PINGUICULA.
Pinguicula vulgaris—Structure of leaves—Number of insects and other
objects caught— Movement of the margins of the leaves—Uses of this
movement—Secretion, digestion, and absorption—Action of the secretion
on various animal and vegetable substances—The effects of substances
not containing soluble nitrogenous matter on the glands—Pinguicula
grandiflora—Pinguicula lusitanica, catches insects—Movement of the
leaves, secretion and digestion.
Pinguicula vulgaris.—This plant grows in moist places, generally on
mountains. It bears on an average eight, rather thick, oblong, light
green leaves, having scarcely any footstalk. A full-sized leaf is about
1 1/2 inch in length and 3/4 inch in breadth. The young central leaves
are deeply concave, and project upwards; the older ones towards the
outside are flat or convex, and lie close to the ground, forming a
rosette from 3 to 4 inches in diameter. The margins of the leaves are
incurved. Their upper surfaces are thickly covered with two sets of
glandular hairs, differing in the size of the glands and in the length
of their pedicels. The larger glands have a circular outline as seen
from above, and are of moderate thickness; they are divided by
radiating partitions into sixteen cells, containing light-green,
homogeneous fluid. They are supported on elongated, unicellular
pedicels (containing a nucleus with a nucleolus) which rest on slight
prominences. The small glands differ only in being formed of about half
the number of cells, containing much paler fluid, and supported on much
shorter pedicels. Near the midrib, towards the base of the leaf, the
[page 369] pedicels are multicellular, are longer than elsewhere, and
bear smaller glands. All the glands secrete a colourless fluid, which
is so viscid that I have seen a fine thread drawn out to a length of 18
inches; but the fluid in this case was secreted by a gland which had
been excited. The edge of the leaf is translucent, and does not bear
any glands; and here the spiral vessels, proceeding from the midrib,
terminate in cells marked by a spiral line, somewhat like those within
the glands of Drosera.
The roots are short. Three plants were dug up in North Wales on June
20, and carefully washed; each bore five or six unbranched roots, the
longest of which was only 1.2 of an inch. Two rather young plants were
examined on September 28; these had a greater number of roots, namely
eight and eighteen, all under 1 inch in length, and very little
branched.
I was led to investigate the habits of this plant by being told by Mr.
W. Marshall that on the mountains of Cumberland many insects adhere to
the leaves.
[A friend sent me on June 23 thirty-nine leaves from North Wales, which
were selected owing to objects of some kind adhering to them. Of these
leaves, thirty-two had caught 142 insects, or on an average 4.4 per
leaf, minute fragments of insects not being included. Besides the
insects, small leaves belonging to four different kinds of plants,
those of Erica tetralix being much the commonest, and three minute
seedling plants, blown by the wind, adhered to nineteen of the leaves.
One had caught as many as ten leaves of the Erica. Seeds or fruits,
commonly of Carex and one of Juncus, besides bits of moss and other
rubbish, likewise adhered to six of the thirty-nine leaves. The same
friend, on June 27, collected nine plants bearing seventy-four leaves,
and all of these, with the exception of three young leaves, had caught
insects; thirty insects were counted on one leaf, eighteen on a second,
and sixteen on a third. Another friend examined on August 22 some
plants in Donegal, Ireland, and found insects on 70 out of 157 leaves;
fifteen of [page 370] these leaves were sent me, each having caught on
an average 2.4 insects. To nine of them, leaves (mostly of Erica
tetralix) adhered; but they had been specially selected on this latter
account. I may add that early in August my son found leaves of this
same Erica and the fruits of a Carex on the leaves of a Pinguicula in
Switzerland, probably Pinguicula alpina; some insects, but no great
number, also adhered to the leaves of this plant, which had much better
developed roots than those of Pinguicula vulgaris. In Cumberland, Mr.
Marshall, on September 3, carefully examined for me ten plants bearing
eighty leaves; and on sixty-three of these (i.e. on 79 per cent.) he
found insects, 143 in number; so that each leaf had on an average 2.27
insects. A few days later he sent me some plants with sixteen seeds or
fruits adhering to fourteen leaves. There was a seed on three leaves on
the same plant. The sixteen seeds belonged to nine different kinds,
which could not be recognised, excepting one of Ranunculus, and several
belonging to three or four distinct species of Carex. It appears that
fewer insects are caught late in the year than earlier; thus in
Cumberland from twenty to twenty-four insects were observed in the
middle of July on several leaves, whereas in the beginning of September
the average number was only 2.27. Most of the insects, in all the
foregoing cases, were Diptera, but with many minute Hymenoptera,
including some ants, a few small Coleoptera, larvæ, spiders, and even
small moths.]
