were inflected. With
a more powerful stimulus than meat, namely a little phosphate of lime
moistened with saliva, I have seen the inflection spread still farther
from a single gland thus treated; but even in this case the three or
four outer rows of tentacles were not affected. From these experiments
it appears that the impulse from a single gland on the disc acts on a
greater number of tentacles than that from a gland of one of the
exterior elongated tentacles; and this probably follows, at least in
part, from the impulse having to travel a very short distance down the
pedicels of the central tentacles, so that it is able to spread to a
considerable distance all round.
Whilst examining these leaves, I was struck with the fact that in six,
perhaps seven, of them the tentacles were much more inflected at the
distal and proximal ends of the leaf (i.e. towards the apex and base)
than on either side; and yet the tentacles on the sides stood as near
to the gland where the bit of meat lay as did those at the two ends. It
thus appeared as [page 237] if the motor impulse was transmitted from
the centre across the disc more readily in a longitudinal than in a
transverse direction; and as this appeared a new and interesting fact
in the physiology of plants, thirty-five fresh experiments were made to
test its truth. Minute bits of meat were placed on a single gland or on
a few glands, on the right or left side of the discs of eighteen
leaves; other bits of the same size being placed on the distal or
proximal ends of seventeen other leaves. Now if the motor impulse were
transmitted with equal force or at an equal rate through the blade in
all directions, a bit of meat placed at one side or at one end of the
disc ought to affect equally all the tentacles situated at an equal
distance from it; but this certainly is not the case. Before giving the
general results, it may be well to describe three or four rather
unusual cases.
[(1) A minute fragment of a fly was placed on one side of the disc, and
after 32 m. seven of the outer tentacles near the fragment were
inflected; after 10 hrs. several more became so, and after 23 hrs. a
still greater number; and now the blade of the leaf on this side was
bent inwards so as to stand up at right angles to the other side.
Neither the blade of the leaf nor a single tentacle on the opposite
side was affected; the line of separation between the two halves
extending from the footstalk to the apex. The leaf remained in this
state for three days, and on the fourth day began to re-expand; not a
single tentacle having been inflected on the opposite side.
(2) I will here give a case not included in the above thirty-five
experiments. A small fly was found adhering by its feet to the left
side of the disc. The tentacles on this side soon closed in and killed
the fly; and owing probably to its struggle whilst alive, the leaf was
so much excited that in about 24 hrs. all the tentacles on the opposite
side became inflected; but as they found no prey, for their glands did
not reach the fly, they re-expanded in the course of 15 hrs.; the
tentacles on the left side remaining clasped for several days.
(3) A bit of meat, rather larger than those commonly used, [page 238]
was placed in a medial line at the basal end of the disc, near the
footstalk; after 2 hrs. 30 m. some neighbouring tentacles were
inflected; after 6 hrs. the tentacles on both sides of the footstalk,
and some way up both sides, were moderately inflected; after 8 hrs. the
tentacles at the further or distal end were more inflected than those
on either side; after 23 hrs. the meat was well clasped by all the
tentacles, excepting by the exterior ones on the two sides.
(4) Another bit of meat was placed at the opposite or distal end of
another leaf, with exactly the same relative results.
(5) A minute bit of meat was placed on one side of the disc; next day
the neighbouring short tentacles were inflected, as well as in a slight
degree three or four on the opposite side near the footstalk. On the
second day these latter tentacles showed signs of re-expanding, so I
added a fresh bit of meat at nearly the same spot, and after two days
some of the short tentacles on the opposite side of the disc were
inflected. As soon as these began to re-expand, I added another bit of
meat, and next day all the tentacles on the opposite side of the disc
were inflected towards the meat; whereas we have seen that those on the
same side were affected by the first bit of meat which was given.]
Now for the general results. Of the eighteen leaves on which bits of
meat were placed on the right or left sides of the disc, eight had a
vast number of tentacles inflected on the same side, and in four of
them the blade itself on this side was likewise inflected; whereas not
a single tentacle nor the blade was affected on the opposite side.
These leaves presented a very curious appearance, as if only the
inflected side was active, and the other paralysed. In the remaining
ten cases, a few tentacles became inflected beyond the medial line, on
the side opposite to that where the meat lay; but, in some of these
cases, only at the proximal or distal ends of the leaves. The
inflection on the opposite side always occurred considerably after that
on the same side, and in one instance not until the fourth day. We have
also seen [page 239] with No. 5 that bits of meat had to be added
thrice before all the short tentacles on the opposite side of the disc
were inflected.
