CHAPTER II.

THE MOVEMENTS OF THE TENTACLES FROM THE CONTACT OF SOLID BODIES.

Inflection of the exterior tentacles owing to the glands of the disc
being excited by repeated touches, or by objects left in contact with
them—Difference in the action of bodies yielding and not yielding
soluble nitrogenous matter—Inflection of the exterior tentacles
directly caused by objects left in contact with their glands—Periods of
commencing inflection and of subsequent re-expansion—Extreme minuteness
of the particles causing inflection—Action under water—Inflection of
the exterior tentacles when their glands are excited by repeated
touches—Falling drops of water do not cause inflection.

I will give in this and the following chapters some of the many
experiments made, which best illustrate the manner and rate of movement
of the tentacles, when excited in various ways. The glands alone in all
ordinary cases are susceptible to excitement. When excited, they do not
themselves move or change form, but transmit a motor impulse to the
bending part of their own and adjoining tentacles, and are thus carried
towards the centre of the leaf. Strictly speaking, the glands ought to
be called irritable, as the term sensitive generally implies
consciousness; but no one supposes that the Sensitive-plant is
conscious, and as I have found the term convenient, I shall use it
without scruple. I will commence with the movements of the exterior
tentacles, when indirectly excited by stimulants applied to the glands
of the short tentacles on the disc. The exterior tentacles may be said
in this case to be indirectly excited, because their own glands are not
directly acted on. The stimulus proceeding from the glands of the disc
acts on the bending part of the [page 20] exterior tentacles, near
their bases, and does not (as will hereafter be proved) first travel up
the pedicels to the glands, to be then reflected back to the bending
place. Nevertheless, some influence does travel up to the glands,
causing them to secrete more copiously, and the secretion to become
acid. This latter fact is, I believe, quite new in the physiology of
plants; it has indeed only recently been established that in the animal
kingdom an influence can be transmitted along the nerves to glands,
modifying their power of secretion, independently of the state of the
blood-vessels.

The Inflection of the Exterior Tentacles from the Glands of the Disc
being excited by Repeated Touches, or by Objects left in Contact with
them.

The central glands of a leaf were irritated with a small stiff
camel-hair brush, and in 70 m. (minutes) several of the outer tentacles
were inflected; in 5 hrs. (hours) all the sub-marginal tentacles were
inflected; next morning after an interval of about 22 hrs. they were
fully re-expanded. In all the following cases the period is reckoned
from the time of first irritation. Another leaf treated in the same
manner had a few tentacles inflected in 20 m.; in 4 hrs. all the
submarginal and some of the extreme marginal tentacles, as well as the
edge of the leaf itself, were inflected; in 17 hrs. they had recovered
their proper, expanded position. I then put a dead fly in the centre of
the last-mentioned leaf, and next morning it was closely clasped; five
days afterwards the leaf re-expanded, and the tentacles, with their
glands surrounded by secretion, were ready to act again.

Particles of meat, dead flies, bits of paper, wood, dried moss, sponge,
cinders, glass, &c., were repeatedly [page 21] placed on leaves, and
these objects were well embraced in various periods from one hr. to as
long as 24 hrs., and set free again, with the leaf fully re-expanded,
in from one or two, to seven or even ten days, according to the nature
of the object. On a leaf which had naturally caught two flies, and
therefore had already closed and reopened either once or more probably
twice, I put a fresh fly: in 7 hrs. it was moderately, and in 21 hrs.
thoroughly well, clasped, with the edges of the leaf inflected. In two
days and a half the leaf had nearly re-expanded; as the exciting object
was an insect, this unusually short period of inflection was, no doubt,
due to the leaf having recently been in action. Allowing this same leaf
to rest for only a single day, I put on another fly, and it again
closed, but now very slowly; nevertheless, in less than two days it
succeeded in thoroughly clasping the fly.