We thus see that numerous insects and other objects are caught by the
viscid leaves; but we have no right to infer from this fact that the
habit is beneficial to the plant, any more than in the before given
case of the Mirabilis, or of the horse-chestnut. But it will presently
be seen that dead insects and other nitrogenous bodies excite the
glands to increased secretion; and that the secretion then becomes acid
and has the power of digesting animal substances, such as albumen,
fibrin, &c. Moreover, the dissolved nitrogenous matter is absorbed by
the glands, as shown by their limpid contents being aggregated into
slowly moving granular masses of protoplasm. The same results follow
when insects are naturally captured, and as the plant lives in poor
soil and has small roots, there can be no [page 371] doubt that it
profits by its power of digesting and absorbing matter from the prey
which it habitually captures in such large numbers. It will, however,
be convenient first to describe the movements of the leaves.
Movements of the Leaves.—That such thick, large leaves as those of
Pinguicula vulgarisshould have the power of curving inwards when
excited has never even been suspected. It is necessary to select for
experiment leaves with their glands secreting freely, and which have
been prevented from capturing many insects; as old leaves, at least
those growing in a state of nature, have their margins already curled
so much inwards that they exhibit little power of movement, or move
very slowly. I will first give in detail the more important experiments
which were tried, and then make some concluding remarks.
[Experiment 1.—A young and almost upright leaf was selected, with its
two lateral edges equally and very slightly incurved. A row of small
flies was placed along one margin. When looked at next day, after 15
hrs., this margin, but not the other, was found folded inwards, like
the helix of the human ear, to the breadth of 1/10 of an inch, so as to
lie partly over the row of flies (fig. 15). The glands on which the
flies rested, as well as those on the over-lapping margin which had
been brought into contact with the flies, were all secreting copiously.
FIG. 15. (Pinguicula vulgaris.) Outline of leaf with left margin
inflected over a row of small flies.
Experiment 2.—A row of flies was placed on one margin of a rather old
leaf, which lay flat on the ground; and in this case the margin, after
the same interval as before, namely 15 hrs., had only just begun to
curl inwards; but so much secretion had been poured forth that the
spoon-shaped tip of the leaf was filled with it.
Experiment 3.—Fragments of a large fly were placed close to the apex of
a vigorous leaf, as well as along half one margin. [page 372] After 4
hrs. 20 m. there was decided incurvation, which increased a little
during the afternoon, but was in the same state on the following
morning. Near the apex both margins were inwardly curved. I have never
seen a case of the apex itself being in the least curved towards the
base of the leaf. After 48 hrs. (always reckoning from the time when
the flies were placed on the leaf) the margin had everywhere begun to
unfold.
Experiment 4.—A large fragment of a fly was placed on a leaf, in a
medial line, a little beneath the apex. Both lateral margins were
perceptibly incurved in 3 hrs., and after 4 hrs. 20 m. to such a degree
that the fragment was clasped by both margins. After 24 hrs. the two
infolded edges near the apex (for the lower part of the leaf was not at
all affected) were measured and found to be .11 of an inch (2.795 mm.)
apart. The fly was now removed, and a stream of water poured over the
leaf so as to wash the surface; and after 24 hrs. the margins were .25
of an inch (6.349 mm.) apart, so that they were largely unfolded. After
an additional 24 hrs. they were completely unfolded. Another fly was
now put on the same spot to see whether this leaf, on which the first
fly had been left 24 hrs., would move again; after 10 hrs. there was a
trace of incurvation, but this did not increase during the next 24 hrs.
A bit of meat was also placed on the margin of a leaf, which four days
previously had become strongly incurved over a fragment of a fly and
had afterwards re-expanded; but the meat did not cause even a trace of
incurvation. On the contrary, the margin became somewhat reflexed, as
if injured, and so remained for the three following days, as long as it
was observed.
Experiment 5.—A large fragment of a fly was placed halfway between the
apex and base of a leaf and halfway between the midrib and one margin.
A short space of this margin, opposite the fly, showed a trace of
incurvation after 3 hrs., and this became strongly pronounced in 7 hrs.