The result was widely different when bits of meat were placed in a
medial line at the distal or proximal ends of the disc. In three of the
seventeen experiments thus made, owing either to the state of the leaf
or to the smallness of the bit of meat, only the immediately adjoining
tentacles were affected; but in the other fourteen cases the tentacles
at the opposite end of the leaf were inflected, though these were as
distant from where the meat lay as were those on one side of the disc
from the meat on the opposite side. In some of the present cases the
tentacles on the sides were not at all affected, or in a less degree,
or after a longer interval of time, than those at the opposite end. One
set of experiments is worth giving in fuller detail. Cubes of meat, not
quite so small as those usually employed, were placed on one side of
the discs of four leaves, and cubes of the same size at the proximal or
distal end of four other leaves. Now, when these two sets of leaves
were compared after an interval of 24 hrs., they presented a striking
difference. Those having the cubes on one side were very slightly
affected on the opposite side; whereas those with the cubes at either
end had almost every tentacle at the opposite end, even the marginal
ones, closely inflected. After 48 hrs. the contrast in the state of the
two sets was still great; yet those with the meat on one side now had
their discal and submarginal tentacles on the opposite side somewhat
inflected, this being due to the large size of the cubes. Finally we
may conclude from these thirty-five experiments, not to mention the six
or seven previous ones, that the motor impulse is transmitted from any
single gland [page 240] or small group of glands through the blade to
the other tentacles more readily and effectually in a longitudinal than
in a transverse direction.
As long as the glands remain excited, and this may last for many days,
even for eleven, as when in contact with phosphate of lime, they
continue to transmit a motor impulse to the basal and bending parts of
their own pedicels, for otherwise they would re-expand. The great
difference in the length of time during which tentacles remain
inflected over inorganic objects, and over objects of the same size
containing soluble nitrogenous matter, proves the same fact. But the
intensity of the impulse transmitted from an excited gland, which has
begun to pour forth its acid secretion and is at the same time
absorbing, seems to be very small compared with that which it transmits
when first excited. Thus, when moderately large bits of meat were
placed on one side of the disc, and the discal and sub-marginal
tentacles on the opposite side became inflected, so that their glands
at last touched the meat and absorbed matter from it, they did not
transmit any motor influence to the exterior rows of tentacles on the
same side, for these never became inflected. If, however, meat had been
placed on the glands of these same tentacles before they had begun to
secrete copiously and to absorb, they undoubtedly would have affected
the exterior rows. Nevertheless, when I gave some phosphate of lime,
which is a most powerful stimulant, to several submarginal tentacles
already considerably inflected, but not yet in contact with some
phosphate previously placed on two glands in the centre of the disc,
the exterior tentacles on the same side were acted on.
When a gland is first excited, the motor impulse is discharged within a
few seconds, as we know from the [page 241] bending of the tentacle;
and it appears to be discharged at first with much greater force than
afterwards. Thus, in the case above given of a small fly naturally
caught by a few glands on one side of a leaf, an impulse was slowly
transmitted from them across the whole breadth of the leaf, causing the
opposite tentacles to be temporarily inflected, but the glands which
remained in contact with the insect, though they continued for several
days to send an impulse down their own pedicels to the bending place,
did not prevent the tentacles on the opposite side from quickly
re-expanding; so that the motor discharge must at first have been more
powerful than afterwards.
When an object of any kind is placed on the disc, and the surrounding
tentacles are inflected, their glands secrete more copiously and the
secretion becomes acid, so that some influence is sent to them from the
discal glands. This change in the nature and amount of the secretion
cannot depend on the bending of the tentacles, as the glands of the
short central tentacles secrete acid when an object is placed on them,
though they do not themselves bend. Therefore I inferred that the
glands of the disc sent some influence up the surrounding tentacles to
their glands, and that these reflected back a motor impulse to their
basal parts; but this view was soon proved erroneous. It was found by
many trials that tentacles with their glands closely cut off by sharp
scissors often become inflected and again re-expand, still appearing
healthy. One which was observed continued healthy for ten days after
the operation. I therefore cut the glands off twenty-five tentacles, at
different times and on different leaves, and seventeen of these soon
became inflected, and afterwards re-expanded. The re-expansion
commenced in about [page 242] 8 hrs. or 9 hrs., and was completed in
from 22 hrs. to 30 hrs. from the time of inflection. After an interval
of a day or two, raw meat with saliva was placed on the discs of these
seventeen leaves, and when observed next day, seven of the headless
tentacles were inflected over the meat as closely as the uninjured ones
on the same leaves; and an eighth headless tentacle became inflected
after three additional days. The meat was removed from one of these
leaves, and the surface washed with a little stream of water, and after
three days the headless tentacle re-expanded for the second time. These
tentacles without glands were, however, in a different state from those
provided with glands and which had absorbed matter from the meat, for
the protoplasm within the cells of the former had undergone far less
aggregation. From these experiments with headless tentacles it is
certain that the glands do not, as far as the motor impulse is
concerned, act in a reflex manner like the nerve-ganglia of animals.
