RECAPITULATION OF THE CHIEF OBSERVATIONS ON DROSERA ROTUNDIFOLIA.
As summaries have been given to most of the chapters, it will be
sufficient here to recapitulate, as briefly as I can, the chief points.
In the first chapter a preliminary sketch was given of the structure of
the leaves, and of the manner in which they capture insects. This is
effected by drops of extremely viscid fluid surrounding the glands and
by the inward movement of the tentacles. As the plants gain most of
their nutriment by this means, their roots are very poorly developed;
and they often grow in places where hardly any other plant except
mosses can exist. The glands have the power of absorption, besides that
of secretion. They are extremely sensitive to various stimulants,
namely repeated touches, the pressure of minute particles, the
absorption of animal matter and of various fluids, heat, and galvanic
action. A tentacle with a bit of raw meat on the gland has been seen to
begin bending in 10 s., to be strongly incurved in 5 m., and to reach
the centre of the leaf in half an hour. The blade of the leaf often
becomes so much inflected that it forms a cup, enclosing any object
placed on it.
A gland, when excited, not only sends some influence down its own
tentacle, causing it to bend, but likewise to the surrounding
tentacles, which become incurved; so that the bending place can be
acted on by an impulse received from opposite directions, [page 263]
namely from the gland on the summit of the same tentacle, and from one
or more glands of the neighbouring tentacles. Tentacles, when
inflected, re-expand after a time, and during this process the glands
secrete less copiously, or become dry. As soon as they begin to secrete
again, the tentacles are ready to re-act; and this may be repeated at
least three, probably many more times.
It was shown in the second chapter that animal substances placed on the
discs cause much more prompt and energetic inflection than do inorganic
bodies of the same size, or mere mechanical irritation; but there is a
still more marked difference in the greater length of time during which
the tentacles remain inflected over bodies yielding soluble and
nutritious matter, than over those which do not yield such matter.
Extremely minute particles of glass, cinders, hair, thread,
precipitated chalk, &c., when placed on the glands of the outer
tentacles, cause them to bend. A particle, unless it sinks through the
secretion and actually touches the surface of the gland with some one
point, does not produce any effect. A little bit of thin human hair
8/1000 of an inch (.203 mm.) in length, and weighing only 1/78740 of a
grain (.000822 mg.), though largely supported by the dense secretion,
suffices to induce movement. It is not probable that the pressure in
this case could have amounted to that from the millionth of a grain.
Even smaller particles cause a slight movement, as could be seen
through a lens. Larger particles than those of which the measurements
have been given cause no sensation when placed on the tongue, one of
the most sensitive parts of the human body.
Movement ensues if a gland is momentarily touched three or four times;
but if touched only once or twice, [page 264] though with considerable
force and with a hard object, the tentacle does not bend. The plant is
thus saved from much useless movement, as during a high wind the glands
can hardly escape being occasionally brushed by the leaves of
surrounding plants. Though insensible to a single touch, they are
exquisitely sensitive, as just stated, to the slightest pressure if
prolonged for a few seconds; and this capacity is manifestly of service
to the plant in capturing small insects. Even gnats, if they rest on
the glands with their delicate feet, are quickly and securely embraced.
The glands are insensible to the weight and repeated blows of drops of
heavy rain, and the plants are thus likewise saved from much useless
movement.
The description of the movements of the tentacles was interrupted in
the third chapter for the sake of describing the process of
aggregation. This process always commences in the cells of the glands,
the contents of which first become cloudy; and this has been observed
within 10 s. after a gland has been excited. Granules just resolvable
under a very high power soon appear, sometimes within a minute, in the
cells beneath the glands; and these then aggregate into minute spheres.