When a small object is placed on the glands of the disc, on one side of
a leaf, as near as possible to its circumference, the tentacles on this
side are first affected, those on the opposite side much later, or, as
often occurred, not at all. This was repeatedly proved by trials with
bits of meat; but I will here give only the case of a minute fly,
naturally caught and still alive, which I found adhering by its
delicate feet to the glands on the extreme left side of the central
disc. The marginal tentacles on this side closed inwards and killed the
fly, and after a time the edge of the leaf on this side also became
inflected, and thus remained for several days, whilst neither the
tentacles nor the edge on the opposite side were in the least affected.

If young and active leaves are selected, inorganic particles not larger
than the head of a small pin, placed on the central glands, sometimes
cause the [page 22] outer tentacles to bend inwards. But this follows
much more surely and quickly, if the object contains nitrogenous matter
which can be dissolved by the secretion. On one occasion I observed the
following unusual circumstance. Small bits of raw meat (which acts more
energetically than any other substance), of paper, dried moss, and of
the quill of a pen were placed on several leaves, and they were all
embraced equally well in about 2 hrs. On other occasions the
above-named substances, or more commonly particles of glass,
coal-cinder (taken from the fire), stone, gold-leaf, dried grass, cork,
blotting-paper, cotton-wool, and hair rolled up into little balls, were
used, and these substances, though they were sometimes well embraced,
often caused no movement whatever in the outer tentacles, or an
extremely slight and slow movement. Yet these same leaves were proved
to be in an active condition, as they were excited to move by
substances yielding soluble nitrogenous matter, such as bits of raw or
roast meat, the yolk or white of boiled eggs, fragments of insects of
all orders, spiders, &c. I will give only two instances. Minute flies
were placed on the discs of several leaves, and on others balls of
paper, bits of moss and quill of about the same size as the flies, and
the latter were well embraced in a few hours; whereas after 25 hrs.
only a very few tentacles were inflected over the other objects. The
bits of paper, moss, and quill were then removed from these leaves, and
bits of raw meat placed on them; and now all the tentacles were soon
energetically inflected.

Again, particles of coal-cinder (weighing rather more than the flies
used in the last experiment) were placed on the centres of three
leaves: after an interval of 19 hrs. one of the particles was tolerably
well embraced; [page 23] a second by a very few tentacles; and a third
by none. I then removed the particles from the two latter leaves, and
put on them recently killed flies. These were fairly well embraced in 7
1/2 hrs. and thoroughly after 20 1/2 hrs.; the tentacles remaining
inflected for many subsequent days. On the other hand, the one leaf
which had in the course of 19 hrs. embraced the bit of cinder
moderately well, and to which no fly was given, after an additional 33
hrs. (i.e. in 52 hrs. from the time when the cinder was put on) was
completely re-expanded and ready to act again.

From these and numerous other experiments not worth giving, it is
certain that inorganic substances, or such organic substances as are
not attacked by the secretion, act much less quickly and efficiently
than organic substances yielding soluble matter which is absorbed.
Moreover, I have met with very few exceptions to the rule, and these
exceptions apparently depended on the leaf having been too recently in
action, that the tentacles remain clasped for a much longer time over
organic bodies of the nature just specified than over those which are
not acted on by the secretion, or over inorganic objects.*

* Owing to the extraordinary belief held by M. Ziegler (‘Comptes
rendus,’ May 1872, p. 122), that albuminous substances, if held for a
moment between the fingers, acquire the property of making the
tentacles of Drosera contract, whereas, if not thus held, they have no
such power, I tried some experiments with great care, but the results
did not confirm this belief. Red-hot cinders were taken out of the
fire, and bits of glass, cotton-thread, blotting paper and thin slices
of cork were immersed in boiling water; and particles were then placed
(every instrument with which they were touched having been previously
immersed in boiling water) on the glands of several leaves, and they
acted in exactly the same manner as other particles, which had been
purposely handled for some time. Bits of a boiled egg, cut with a knife
which had been washed in boiling water, also acted like any other
animal substance. I breathed on some leaves for above a minute, and
repeated the act two or three times, with my mouth close to [[page 24]]
them, but this produced no effect. I may here add, as showing that the
leaves are not acted on by the odour of nitrogenous substances, that
pieces of raw meat stuck on needles were fixed as close as possible,
without actual contact, to several leaves, but produced no effect
whatever. On the other hand, as we shall hereafter see, the vapours of
certain volatile substances and fluids, such as of carbonate of
ammonia, chloroform, certain essential oils, &c., cause inflection. M.
Ziegler makes still more extraordinary statements with respect to the
power of animal substances, which have been left close to, but not in
contact with, sulphate of quinine. The action of salts of quinine will
be described in a future chapter. Since the appearance of the paper
above referred to, M. Ziegler has published a book on the same subject,
entitled ‘Atonicit et Zoicit,’ 1874.) [page 24]