After 24 hrs. the infolded edge was only .16 of an inch (4.064 mm.)
from the midrib. The margin now began to unfold, though the fly was
left on the leaf; so that by the next morning (i.e. 48 hrs. from the
time when the fly was first put on) the infolded edge had almost
completely recovered its original position, being now .3 of an inch
(7.62 mm.), instead of .16 of an inch, from the midrib. A trace of
flexure was, however, still visible.
Experiment 6.—A young and concave leaf was selected with its margins
slightly and naturally incurved. Two rather large, oblong, rectangular
pieces of roast meat were placed with their ends touching the infolded
edge, and .46 of an inch (11.68 mm.) [page 373] apart from one another.
After 24 hrs. the margin was greatly and equally incurved (see fig. 16)
throughout this space, and for a length of .12 or .13 of an inch (3.048
or 3.302 mm.) above and below each bit; so that the margin had been
affected over a greater length between the two bits, owing to their
conjoint action, than beyond them. The bits of meat were too large to
be clasped by the margin, but they were tilted up, one of them so as to
stand almost vertically. After 48 hrs. the margin was almost unfolded,
and the bits had sunk down. When again examined after two days, the
margin was quite unfolded, with the exception of the naturally
inflected edge; and one of the bits of meat, the end of which had at
first touched the edge, was now .067 of an inch (1.70 mm.) distant from
it; so that this bit had been pushed thus far across the blade of the
leaf.
FIG. 16. (Pinguicula vulgaris.) Outline of leaf, with right margin
inflected against two square bits of meat.
Experiment 7.—A bit of meat was placed close to the incurved edge of a
rather young leaf, and after it had re-expanded, the bit was left lying
.11 of an inch (2.795 mm.) from the edge. The distance from the edge to
the midrib of the fully expanded leaf was .35 of an inch (8.89 mm.); so
that the bit had been pushed inwards and across nearly one-third of its
semi-diameter.
Experiment 8.—Cubes of sponge, soaked in a strong infusion of raw meat,
were placed in close contact with the incurved edges of two leaves,—an
older and younger one. The distance from the edges to the midribs was
carefully measured. After 1 hr. 17 m. there appeared to be a trace of
incurvation. After 2 hrs. 17 m. both leaves were plainly inflected; the
distance between the edges and midribs being now only half what it was
at first. The incurvation increased slightly during the next 4 1/2
hrs., but remained nearly the same for the next 17 hrs. 30 m. In 35
hrs. from the time when the sponges were placed on the leaves, the
margins were a little unfolded—to a greater degree in the younger than
in the older leaf. The latter was not quite unfolded until the third
day, and now both bits of sponge were left at the distance of .1 of an
inch (2.54 mm.) from the edges; or about a quarter of the distance
between the edge and midrib. A third bit of sponge adhered to the edge,
and, as the margin unfolded, was dragged backwards, into its original
position. [page 374]
Experiment 9.—A chain of fibres of roast meat, as thin as bristles and
moistened with saliva, were placed down one whole side, close to the
narrow, naturally incurved edge of a leaf. In 3 hrs. this side was
greatly incurved along its whole length, and after 8 hrs. formed a
cylinder, about 1/20 of an inch (1.27 mm) in diameter, quite concealing
the meat. This cylinder remained closed for 32 hrs., but after 48 hrs.
was half unfolded, and in 72 hrs. was as open as the opposite margin
where no meat had been placed. As the thin fibres of meat were
completely overlapped by the margin, they were not pushed at all
inwards, across the blade.
Experiment 10.—Six cabbage seeds, soaked for a night in water, were
placed in a row close to the narrow incurved edge of a leaf. We shall
hereafter see that these seeds yield soluble matter to the glands. In 2
hrs. 25 m. the margin was decidedly inflected; in 4 hrs. it extended
over the seeds for about half their breadth, and in 7 hrs. over
three-fourths of their breadth, forming a cylinder not quite closed
along the inner side, and about .7 of an inch (1.778 mm.) in diameter.
After 24 hrs. the inflection had not increased, perhaps had decreased.
The glands which had been brought into contact with the upper surfaces
of the seeds were now secreting freely. In 36 hrs. from the time when
the seeds were put on the leaf the margin had greatly, and after 48
hrs. had completely, re-expanded. As the seeds were no longer held by
the inflected margin, and as the secretion was beginning to fail, they
rolled some way down the marginal channel.