The process afterwards travels down the tentacles, being arrested for a
short time at each transverse partition. The small spheres coalesce
into larger spheres, or into oval, club-headed, thread- or
necklace-like, or otherwise shaped masses of protoplasm, which,
suspended in almost colourless fluid, exhibit incessant spontaneous
changes of form. These frequently coalesce and again separate. If a
gland has been powerfully excited, all the cells down to the base of
the tentacle are affected. In cells, especially if filled with dark red
fluid, the first step in the [page 265] process often is the formation
of a dark red, bag-like mass of protoplasm, which afterwards divides
and undergoes the usual repeated changes of form. Before any
aggregation has been excited, a sheet of colourless protoplasm,
including granules (the primordial utricle of Mohl), flows round the
walls of the cells; and this becomes more distinct after the contents
have been partially aggregated into spheres or bag-like masses. But
after a time the granules are drawn towards the central masses and
unite with them; and then the circulating sheet can no longer be
distinguished, but there is still a current of transparent fluid within
the cells.
Aggregation is excited by almost all the stimulants which induce
movement; such as the glands being touched two or three times, the
pressure of minute inorganic particles, the absorption of various
fluids, even long immersion in distilled water, exosmose, and heat. Of
the many stimulants tried, carbonate of ammonia is the most energetic
and acts the quickest: a dose of 1/134400 of a grain (.00048 mg.) given
to a single gland suffices to cause in one hour well-marked aggregation
in the upper cells of the tentacle. The process goes on only as long as
the protoplasm is in a living, vigorous, and oxygenated condition.
The result is in all respects exactly the same, whether a gland has
been excited directly, or has received an influence from other and
distant glands. But there is one important difference: when the central
glands are irritated, they transmit centrifugally an influence up the
pedicels of the exterior tentacles to their glands; but the actual
process of aggregation travels centripetally, from the glands of the
exterior tentacles down their pedicels. The exciting influence,
therefore, which is transmitted from [page 266] one part of the leaf to
another must be different from that which actually induces aggregation.
The process does not depend on the glands secreting more copiously than
they did before; and is independent of the inflection of the tentacles.
It continues as long as the tentacles remain inflected, and as soon as
these are fully re-expanded, the little masses of protoplasm are all
redissolved; the cells becoming filled with homogeneous purple fluid,
as they were before the leaf was excited.
As the process of aggregation can be excited by a few touches, or by
the pressure of insoluble particles, it is evidently independent of the
absorption of any matter, and must be of a molecular nature. Even when
caused by the absorption of the carbonate or other salt of ammonia, or
an infusion of meat, the process seems to be of exactly the same
nature. The protoplasmic fluid must, therefore, be in a singularly
unstable condition, to be acted on by such slight and varied causes.
Physiologists believe that when a nerve is touched, and it transmits an
influence to other parts of the nervous system, a molecular change is
induced in it, though not visible to us. Therefore it is a very
interesting spectacle to watch the effects on the cells of a gland, of
the pressure of a bit of hair, weighing only 1/78700 of a grain and
largely supported by the dense secretion, for this excessively slight
pressure soon causes a visible change in the protoplasm, which change
is transmitted down the whole length of the tentacle, giving it at last
a mottled appearance, distinguishable even by the naked eye.
In the fourth chapter it was shown that leaves placed for a short time
in water at a temperature of 110° Fahr. (43°.3 Cent.) become somewhat
inflected; they are thus also rendered more sensitive to the action
[page 267] of meat than they were before. If exposed to a temperature
of between 115° and 125°(46°.1-51°.6 Cent.), they are quickly
inflected, and their protoplasm undergoes aggregation; when afterwards
placed in cold water, they re-expand. Exposed to 130° (54°.4 Cent.), no
inflection immediately occurs, but the leaves are only temporarily
paralysed, for on being left in cold water, they often become inflected
and afterwards re-expand. In one leaf thus treated, I distinctly saw
the protoplasm in movement. In other leaves, treated in the same
manner, and then immersed in a solution of carbonate of ammonia, strong
aggregation ensued. Leaves placed in cold water, after an exposure to
so high a temperature as 145° (62°.7 Cent.), sometimes become slightly,
though slowly, inflected; and afterwards have the contents of their
cells strongly aggregated by carbonate of ammonia. But the duration of
the immersion is an important element, for if left in water at 145°
(62°.7 Cent.), or only at 140° (60° Cent.), until it becomes cool, they
are killed, and the contents of the glands are rendered white and
opaque. This latter result seems to be due to the coagulation of the
albumen, and was almost always caused by even a short exposure to 150°
(65.5 Cent.); but different leaves, and even the separate cells in the
same tentacle, differ considerably in their power of resisting heat.