But there is another action, namely that of aggregation, which in
certain cases may be called reflex, and it is the only known instance
in the vegetable kingdom. We should bear in mind that the process does
not depend on the previous bending of the tentacles, as we clearly see
when leaves are immersed in certain strong solutions. Nor does it
depend on increased secretion from the glands, and this is shown by
several facts, more especially by the papillae, which do not secrete,
yet undergoing aggregation, if given carbonate of ammonia or an
infusion of raw meat. When a gland is directly stimulated in any way,
as by the pressure of a minute particle of glass, the protoplasm within
the cells of the gland first becomes aggregated, then that in the cells
immediately beneath the gland, and so lower and lower down the
tentacles to their bases;— [page 243] that is, if the stimulus has been
sufficient and not injurious. Now, when the glands of the disc are
excited, the exterior tentacles are affected in exactly the same
manner: the aggregation always commences in their glands, though these
have not been directly excited, but have only received some influence
from the disc, as shown by their increased acid secretion. The
protoplasm within the cells immediately beneath the glands are next
affected, and so downwards from cell to cell to the bases of the
tentacles. This process apparently deserves to be called a reflex
action, in the same manner as when a sensory nerve is irritated, and
carries an impression to a ganglion which sends back some influence to
a muscle or gland, causing movement or increased secretion; but the
action in the two cases is probably of a widely different nature. After
the protoplasm in a tentacle has been aggregated, its redissolution
always begins in the lower part, and slowly travels up the pedicel to
the gland, so that the protoplasm last aggregated is first redissolved.
This probably depends merely on the protoplasm being less and less
aggregated, lower and lower down in the tentacles, as can be seen
plainly when the excitement has been slight. As soon, therefore, as the
aggregating action altogether ceases, redissolution naturally commences
in the less strongly aggregated matter in the lowest part of the
tentacle, and is there first completed.
Direction of the Inflected Tentacles.—When a particle of any kind is
placed on the gland of one of the outer tentacles, this invariably
moves towards the centre of the leaf; and so it is with all the
tentacles of a leaf immersed in any exciting fluid. The glands of the
exterior tentacles then form a ring round the middle part of the disc,
as shown in a previous figure (fig. 4, [page 244] p. 10). The short
tentacles within this ring still retain their vertical position, as
they likewise do when a large object is placed on their glands, or when
an insect is caught by them. In this latter case we can see that the
inflection of the short central tentacles would be useless, as their
glands are already in contact with their prey.
FIG. 10. (Drosera rotundifolia.) Leaf (enlarged) with the tentacles
inflected over a bit of meat placed on one side of the disc.
The result is very different when a single gland on one side of the
disc is excited, or a few in a group. These send an impulse to the
surrounding tentacles, which do not now bend towards the centre of the
leaf, but to the point of excitement. We owe this capital observation
to Nitschke,* and since reading his paper a few years ago, I have
repeatedly verified it. If a minute bit of meat be placed by the aid of
a needle on a single gland, or on three or four together, halfway
between the centre and the circumference of the disc, the directed
movement of the surrounding tentacles is well exhibited. An accurate
drawing of a leaf with meat in this position is here reproduced (fig.
10), and we see the tentacles, including some of the exterior ones,
accurately directed to the point where the meat lay. But a much better
* ‘Bot. Zeitung,’ 1860, p. 240. [page 245]
plan is to place a particle of the phosphate of lime moistened with
saliva on a single gland on one side of the disc of a large leaf, and
another particle on a single gland on the opposite side. In four such
trials the excitement was not sufficient to affect the outer tentacles,
but all those near the two points were directed to them, so that two
wheels were formed on the disc of the same leaf; the pedicels of the
tentacles forming the spokes, and the glands united in a mass over the
phosphate representing the axles. The precision with which each
tentacle pointed to the particle was wonderful; so that in some cases I
could detect no deviation from perfect accuracy. Thus, although the
short tentacles in the middle of the disc do not bend when their glands
are excited in a direct manner, yet if they receive a motor impulse
from a point on one side, they direct themselves to the point equally
well with the tentacles on the borders of the disc.
In these experiments, some of the short tentacles on the disc, which
would have been directed to the centre, had the leaf been immersed in
an exciting fluid, were now inflected in an exactly opposite direction,
viz. towards the circumference. These tentacles, therefore, had
deviated as much as 180o from the direction which they would have
assumed if their own glands had been stimulated, and which may be
considered as the normal one. Between this, the greatest possible and
no deviation from the normal direction, every degree could be observed
in the tentacles on these several leaves. Notwithstanding the precision
with which the tentacles generally were directed, those near the
circumference of one leaf were not accurately directed towards some
phosphate of lime at a rather distant point on the opposite side of the
disc. It appeared as if the motor [page 246] impulse in passing
transversely across nearly the whole width of the disc had departed
somewhat from a true course. This accords with what we have already
seen of the impulse travelling less readily in a transverse than in a
longitudinal direction. In some other cases, the exterior tentacles did
not seem capable of such accurate movement as the shorter and more
central ones.