The Inflection of the Exterior Tentacles as directly caused by Objects
left in Contact with their Glands.

I made a vast number of trials by placing, by means of a fine needle
moistened with distilled water, and with the aid of a lens, particles
of various substances on the viscid secretion surrounding the glands of
the outer tentacles. I experimented on both the oval and long-headed
glands. When a particle is thus placed on a single gland, the movement
of the tentacle is particularly well seen in contrast with the
stationary condition of the surrounding tentacles. (See previous fig.
6.) In four cases small particles of raw meat caused the tentacles to
be greatly inflected in between 5 and 6 m. Another tentacle similarly
treated, and observed with special care, distinctly, though slightly,
changed its position in 10 s. (seconds); and this is the quickest
movement seen by me. In 2 m. 30 s. it had moved through an angle of
about 45o. The movement as seen through a lens resembled that of the
hand of a large clock. In 5 m. it had moved through 90o, and when I
looked again after 10 m., the particle had reached the centre of the
leaf; so that the whole movement was completed in less [page 25] than
17 m. 30 s. In the course of some hours this minute bit of meat, from
having been brought into contact with some of the glands of the central
disc, acted centrifugally on the outer tentacles, which all became
closely inflected. Fragments of flies were placed on the glands of four
of the outer tentacles, extended in the same plane with that of the
blade, and three of these fragments were carried in 35 m. through an
angle of 180o to the centre. The fragment on the fourth tentacle was
very minute, and it was not carried to the centre until 3 hrs. had
elapsed. In three other cases minute flies or portions of larger ones
were carried to the centre in 1 hr. 30 s. In these seven cases, the
fragments or small flies, which had been carried by a single tentacle
to the central glands, were well embraced by the other tentacles after
an interval of from 4 to 10 hrs.

I also placed in the manner just described six small balls of
writing-paper (rolled up by the aid of pincers, so that they were not
touched by my fingers) on the glands of six exterior tentacles on
distinct leaves; three of these were carried to the centre in about 1
hr., and the other three in rather more than 4 hrs.; but after 24 hrs.
only two of the six balls were well embraced by the other tentacles. It
is possible that the secretion may have dissolved a trace of glue or
animalised matter from the balls of paper. Four particles of
coal-cinder were then placed on the glands of four exterior tentacles;
one of these reached the centre in 3 hrs. 40 m.; the second in 9 hrs.;
the third within 24 hrs., but had moved only part of the way in 9 hrs.;
whilst the fourth moved only a very short distance in 24 hrs., and
never moved any farther. Of the above three bits of cinder which were
ultimately carried to the centre, one alone was well embraced by [page
26] many of the other tentacles. We here see clearly that such bodies
as particles of cinder or little balls of paper, after being carried by
the tentacles to the central glands, act very differently from
fragments of flies, in causing the movement of the surrounding
tentacles.