Unless the heat has been sufficient to coagulate the albumen, carbonate
of ammonia subsequently induces aggregation.
In the fifth chapter, the results of placing drops of various
nitrogenous and non-nitrogenous organic fluids on the discs of leaves
were given, and it was shown that they detect with almost unerring
certainty the presence of nitrogen. A decoction of green peas or of
fresh cabbage-leaves acts almost as powerfully as an infusion of raw
meat; whereas an infusion of cabbage- [page 268] leaves made by keeping
them for a long time in merely warm water is far less efficient. A
decoction of grass-leaves is less powerful than one of green peas or
cabbage-leaves.
These results led me to inquire whether Drosera possessed the power of
dissolving solid animal matter. The experiments proving that the leaves
are capable of true digestion, and that the glands absorb the digested
matter, are given in detail in the sixth chapter. These are, perhaps,
the most interesting of all my observations on Drosera, as no such
power was before distinctly known to exist in the vegetable kingdom. It
is likewise an interesting fact that the glands of the disc, when
irritated, should transmit some influence to the glands of the exterior
tentacles, causing them to secrete more copiously and the secretion to
become acid, as if they had been directly excited by an object placed
on them. The gastric juice of animals contains, as is well known, an
acid and a ferment, both of which are indispensable for digestion, and
so it is with the secretion of Drosera. When the stomach of an animal
is mechanically irritated, it secretes an acid, and when particles of
glass or other such objects were placed on the glands of Drosera, the
secretion, and that of the surrounding and untouched glands, was
increased in quantity and became acid. But, according to Schiff, the
stomach of an animal does not secrete its proper ferment, pepsin, until
certain substances, which he calls peptogenes, are absorbed; and it
appears from my experiments that some matter must be absorbed by the
glands of Drosera before they secrete their proper ferment. That the
secretion does contain a ferment which acts only in the presence of an
acid on solid animal matter, was clearly proved by adding minute doses
of [page 269] an alkali, which entirely arrested the process of
digestion, this immediately recommencing as soon as the alkali was
neutralised by a little weak hydrochloric acid. From trials made with a
large number of substances, it was found that those which the secretion
of Drosera dissolves completely, or partially, or not at all, are acted
on in exactly the same manner by gastric juice. We may, therefore,
conclude that the ferment of Drosera is closely analogous to, or
identical with, the pepsin of animals.
The substances which are digested by Drosera act on the leaves very
differently. Some cause much more energetic and rapid inflection of the
tentacles, and keep them inflected for a much longer time, than do
others. We are thus led to believe that the former are more nutritious
than the latter, as is known to be the case with some of these same
substances when given to animals; for instance, meat in comparison with
gelatine. As cartilage is so tough a substance and is so little acted
on by water, its prompt dissolution by the secretion of Drosera, and
subsequent absorption is, perhaps, one of the most striking cases. But
it is not really more remarkable than the digestion of meat, which is
dissolved by this secretion in the same manner and by the same stages
as by gastric juice. The secretion dissolves bone, and even the enamel
of teeth, but this is simply due to the large quantity of acid
secreted, owing, apparently, to the desire of the plant for phosphorus.
In the case of bone, the ferment does not come into play until all the
phosphate of lime has been decomposed and free acid is present, and
then the fibrous basis is quickly dissolved. Lastly, the secretion
attacks and dissolves matter out of living seeds, which it sometimes
kills, or injures, as shown by the diseased state [page 270] of the
seedlings. It also absorbs matter from pollen, and from fragments of
leaves.