Nothing could be more striking than the appearance of the above four
leaves, each with their tentacles pointing truly to the two little
masses of the phosphate on their discs. We might imagine that we were
looking at a lowly organised animal seizing prey with its arms. In the
case of Drosera the explanation of this accurate power of movement, no
doubt, lies in the motor impulse radiating in all directions, and
whichever side of a tentacle it first strikes, that side contracts, and
the tentacle consequently bends towards the point of excitement. The
pedicels of the tentacles are flattened, or elliptic in section. Near
the bases of the short central tentacles, the flattened or broad face
is formed of about five longitudinal rows of cells; in the outer
tentacles of the disc it consists of about six or seven rows; and in
the extreme marginal tentacles of above a dozen rows. As the flattened
bases are thus formed of only a few rows of cells, the precision of the
movements of the tentacles is the more remarkable; for when the motor
impulse strikes the base of a tentacle in a very oblique direction
relatively to its broad face, scarcely more than one or two cells
towards one end can be affected at first, and the contraction of these
cells must draw the whole tentacle into the proper direction. It is,
perhaps, owing to the exterior pedicels being much flattened that they
do not bend quite so accurately to the point of excitement as the [page
247] more central ones. The properly directed movement of the tentacles
is not an unique case in the vegetable kingdom, for the tendrils of
many plants curve towards the side which is touched; but the case of
Drosera is far more interesting, as here the tentacles are not directly
excited, but receive an impulse from a distant point; nevertheless,
they bend accurately towards this point.
FIG. 11. (Drosera rotundifolia.) Diagram showing the distribution of
the vascular tissue in a small leaf.
On the Nature of the Tissues through which the Motor Impulse is
Transmitted.—It will be necessary first to describe briefly the course
of the main fibro-vascular bundles. These are shown in the accompanying
sketch (fig. 11) of a small leaf. Little vessels from the neighbouring
bundles enter all the many tentacles with which the surface is studded;
but these are not here represented. The central trunk, which runs up
the footstalk, bifurcates near the centre of the leaf, each branch
bifurcating again and again according to the size of the leaf. This
central trunk sends off, low down on each side, a delicate branch,
which may be called the sublateral branch. There is also, on each side,
a main lateral branch or bundle, which bifurcates in the same manner as
the others. Bifurcation does not imply that any single vessel divides,
but that a bundle [page 248] divides into two. By looking to either
side of the leaf, it will be seen that a branch from the great central
bifurcation inosculates with a branch from the lateral bundle, and that
there is a smaller inosculation between the two chief branches of the
lateral bundle. The course of the vessels is very complex at the larger
inosculation; and here vessels, retaining the same diameter, are often
formed by the union of the bluntly pointed ends of two vessels, but
whether these points open into each other by their attached surfaces, I
do not know. By means of the two inosculations all the vessels on the
same side of the leaf are brought into some sort of connection. Near
the circumference of the larger leaves the bifurcating branches also
come into close union, and then separate again, forming a continuous
zigzag line of vessels round the whole circumference. But the union of
the vessels in this zigzag line seems to be much less intimate than at
the main inosculation. It should be added that the course of the
vessels differs somewhat in different leaves, and even on opposite
sides of the same leaf, but the main inosculation is always present.
Now in my first experiments with bits of meat placed on one side of the
disc, it so happened that not a single tentacle was inflected on the
opposite side; and when I saw that the vessels on the same side were
all connected together by the two inosculations, whilst not a vessel
passed over to the opposite side, it seemed probable that the motor
impulse was conducted exclusively along them.
In order to test this view, I divided transversely with the point of a
lancet the central trunks of four leaves, just beneath the main
bifurcation; and two days afterwards placed rather large bits of raw
meat [page 249] (a most powerful stimulant) near the centre of the disc
above the incision—that is, a little towards the apex—with the
following results:—
[(1) This leaf proved rather torpid: after 4 hrs. 40 m. (in all cases
reckoning from the time when the meat was given) the tentacles at the
distal end were a little inflected, but nowhere else; they remained so
for three days, and re-expanded on the fourth day. The leaf was then
dissected, and the trunk, as well as the two sublateral branches, were
found divided.