I made, without carefully recording the times of movement, many similar
trials with other substances, such as splinters of white and blue
glass, particles of cork, minute bits of gold-leaf, &c.; and the
proportional number of cases varied much in which the tentacles reached
the centre, or moved only slightly, or not at all. One evening,
particles of glass and cork, rather larger than those usually employed,
were placed on about a dozen glands, and next morning, after 13 hrs.,
every single tentacle had carried its little load to the centre; but
the unusually large size of the particles will account for this result.
In another case 6/7 of the particles of cinder, glass, and thread,
placed on separate glands, were carried towards, or actually to, the
centre; in another case 7/9, in another 7/12, and in the last case only
7/26 were thus carried inwards, the small proportion being here due, at
least in part, to the leaves being rather old and inactive.
Occasionally a gland, with its light load, could be seen through a
strong lens to move an extremely short distance and then stop; this was
especially apt to occur when excessively minute particles, much less
than those of which the measurements will be immediately given, were
placed on glands; so that we here have nearly the limit of any action.

I was so much surprised at the smallness of the particles which caused
the tentacles to become greatly inflected that it seemed worth while
carefully to ascertain how minute a particle would plainly act. [page
27] Accordingly measured lengths of a narrow strip of blotting paper,
of fine cotton-thread, and of a woman’s hair, were carefully weighed
for me by Mr. Trenham Reeks, in an excellent balance, in the laboratory
in Jermyn Street. Short bits of the paper, thread, and hair were then
cut off and measured by a micrometer, so that their weights could be
easily calculated. The bits were placed on the viscid secretion
surrounding the glands of the exterior tentacles, with the precautions
already stated, and I am certain that the gland itself was never
touched; nor indeed would a single touch have produced any effect. A
bit of the blotting-paper, weighing 1/465 of a grain, was placed so as
to rest on three glands together, and all three tentacles slowly curved
inwards; each gland, therefore, supposing the weight to be distributed
equally, could have been pressed on by only 1/1395 of a grain, or .0464
of a milligramme. Five nearly equal bits of cotton-thread were tried,
and all acted. The shortest of these was 1/50 of an inch in length, and
weighed 1/8197 of a grain. The tentacle in this case was considerably
inflected in 1 hr. 30 m., and the bit of thread was carried to the
centre of the leaf in 1 hr. 40 m. Again, two particles of the thinner
end of a woman’s hair, one of these being 18/1000 of an inch in length,
and weighing 1/35714 of a grain, the other 19/1000 of an inch in
length, and weighing of course a little more, were placed on two glands
on opposite sides of the same leaf, and these two tentacles were
inflected halfway towards the centre in 1 hr. 10 m.; all the many other
tentacles round the same leaf remaining motionless. The appearance of
this one leaf showed in an unequivocal manner that these minute
particles sufficed to cause the tentacles to bend. Altogether, ten such
particles of hair were placed on ten glands on several leaves, and
seven of them caused [page 28] the tentacles to move in a conspicuous
manner. The smallest particle which was tried, and which acted plainly,
was only 8/1000 of an inch (.203 millimetre) in length, and weighed the
1/78740 of a grain, or .000822 milligramme. In these several cases, not
only was the inflection of the tentacles conspicuous, but the purple
fluid within their cells became aggregated into little masses of
protoplasm, in the manner to be described in the next chapter; and the
aggregation was so plain that I could, by this clue alone, have readily
picked out under the microscope all the tentacles which had carried
their light loads towards the centre, from the hundreds of other
tentacles on the same leaves which had not thus acted.

My surprise was greatly excited, not only by the minuteness of the
particles which caused movement, but how they could possibly act on the
glands; for it must be remembered that they were laid with the greatest
care on the convex surface of the secretion. At first I thought—but, as
I now know, erroneously—that particles of such low specific gravity as
those of cork, thread, and paper, would never come into contact with
the surfaces of the glands. The particles cannot act simply by their
weight being added to that of the secretion, for small drops of water,
many times heavier than the particles, were repeatedly added, and never
produced any effect. Nor does the disturbance of the secretion produce
any effect, for long threads were drawn out by a needle, and affixed to
some adjoining object, and thus left for hours; but the tentacles
remained motionless.