The seventh chapter was devoted to the action of the salts of ammonia.
These all cause the tentacles, and often the blade of the leaf, to be
inflected, and the protoplasm to be aggregated. They act with very
different power; the citrate being the least powerful, and the
phosphate, owing, no doubt, to the presence of phosphorus and nitrogen,
by far the most powerful. But the relative efficiency of only three
salts of ammonia was carefully determined, namely the carbonate,
nitrate, and phosphate. The experiments were made by placing
half-minims (.0296 ml.) of solutions of different strengths on the
discs of the leaves,—by applying a minute drop (about the 1/20 of a
minim, or .00296 ml.) for a few seconds to three or four glands,—and by
the immersion of whole leaves in a measured quantity. In relation to
these experiments it was necessary first to ascertain the effects of
distilled water, and it was found, as described in detail, that the
more sensitive leaves are affected by it, but only in a slight degree.
A solution of the carbonate is absorbed by the roots and induces
aggregation in their cells, but does not affect the leaves. The vapour
is absorbed by the glands, and causes inflection as well as
aggregation. A drop of a solution containing 1/960 of a grain (.0675
mg.) is the least quantity which, when placed on the glands of the
disc, excites the exterior tentacles to bend inwards. But a minute
drop, containing 1/14400 of a grain (.00445 mg.), if applied for a few
seconds to the secretion surrounding a gland, causes the inflection of
the same tentacle. When a highly sensitive leaf is immersed in a
solution, and there is ample time for absorption, the 1/268800 of a
grain [page 271] (.00024 mg.) is sufficient to excite a single tentacle
into movement.
The nitrate of ammonia induces aggregation of the protoplasm much less
quickly than the carbonate, but is more potent in causing inflection. A
drop containing 1/2400 of a grain (.027 mg.) placed on the disc acts
powerfully on all the exterior tentacles, which have not themselves
received any of the solution; whereas a drop with 1/2800 of a grain
caused only a few of these tentacles to bend, but affected rather more
plainly the blade. A minute drop applied as before, and containing
1/28800 of a grain (.0025 mg.), caused the tentacle bearing this gland
to bend. By the immersion of whole leaves, it was proved that the
absorption by a single gland of 1/691200 of a grain (.0000937 mg.) was
sufficient to set the same tentacle into movement.
The phosphate of ammonia is much more powerful than the nitrate. A drop
containing 1/3840 of a grain (.0169 mg.) placed on the disc of a
sensitive leaf causes most of the exterior tentacles to be inflected,
as well as the blade of the leaf. A minute drop containing 1/153600 of
a grain (.000423 mg.), applied for a few seconds to a gland, acts, as
shown by the movement of the tentacle. When a leaf is immersed in
thirty minims (1.7748 ml.) of a solution of one part by weight of the
salt to 21,875,000 of water, the absorption by a gland of only the
1/19760000 of a grain (.00000328 mg.), that is, about the
one-twenty-millionth of a grain, is sufficient to cause the tentacle
bearing this gland to bend to the centre of the leaf. In this
experiment, owing to the presence of the water of crystallisation, less
than the one-thirty-millionth of a grain of the efficient elements
could have been absorbed. There is nothing remarkable in such minute
quantities being absorbed by the glands, [page 272] for all
physiologists admit that the salts of ammonia, which must be brought in
still smaller quantity by a single shower of rain to the roots, are
absorbed by them. Nor is it surprising that Drosera should be enabled
to profit by the absorption of these salts, for yeast and other low
fungoid forms flourish in solutions of ammonia, if the other necessary
elements are present. But it is an astonishing fact, on which I will
not here again enlarge, that so inconceivably minute a quantity as the
one-twenty-millionth of a grain of phosphate of ammonia should induce
some change in a gland of Drosera, sufficient to cause a motor impulse
to be sent down the whole length of the tentacle; this impulse exciting
movement often through an angle of above 180o. I know not whether to be
most astonished at this fact, or that the pressure of a minute bit of
hair, supported by the dense secretion, should quickly cause
conspicuous movement. Moreover, this extreme sensitiveness, exceeding
that of the most delicate part of the human body, as well as the power
of transmitting various impulses from one part of the leaf to another,
have been acquired without the intervention of any nervous system.