(2) After 4 hrs. 30 m. many of the tentacles at the distal end were
well inflected. Next day the blade and all the tentacles at this end
were strongly inflected, and were separated by a distinct transverse
line from the basal half of the leaf, which was not in the least
affected. On the third day, however, some of the short tentacles on the
disc near the base were very slightly inflected. The incision was found
on dissection to extend across the leaf as in the last case.
(3) After 4 hrs. 30 m. strong inflection of the tentacles at the distal
end, which during the next two days never extended in the least to the
basal end. The incision as before.
(4) This leaf was not observed until 15 hrs. had elapsed, and then all
the tentacles, except the extreme marginal ones, were found equally
well inflected all round the leaf. On careful examination the spiral
vessels of the central trunk were certainly divided; but the incision
on one side had not passed through the fibrous tissue surrounding these
vessels, though it had passed through the tissue on the other side.*]
The appearance presented by the leaves (2) and (3) was very curious,
and might be aptly compared with that of a man with his backbone broken
and lower extremities paralysed. Excepting that the line between the
two halves was here transverse instead of longitudinal, these leaves
were in the same state as some of those in the former experiments, with
bits of meat placed on one side of the disc. The case of leaf (4)
* M. Ziegler made similar experiments by cutting the spiral vessels of
Drosera intermedia(‘Comptes rendus,’ 1874, p. 1417), but arrived at
conclusions widely different from mine. [page 250]
proves that the spiral vessels of the central trunk may be divided, and
yet the motor impulse be transmitted from the distal to the basal end;
and this led me at first to suppose that the motor force was sent
through the closely surrounding fibrous tissue; and that if one half of
this tissue was left undivided, it sufficed for complete transmission.
But opposed to this conclusion is the fact that no vessels pass
directly from one side of the leaf to the other, and yet, as we have
seen, if a rather large bit of meat is placed on one side, the motor
impulse is sent, though slowly and imperfectly, in a transverse
direction across the whole breadth of the leaf. Nor can this latter
fact be accounted for by supposing that the transmission is effected
through the two inosculations, or through the circumferential zigzag
line of union, for had this been the case, the exterior tentacles on
the opposite side of the disc would have been affected before the more
central ones, which never occurred. We have also seen that the extreme
marginal tentacles appear to have no power to transmit an impulse to
the adjoining tentacles; yet the little bundle of vessels which enters
each marginal tentacle sends off a minute branch to those on both
sides, and this I have not observed in any other tentacles; so that the
marginal ones are more closely connected together by spiral vessels
than are the others, and yet have much less power of communicating a
motor impulse to one another.
But besides these several facts and arguments we have conclusive
evidence that the motor impulse is not sent, at least exclusively,
through the spiral vessels, or through the tissue immediately
surrounding them. We know that if a bit of meat is placed on a gland
(the immediately adjoining ones having been removed) on any part of the
disc, all the short sur- [page 251] rounding tentacles bend almost
simultaneously with great precision towards it. Now there are tentacles
on the disc, for instance near the extremities of the sublateral
bundles (fig. 11), which are supplied with vessels that do not come
into contact with the branches that enter the surrounding tentacles,
except by a very long and extremely circuitous course. Nevertheless, if
a bit of meat is placed on the gland of a tentacle of this kind, all
the surrounding ones are inflected towards it with great precision. It
is, of course, possible that an impulse might be sent through a long
and circuitous course, but it is obviously impossible that the
direction of the movement could be thus communicated, so that all the
surrounding tentacles should bend precisely to the point of excitement.
The impulse no doubt is transmitted in straight radiating lines from
the excited gland to the surrounding tentacles; it cannot, therefore,
be sent along the fibro-vascular bundles. The effect of cutting the
central vessels, in the above cases, in preventing the transmission of
the motor impulse from the distal to the basal end of a leaf, may be
attributed to a considerable space of the cellular tissue having been
divided. We shall hereafter see, when we treat of Dionaea, that this
same conclusion, namely that the motor impulse is not transmitted by
the fibro-vascular bundles, is plainly confirmed; and Prof. Cohn has
come to the same conclusion with respect to Aldrovanda—both members of
the Droseraceae.
As the motor impulse is not transmitted along the vessels, there
remains for its passage only the cellular tissue; and the structure of
this tissue explains to a certain extent how it travels so quickly down
the long exterior tentacles, and much more slowly across the blade of
the leaf. We shall also see why it crosses [page 252] the blade more
quickly in a longitudinal than in a transverse direction; though with
time it can pass in any direction. We know that the same stimulus
causes movement of the tentacles and aggregation of the protoplasm, and
that both influences originate in and proceed from the glands within
the same brief space of time. It seems therefore probable that the
motor impulse consists of the first commencement of a molecular change
in the protoplasm, which, when well developed, is plainly visible, and
has been designated aggregation; but to this subject I shall return. We
further know that in the transmission of the aggregating process the
chief delay is caused by the passage of the transverse cell-walls; for
as the aggregation travels down the tentacles, the contents of each
successive cell seem almost to flash into a cloudy mass. We may
therefore infer that the motor impulse is in like manner delayed
chiefly by passing through the cell-walls.