I also carefully removed the secretion from four glands with a sharply
pointed piece of blotting-paper, so that they were exposed for a time
naked to the air, but this caused no movement; yet these glands were
[page 29] in an efficient state, for after 24 hrs. had elapsed, they
were tried with bits of meat, and all became quickly inflected. It then
occurred to me that particles floating on the secretion would cast
shadows on the glands, which might be sensitive to the interception of
the light. Although this seemed highly improbable, as minute and thin
splinters of colourless glass acted powerfully, nevertheless, after it
was dark, I put on, by the aid of a single tallow candle, as quickly as
possible, particles of cork and glass on the glands of a dozen
tentacles, as well as some of meat on other glands, and covered them up
so that not a ray of light could enter; but by the next morning, after
an interval of 13 hrs., all the particles were carried to the centres
of the leaves.

These negative results led me to try many more experiments, by placing
particles on the surface of the drops of secretion, observing, as
carefully as I could, whether they penetrated it and touched the
surface of the glands. The secretion, from its weight, generally forms
a thicker layer on the under than on the upper sides of the glands,
whatever may be the position of the tentacles. Minute bits of dry cork,
thread, blotting paper, and coal cinders were tried, such as those
previously employed; and I now observed that they absorbed much more of
the secretion, in the course of a few minutes, than I should have
thought possible; and as they had been laid on the upper surface of the
secretion, where it is thinnest, they were often drawn down, after a
time, into contact with at least some one point of the gland. With
respect to the minute splinters of glass and particles of hair, I
observed that the secretion slowly spread itself a little over their
surfaces, by which means they were likewise drawn downwards or
sideways, and thus one end, or some minute [page 30] prominence, often
came to touch, sooner or later, the gland.

In the foregoing and following cases, it is probable that the
vibrations, to which the furniture in every room is continually liable,
aids in bringing the particles into contact with the glands. But as it
was sometimes difficult, owing to the refraction of the secretion, to
feel sure whether the particles were in contact, I tried the following
experiment. Unusually minute particles of glass, hair, and cork, were
gently placed on the drops round several glands, and very few of the
tentacles moved. Those which were not affected were left for about half
an hour, and the particles were then disturbed or tilted up several
times with a fine needle under the microscope, the glands not being
touched. And now in the course of a few minutes almost all the hitherto
motionless tentacles began to move; and this, no doubt, was caused by
one end or some prominence of the particles having come into contact
with the surface of the glands. But as the particles were unusually
minute, the movement was small.

Lastly, some dark blue glass pounded into fine splinters was used, in
order that the points of the particles might be better distinguished
when immersed in the secretion; and thirteen such particles were placed
in contact with the depending and therefore thicker part of the drops
round so many glands. Five of the tentacles began moving after an
interval of a few minutes, and in these cases I clearly saw that the
particles touched the lower surface of the gland. A sixth tentacle
moved after 1 hr. 45 m., and the particle was now in contact with the
gland, which was not the case at first. So it was with the seventh
tentacle, but its movement did not begin until 3 hrs. 45 m. had [page
31] elapsed. The remaining six tentacles never moved as long as they
were observed; and the particles apparently never came into contact
with the surfaces of the glands.

From these experiments we learn that particles not containing soluble
matter, when placed on glands, often cause the tentacles to begin
bending in the course of from one to five minutes; and that in such
cases the particles have been from the first in contact with the
surfaces of the glands. When the tentacles do not begin moving for a
much longer time, namely, from half an hour to three or four hours, the
particles have been slowly brought into contact with the glands, either
by the secretion being absorbed by the particles or by its gradual
spreading over them, together with its consequent quicker evaporation.
When the tentacles do not move at all, the particles have never come
into contact with the glands, or in some cases the tentacles may not
have been in an active condition. In order to excite movement, it is
indispensable that the particles should actually rest on the glands;
for a touch once, twice, or even thrice repeated by any hard body is
not sufficient to excite movement.