As few plants are at present known to possess glands specially adapted
for absorption, it seemed worth while to try the effects on Drosera of
various other salts, besides those of ammonia, and of various acids.
Their action, as described in the eighth chapter, does not correspond
at all strictly with their chemical affinities, as inferred from the
classification commonly followed. The nature of the base is far more
influential than that of the acid; and this is known to hold good with
animals. For instance, nine salts of sodium all caused well-marked
inflection, and none of them were poisonous in small doses; whereas
seven of the nine corre- [page 273] sponding salts of potassium
produced no effect, two causing slight inflection. Small doses,
moreover, of some of the latter salts were poisonous. The salts of
sodium and potassium, when injected into the veins of animals, likewise
differ widely in their action. The so-called earthy salts produce
hardly any effect on Drosera. On the other hand, most of the metallic
salts cause rapid and strong inflection, and are highly poisonous; but
there are some odd exceptions to this rule; thus chloride of lead and
zinc, as well as two salts of barium, did not cause inflection, and
were not poisonous.
Most of the acids which were tried, though much diluted (one part to
437 of water), and given in small doses, acted powerfully on Drosera;
nineteen, out of the twenty-four, causing the tentacles to be more or
less inflected. Most of them, even the organic acids, are poisonous,
often highly so; and this is remarkable, as the juices of so many
plants contain acids. Benzoic acid, which is innocuous to animals,
seems to be as poisonous to Drosera as hydrocyanic. On the other hand,
hydrochloric acid is not poisonous either to animals or to Drosera, and
induces only a moderate amount of inflection. Many acids excite the
glands to secrete an extraordinary quantity of mucus; and the
protoplasm within their cells seems to be often killed, as may be
inferred from the surrounding fluid soon becoming pink. It is strange
that allied acids act very differently: formic acid induces very slight
inflection, and is not poisonous; whereas acetic acid of the same
strength acts most powerfully and is poisonous. Lactic acid is also
poisonous, but causes inflection only after a considerable lapse of
time. Malic acid acts slightly, whereas citric and tartaric acids
produce no effect. [page 274]
In the ninth chapter the effects of the absorption of various alkaloids
and certain other substances were described. Although some of these are
poisonous, yet as several, which act powerfully on the nervous system
of animals, produce no effect on Drosera, we may infer that the extreme
sensibility of the glands, and their power of transmitting an influence
to other parts of the leaf, causing movement, or modified secretion, or
aggregation, does not depend on the presence of a diffused element,
allied to nerve-tissue. One of the most remarkable facts is that long
immersion in the poison of the cobra-snake does not in the least check,
but rather stimulates, the spontaneous movements of the protoplasm in
the cells of the tentacles. Solutions of various salts and acids behave
very differently in delaying or in quite arresting the subsequent
action of a solution of phosphate of ammonia. Camphor dissolved in
water acts as a stimulant, as do small doses of certain essential oils,
for they cause rapid and strong inflection. Alcohol is not a stimulant.
The vapours of camphor, alcohol, chloroform, sulphuric and nitric
ether, are poisonous in moderately large doses, but in small doses
serve as narcotics or, anaesthetics, greatly delaying the subsequent
action of meat. But some of these vapours also act as stimulants,
exciting rapid, almost spasmodic movements in the tentacles. Carbonic
acid is likewise a narcotic, and retards the aggregation of the
protoplasm when carbonate of ammonia is subsequently given. The first
access of air to plants which have been immersed in this gas sometimes
acts as a stimulant and induces movement. But, as before remarked, a
special pharmacopoeia would be necessary to describe the diversified
effects of various substances on the leaves of Drosera.