The greater celerity with which the impulse is transmitted down the
long exterior tentacles than across the disc may be largely attributed
to its being closely confined within the narrow pedicel, instead of
radiating forth on all sides as on the disc. But besides this
confinement, the exterior cells of the tentacles are fully twice as
long as those of the disc; so that only half the number of transverse
partitions have to be traversed in a given length of a tentacle,
compared with an equal space on the disc; and there would be in the
same proportion less retardation of the impulse. Moreover, in sections
of the exterior tentacles given by Dr. Warming,* the parenchymatous
* ‘Videnskabelige Meddelelser de la Soc. d’Hist. nat. de Copenhague,’
Nos. 10-12, 1872, woodcuts iv. and v. [page 253]
cells are shown to be still more elongated; and these would form the
most direct line of communication from the gland to the bending place
of the tentacle. If the impulse travels down the exterior cells, it
would have to cross from between twenty to thirty transverse
partitions; but rather fewer if down the inner parenchymatous tissue.
In either case it is remarkable that the impulse is able to pass
through so many partitions down nearly the whole length of the pedicel,
and to act on the bending place, in ten seconds. Why the impulse, after
having passed so quickly down one of the extreme marginal tentacles
(about 1/20 of an inch in length), should never, as far as I have seen,
affect the adjoining tentacles, I do not understand. It may be in part
accounted for by much energy being expended in the rapidity of the
transmission.
Most of the cells of the disc, both the superficial ones and the larger
cells which form the five or six underlying layers, are about four
times as long as broad. They are arranged almost longitudinally,
radiating from the footstalk. The motor impulse, therefore, when
transmitted across the disc, has to cross nearly four times as many
cell-walls as when transmitted in a longitudinal direction, and would
consequently be much delayed in the former case. The cells of the disc
converge towards the bases of the tentacles, and are thus fitted to
convey the motor impulse to them from all sides. On the whole, the
arrangement and shape of the cells, both those of the disc and
tentacles, throw much light on the rate and manner of diffusion of the
motor impulse. But why the impulse proceeding from the glands of the
exterior rows of tentacles tends to travel laterally and towards the
centre of the leaf, but not centrifugally, is by no means clear. [page
254]
Mechanism of the Movements, and Nature of the Motor Impulse.—Whatever
may be the means of movement, the exterior tentacles, considering their
delicacy, are inflected with much force. A bristle, held so that a
length of 1 inch projected from a handle, yielded when I tried to lift
with it an inflected tentacle, which was somewhat thinner than the
bristle. The amount or extent, also, of the movement is great. Fully
expanded tentacles in becoming inflected sweep through an angle of
180o; and if they are beforehand reflexed, as often occurs, the angle
is considerably greater. It is probably the superficial cells at the
bending place which chiefly or exclusively contract; for the interior
cells have very delicate walls, and are so few in number that they
could hardly cause a tentacle to bend with precision to a definite
point. Though I carefully looked, I could never detect any wrinkling of
the surface at the bending place, even in the case of a tentacle
abnormally curved into a complete circle, under circumstances hereafter
to be mentioned.
All the cells are not acted on, though the motor impulse passes through
them. When the gland of one of the long exterior tentacles is excited,
the upper cells are not in the least affected; about halfway down there
is a slight bending, but the chief movement is confined to a short
space near the base; and no part of the inner tentacles bends except
the basal portion. With respect to the blade of the leaf, the motor
impulse may be transmitted through many cells, from the centre to the
circumference, without their being in the least affected, or they may
be strongly acted on and the blade greatly inflected. In the latter
case the movement seems to depend partly on the strength of the
stimulus, and partly on [page 255] its nature, as when leaves are
immersed in certain fluids.