Another experiment, showing that extremely minute particles act on the
glands when immersed in water, may here be given. A grain of sulphate
of quinine was added to an ounce of water, which was not afterwards
filtered; and on placing three leaves in ninety minims of this fluid, I
was much surprised to find that all three leaves were greatly inflected
in 15 m.; for I knew from previous trials that the solution does not
act so quickly as this. It immediately occurred to me that the
particles of the undissolved salt, which were so light as to float
about, might have come [page 32] into contact with the glands, and
caused this rapid movement. Accordingly I added to some distilled water
a pinch of a quite innocent substance, namely, precipitated carbonate
of lime, which consists of an impalpable powder; I shook the mixture,
and thus got a fluid like thin milk. Two leaves were immersed in it,
and in 6 m. almost every tentacle was much inflected. I placed one of
these leaves under the microscope, and saw innumerable atoms of lime
adhering to the external surface of the secretion. Some, however, had
penetrated it, and were lying on the surfaces of the glands; and no
doubt it was these particles which caused the tentacles to bend. When a
leaf is immersed in water, the secretion instantly swells much; and I
presume that it is ruptured here and there, so that little eddies of
water rush in. If so, we can understand how the atoms of chalk, which
rested on the surfaces of the glands, had penetrated the secretion.
Anyone who has rubbed precipitated chalk between his fingers will have
perceived how excessively fine the powder is. No doubt there must be a
limit, beyond which a particle would be too small to act on a gland;
but what this limit is, I know not. I have often seen fibres and dust,
which had fallen from the air, on the glands of plants kept in my room,
and these never induced any movement; but then such particles lay on
the surface of the secretion and never reached the gland itself.

Finally, it is an extraordinary fact that a little bit of soft thread,
1/50 of an inch in length and weighing 1/8197 of a grain, or of a human
hair, 8/1000 of an inch in length and weighing only 1/78740 of a grain
(.000822 milligramme), or particles of precipitated chalk, after
resting for a short time on a gland, should induce some change in its
cells, exciting them [page 33] to transmit a motor impulse throughout
the whole length of the pedicel, consisting of about twenty cells, to
near its base, causing this part to bend, and the tentacle to sweep
through an angle of above 180o. That the contents of the cells of the
glands, and afterwards those of the pedicels, are affected in a plainly
visible manner by the pressure of minute particles, we shall have
abundant evidence when we treat of the aggregation of protoplasm. But
the case is much more remarkable than as yet stated; for the particles
are supported by the viscid and dense secretion; nevertheless, even
smaller ones than those of which the measurements have been given, when
brought by an insensibly slow movement, through the means above
specified, into contact with the surface of a gland, act on it, and the
tentacle bends. The pressure exerted by the particle of hair, weighing
only 1/78740 of a grain and supported by a dense fluid, must have been
inconceivably slight. We may conjecture that it could hardly have
equalled the millionth of a grain; and we shall hereafter see that far
less than the millionth of a grain of phosphate of ammonia in solution,
when absorbed by a gland, acts on it and induces movement. A bit of
hair, 1/50 of an inch in length, and therefore much larger than those
used in the above experiments, was not perceived when placed on my
tongue; and it is extremely doubtful whether any nerve in the human
body, even if in an inflamed condition, would be in any way affected by
such a particle supported in a dense fluid, and slowly brought into
contact with the nerve. Yet the cells of the glands of Drosera are thus
excited to transmit a motor impulse to a distant point, inducing
movement. It appears to me that hardly any more remarkable fact than
this has been observed in the vegetable kingdom. [page 34]

The Inflection of the Exterior Tentacles, when their Glands are excited
by Repeated Touches.