In the tenth chapter it was shown that the sensitive- [page 275] ness
of the leaves appears to be wholly confined to the glands and to the
immediately underlying cells. It was further shown that the motor
impulse and other forces or influences, proceeding from the glands when
excited, pass through the cellular tissue, and not along the
fibro-vascular bundles. A gland sends its motor impulse with great
rapidity down the pedicel of the same tentacle to the basal part which
alone bends. The impulse, then passing onwards, spreads on all sides to
the surrounding tentacles, first affecting those which stand nearest
and then those farther off. But by being thus spread out, and from the
cells of the disc not being so much elongated as those of the
tentacles, it loses force, and here travels much more slowly than down
the pedicels. Owing also to the direction and form of the cells, it
passes with greater ease and celerity in a longitudinal than in a
transverse line across the disc. The impulse proceeding from the glands
of the extreme marginal tentacles does not seem to have force enough to
affect the adjoining tentacles; and this may be in part due to their
length. The impulse from the glands of the next few inner rows spreads
chiefly to the tentacles on each side and towards the centre of the
leaf; but that proceeding from the glands of the shorter tentacles on
the disc radiates almost equally on all sides.
When a gland is strongly excited by the quantity or quality of the
substance placed on it, the motor impulse travels farther than from one
slightly excited; and if several glands are simultaneously excited, the
impulses from all unite and spread still farther. As soon as a gland is
excited, it discharges an impulse which extends to a considerable
distance; but afterwards, whilst the gland is secreting and absorbing,
the impulse suffices only to keep the same tentacle [page 276]
inflected; though the inflection may last for many days.
If the bending place of a tentacle receives an impulse from its own
gland, the movement is always towards the centre of the leaf; and so it
is with all the tentacles, when their glands are excited by immersion
in a proper fluid. The short ones in the middle part of the disc must
be excepted, as these do not bend at all when thus excited. On the
other hand, when the motor impulse comes from one side of the disc, the
surrounding tentacles, including the short ones in the middle of the
disc, all bend with precision towards the point of excitement, wherever
this may be seated. This is in every way a remarkable phenomenon; for
the leaf falsely appears as if endowed with the senses of an animal. It
is all the more remarkable, as when the motor impulse strikes the base
of a tentacle obliquely with respect to its flattened surface, the
contraction of the cells must be confined to one, two, or a very few
rows at one end. And different sides of the surrounding tentacles must
be acted on, in order that all should bend with precision to the point
of excitement.
The motor impulse, as it spreads from one or more glands across the
disc, enters the bases of the surrounding tentacles, and immediately
acts on the bending place. It does not in the first place proceed up
the tentacles to the glands, exciting them to reflect back an impulse
to their bases. Nevertheless, some influence is sent up to the glands,
as their secretion is soon increased and rendered acid; and then the
glands, being thus excited, send back some other influence (not
dependent on increased secretion, nor on the inflection of the
tentacles), causing the protoplasm to aggregate in cell beneath cell.
This may [page 277] be called a reflex action, though probably very
different from that proceeding from the nerve-ganglion of an animal;
and it is the only known case of reflex action in the vegetable
kingdom.
About the mechanism of the movements and the nature of the motor
impulse we know very little. During the act of inflection fluid
certainly travels from one part to another of the tentacles. But the
hypothesis which agrees best with the observed facts is that the motor
impulse is allied in nature to the aggregating process; and that this
causes the molecules of the cell-walls to approach each other, in the
same manner as do the molecules of the protoplasm within the cells; so
that the cell-walls contract. But some strong objections may be urged
against this view. The re-expansion of the tentacles is largely due to
the elasticity of their outer cells, which comes into play as soon as
those on the inner side cease contracting with prepotent force; but we
have reason to suspect that fluid is continually and slowly attracted
into the outer cells during the act of re-expansion, thus increasing
their tension.
I have now given a brief recapitulation of the chief points observed by
me, with respect to the structure, movements, constitution, and habits
of Drosera rotundifolia; and we see how little has been made out in
comparison with what remains unexplained and unknown. [page 278]