The power of movement which various plants possess, when irritated, has
been attributed by high authorities to the rapid passage of fluid out
of certain cells, which, from their previous state of tension,
immediately contract.* Whether or not this is the primary cause of such
movements, fluid must pass out of closed cells when they contract or
are pressed together in one direction, unless they at the same time
expand in some other direction. For instance, fluid can be seen to ooze
from the surface of any young and vigorous shoot if slowly bent into a
semi-circle.** In the case of Drosera there is certainly much movement
of the fluid throughout the tentacles whilst they are undergoing
inflection. Many leaves can be found in which the purple fluid within
the cells is of an equally dark tint on the upper and lower sides of
the tentacles, extending also downwards on both sides to equally near
their bases. If the tentacles of such a leaf are excited into movement,
it will generally be found after some hours that the cells on the
concave side are much paler than they were before, or are quite
colourless, those on the convex side having become much darker. In two
instances, after particles of hair had been placed on glands, and when
in the course of 1 hr. 10 m. the tentacles were incurved halfway
towards the centre of the leaf, this change of colour in the two sides
was conspicuously plain. In another case, after a bit of meat had been
placed on a gland, the purple colour was observed at intervals to be
slowly travelling from the upper to the lower part, down the convex
side of
* Sachs, ‘Traité de Bot.’ 3rd edit. 1874, p. 1038. This view was, I
believe, first suggested by Lamarck.
** Sachs, ibid. p. 919. [page 256]
the bending tentacle. But it does not follow from these observations
that the cells on the convex side become filled with more fluid during
the act of inflection than they contained before; for fluid may all the
time be passing into the disc or into the glands which then secrete
freely.
The bending of the tentacles, when leaves are immersed in a dense
fluid, and their subsequent re-expansion in a less dense fluid, show
that the passage of fluid from or into the cells can cause movements
like the natural ones. But the inflection thus caused is often
irregular; the exterior tentacles being sometimes spirally curved.
Other unnatural movements are likewise caused by the application of
dense fluids, as in the case of drops of syrup placed on the backs of
leaves and tentacles. Such movements may be compared with the
contortions which many vegetable tissues undergo when subjected to
exosmose. It is therefore doubtful whether they throw any light on the
natural movements.
If we admit that the outward passage of fluid is the cause of the
bending of the tentacles, we must suppose that the cells, before the
act of inflection, are in a high state of tension, and that they are
elastic to an extraordinary degree; for otherwise their contraction
could not cause the tentacles often to sweep through an angle of above
180o. Prof. Cohn, in his interesting paper* on the movements of the
stamens of certain Compositae, states that these organs, when dead, are
as elastic as threads of india-rubber, and are then only half as long
as they were when alive. He believes that the living protoplasm
* ‘Abhand. der Schles. Gesell. fr vaterl. Cultur,’ 1861, Heft i. An
excellent abstract of this paper is given in the ‘Annals and Mag. of
Nat. Hist.’ 3rd series, 1863, vol. xi. pp. 188-197. [page 257]
within their cells is ordinarily in a state of expansion, but is
paralysed by irritation, or may be said to suffer temporary death; the
elasticity of the cell-walls then coming into play, and causing the
contraction of the stamens. Now the cells on the upper or concave side
of the bending part of the tentacles of Drosera do not appear to be in
a state of tension, nor to be highly elastic; for when a leaf is
suddenly killed, or dies slowly, it is not the upper but the lower
sides of the tentacles which contract from elasticity. We may,
therefore, conclude that their movements cannot be accounted for by the
inherent elasticity of certain cells, opposed as long as they are alive
and not irritated by the expanded state of their contents.
A somewhat different view has been advanced by other
physiologists—namely that the protoplasm, when irritated, contracts
like the soft sarcode of the muscles of animals. In Drosera the fluid
within the cells of the tentacles at the bending place appears under
the microscope thin and homogeneous, and after aggregation consists of
small, soft masses of matter, undergoing incessant changes of form and
floating in almost colourless fluid. These masses are completely
redissolved when the tentacles re-expand. Now it seems scarcely
possible that such matter should have any direct mechanical power; but
if through some molecular change it were to occupy less space than it
did before, no doubt the cell-walls would close up and contract. But in
this case it might be expected that the walls would exhibit wrinkles,
and none could ever be seen. Moreover, the contents of all the cells
seem to be of exactly the same nature, both before and after
aggregation; and yet only a few of the basal cells contract, the rest
of the tentacle remaining straight.
A third view maintained by some physiologists, [page 258] though
rejected by most others, is that the whole cell, including the walls,
actively contracts. If the walls are composed solely of non-nitrogenous
cellulose, this view is highly improbable; but it can hardly be doubted
that they must be permeated by proteid matter, at least whilst they are
growing. Nor does there seem any inherent improbability in the
cell-walls of Drosera contracting, considering their high state of
organisation; as shown in the case of the glands by their power of
absorption and secretion, and by being exquisitely sensitive so as to
be affected by the pressure of the most minute particles. The
cell-walls of the pedicels also allow various impulses to pass through
them, inducing movement, increased secretion and aggregation. On the
whole the belief that the walls of certain cells contract, some of
their contained fluid being at the same time forced outwards, perhaps
accords best with the observed facts. If this view is rejected, the
next most probable one is that the fluid contents of the cells shrink,
owing to a change in their molecular state, with the consequent closing
in of the walls. Anyhow, the movement can hardly be attributed to the
elasticity of the walls, together with a previous state of tension.