We have already seen that, if the central glands are excited by being
gently brushed, they transmit a motor impulse to the exterior
tentacles, causing them to bend; and we have now to consider the
effects which follow from the glands of the exterior tentacles being
themselves touched. On several occasions, a large number of glands were
touched only once with a needle or fine brush, hard enough to bend the
whole flexible tentacle; and though this must have caused a
thousand-fold greater pressure than the weight of the above described
particles, not a tentacle moved. On another occasion forty-five glands
on eleven leaves were touched once, twice, or even thrice, with a
needle or stiff bristle. This was done as quickly as possible, but with
force sufficient to bend the tentacles; yet only six of them became
inflected,—three plainly, and three in a slight degree. In order to
ascertain whether these tentacles which were not affected were in an
efficient state, bits of meat were placed on ten of them, and they all
soon became greatly incurved. On the other hand, when a large number of
glands were struck four, five, or six times with the same force as
before, a needle or sharp splinter of glass being used, a much larger
proportion of tentacles became inflected; but the result was so
uncertain as to seem capricious. For instance, I struck in the above
manner three glands, which happened to be extremely sensitive, and all
three were inflected almost as quickly, as if bits of meat had been
placed on them. On another occasion I gave a single for- [page 35]
cible touch to a considerable number of glands, and not one moved; but
these same glands, after an interval of some hours, being touched four
or five times with a needle, several of the tentacles soon became
inflected.

The fact of a single touch or even of two or three touches not causing
inflection must be of some service to the plant; as during stormy
weather, the glands cannot fail to be occasionally touched by the tall
blades of grass, or by other plants growing near; and it would be a
great evil if the tentacles were thus brought into action, for the act
of re-expansion takes a considerable time, and until the tentacles are
re-expanded they cannot catch prey. On the other hand, extreme
sensitiveness to slight pressure is of the highest service to the
plant; for, as we have seen, if the delicate feet of a minute
struggling insect press ever so lightly on the surfaces of two or three
glands, the tentacles bearing these glands soon curl inwards and carry
the insect with them to the centre, causing, after a time, all the
circumferential tentacles to embrace it. Nevertheless, the movements of
the plant are not perfectly adapted to its requirements; for if a bit
of dry moss, peat, or other rubbish, is blown on to the disc, as often
happens, the tentacles clasp it in a useless manner. They soon,
however, discover their mistake and release such innutritious objects.

It is also a remarkable fact, that drops of water falling from a
height, whether under the form of natural or artificial rain, do not
cause the tentacles to move; yet the drops must strike the glands with
considerable force, more especially after the secretion has been all
washed away by heavy rain; and this often occurs, [page 36] though the
secretion is so viscid that it can be removed with difficulty merely by
waving the leaves in water. If the falling drops of water are small,
they adhere to the secretion, the weight of which must be increased in
a much greater degree, as before remarked, than by the addition of
minute particles of solid matter; yet the drops never cause the
tentacles to become inflected. It would obviously have been a great
evil to the plant (as in the case of occasional touches) if the
tentacles were excited to bend by every shower of rain; but this evil
has been avoided by the glands either having become through habit
insensible to the blows and prolonged pressure of drops of water, or to
their having been originally rendered sensitive solely to the contact
of solid bodies. We shall hereafter see that the filaments on the
leaves of Dionaea are likewise insensible to the impact of fluids,
though exquisitely sensitive to momentary touches from any solid body.

When the pedicel of a tentacle is cut off by a sharp pair of scissors
quite close beneath the gland, the tentacle generally becomes
inflected. I tried this experiment repeatedly, as I was much surprised
at the fact, for all other parts of the pedicels are insensible to any
stimulus. These headless tentacles after a time re-expand; but I shall
return to this subject. On the other hand, I occasionally succeeded in
crushing a gland between a pair of pincers, but this caused no
inflection. In this latter case the tentacles seem paralysed, as
likewise follows from the action of too strong solutions of certain
salts, and by too great heat, whilst weaker solutions of the same salts
and a more gentle heat cause movement. We shall also see in future
chapters that various other fluids, some [page 37] vapours, and oxygen
(after the plant has been for some time excluded from its action), all
induce inflection, and this likewise results from an induced galvanic
current.*

* My son Francis, guided by the observations of Dr. Burdon Sanderson on
Dionaea, finds that if two needles are inserted into the blade of a
leaf of Drosera, the tentacles do not move; but that if similar needles
in connection with the secondary coil of a Du Bois inductive apparatus
are inserted, the tentacles curve inwards in the course of a few
minutes. My son hopes soon to publish an account of his observations.
[page 38]