With respect to the nature of the motor impulse which is transmitted
from the glands down the pedicels and across the disc, it seems not
improbable that it is closely allied to that influence which causes the
protoplasm within the cells of the glands and tentacles to aggregate.
We have seen that both forces originate in and proceed from the glands
within a few seconds of the same time, and are excited by the same
causes. The aggregation of the protoplasm lasts almost as long as the
tentacles remain inflected, even though this be for more than a week;
but the [page 259] protoplasm is redissolved at the bending place
shortly before the tentacles re-expand, showing that the exciting cause
of the aggregating process has then quite ceased. Exposure to carbonic
acid causes both the latter process and the motor impulse to travel
very slowly down the tentacles. We know that the aggregating process is
delayed in passing through the cell- walls, and we have good reason to
believe that this holds good with the motor impulse; for we can thus
understand the different rates of its transmission in a longitudinal
and transverse line across the disc. Under a high power the first sign
of aggregation is the appearance of a cloud, and soon afterwards of
extremely fine granules, in the homogeneous purple fluid within the
cells; and this apparently is due to the union of molecules of
protoplasm. Now it does not seem an improbable view that the same
tendency—namely for the molecules to approach each other—should be
communicated to the inner surfaces of the cell-walls which are in
contact with the protoplasm; and if so, their molecules would approach
each other, and the cell-wall would contract.
To this view it may with truth be objected that when leaves are
immersed in various strong solutions, or are subjected to a heat of
above 130° Fahr. (54°.4 Cent.), aggregation ensues, but there is no
movement. Again, various acids and some other fluids cause rapid
movement, but no aggregation, or only of an abnormal nature, or only
after a long interval of time; but as most of these fluids are more or
less injurious, they may check or prevent the aggregating process by
injuring or killing the protoplasm. There is another and more important
difference in the two processes: when the glands on the disc are
excited, they transmit some influence up the surrounding [page 260]
tentacles, which acts on the cells at the bending place, but does not
induce aggregation until it has reached the glands; these then send
back some other influence, causing the protoplasm to aggregate, first
in the upper and then in the lower cells.
The Re-expansion of the Tentacles.—This movement is always slow and
gradual. When the centre of the leaf is excited, or a leaf is immersed
in a proper solution, all the tentacles bend directly towards the
centre, and afterwards directly back from it. But when the point of
excitement is on one side of the disc, the surrounding tentacles bend
towards it, and therefore obliquely with respect to their normal
direction; when they afterwards re-expand, they bend obliquely back, so
as to recover their original positions. The tentacles farthest from an
excited point, wherever that may be, are the last and the least
affected, and probably in consequence of this they are the first to
re-expand. The bent portion of a closely inflected tentacle is in a
state of active contraction, as shown by the following experiment. Meat
was placed on a leaf, and after the tentacles were closely inflected
and had quite ceased to move, narrow strips of the disc, with a few of
the outer tentacles attached to it, were cut off and laid on one side
under the microscope. After several failures, I succeeded in cutting
off the convex surface of the bent portion of a tentacle. Movement
immediately recommenced, and the already greatly bent portion went on
bending until it formed a perfect circle; the straight distal portion
of the tentacle passing on one side of the strip. The convex surface
must therefore have previously been in a state of tension, sufficient
to counter-balance that of the concave surface, which, when free,
curled into a complete ring.
The tentacles of an expanded and unexcited leaf [page 261] are
moderately rigid and elastic; if bent by a needle, the upper end yields
more easily than the basal and thicker part, which alone is capable of
becoming inflected. The rigidity of this basal part seems due to the
tension of the outer surface balancing a state of active and persistent
contraction of the cells of the inner surface. I believe that this is
the case, because, when a leaf is dipped into boiling water, the
tentacles suddenly become reflexed, and this apparently indicates that
the tension of the outer surface is mechanical, whilst that of the
inner surface is vital, and is instantly destroyed by the boiling
water. We can thus also understand why the tentacles as they grow old
and feeble slowly become much reflexed. If a leaf with its tentacles
closely inflected is dipped into boiling water, these rise up a little,
but by no means fully re-expand. This may be owing to the heat quickly
destroying the tension and elasticity of the cells of the convex
surface; but I can hardly believe that their tension, at any one time,
would suffice to carry back the tentacles to their original position,
often through an angle of above 180o. It is more probable that fluid,
which we know travels along the tentacles during the act of inflection,
is slowly re-attracted into the cells of the convex surface, their
tension being thus gradually and continually increased.
A recapitulation of the chief facts and discussions in this chapter
will be given at the close of the next chapter. [page 262]