Experiment 11.—Fragments of glass were placed on the margins of two
fine young leaves. After 2 hrs. 30 m. the margin of one certainly
became slightly incurved; but the inflection never increased, and
disappeared in 16 hrs. 30 m. from the time when the fragments were
first applied. With the second leaf there was a trace of incurvation in
2 hrs. 15 m., which became decided in 4 hrs. 30 m., and still more
strongly pronounced in 7 hrs., but after 19 hrs. 30 m. had plainly
decreased. The fragments excited at most a slight and doubtful increase
of the secretion; and in two other trials, no increase could be
perceived. Bits of coal-cinders, placed on a leaf, produced no effect,
either owing to their lightness or to the leaf being torpid.
Experiment 12.—We now turn to fluids. A row of drops of a strong
infusion of raw meat were placed along the margins of two leaves;
squares of sponge soaked in the same infusion being placed on the
opposite margins. My object was to ascer- [page 375] tain whether a
fluid would act as energetically as a substance yielding the same
soluble matter to the glands. No distinct difference was perceptible;
certainly none in the degree of incurvation; but the incurvation round
the bits of sponge lasted rather longer, as might perhaps have been
expected from the sponge remaining damp and supplying nitrogenous
matter for a longer time. The margins, with the drops, became plainly
incurved in 2 hrs. 17 m. The incurvation subsequently increased
somewhat, but after 24 hrs. had greatly decreased.
Experiment 13.—Drops of the same strong infusion of raw meat were
placed along the midrib of a young and rather deeply concave leaf. The
distance across the broadest part of the leaf, between the naturally
incurved edges, was .55 of an inch (13.97 mm.). In 3 hrs. 27 m. this
distance was a trace less; in 6 hrs. 27 m. it was exactly .45 of an
inch (11.43 mm.), and had therefore decreased by .1 of an inch (2.54
mm.). After only 10 hrs. 37 m. the margin began to re-expand, for the
distance from edge to edge was now a trace wider, and after 24 hrs. 20
m. was as great, within a hair’s breadth, as when the drops were first
placed on the leaf. From this experiment we learn that the motor
impulse can be transmitted to a distance of .22 of an inch (5.590 mm.)
in a transverse direction from the midrib to both margins; but it would
be safer to say .2 of an inch (5.08 mm.) as the drops spread a little
beyond the midrib. The incurvation thus caused lasted for an unusually
short time.
Experiment 14.—Three drops of a solution of one part of carbonate of
ammonia to 218 of water (2 grs. to 1 oz.) were placed on the margin of
a leaf. These excited so much secretion that in 1 h. 22 m. all three
drops ran together; but although the leaf was observed for 24 hrs.,
there was no trace of inflection. We know that a rather strong solution
of this salt, though it does not injure the leaves of Drosera,
paralyses their power of movement, and I have no doubt, from the
following case, that this holds good with Pinguicula.
Experiment 15.—A row of drops of a solution of one part of carbonate of
ammonia to 875 of water (1 gr. to 2 oz.) was placed on the margin of a
leaf. In 1 hr. there was apparently some slight incurvation, and this
was well-marked in 3 hrs. 30 m. After 24 hrs. the margin was almost
completely re-expanded.
Experiment 16.—A row of large drops of a solution of one part of
phosphate of ammonia to 4375 of water (1 gr. to 10 oz.) was placed
along the margin of a leaf. No effect was produced, and after 8 hrs.
fresh drops were added along the same margin without the least effect.
We know that a solution of this [page 376] strength acts powerfully on
Drosera, and it is just possible that the solution was too strong. I
regret that I did not try a weaker solution.
Experiment 17.—As the pressure from bits of glass causes incurvation, I
scratched the margins of two leaves for some minutes with a blunt
needle, but no effect was produced. The surface of a leaf beneath a
drop of a strong infusion of raw meat was also rubbed for 10. m. with
the end of a bristle, so as to imitate the struggles of a captured
insect; but this part of the margin did not bend sooner than the other
parts with undisturbed drops of the infusion.]
We learn from the foregoing experiments that the margins of the leaves
curl inwards when excited by the mere pressure of objects not yielding
any soluble matter, by objects yielding such matter, and by some
fluids—namely an infusion of raw meat and a week solution of carbonate
of ammonia. A stronger solution of two grains of this salt to an ounce
of water, though exciting copious secretion, paralyses the leaf. Drops
of water and of a solution of sugar or gum did not cause any movement.
Scratching the surface of the leaf for some minutes produced no effect.
Therefore, as far as we at present know, only two causes—namely slight
continued pressure and the absorption of nitrogenous matter—excite
movement. It is only the margins of the leaf which bend, for the apex
never curves towards the base. The pedicels of the glandular hairs have
no power of movement. I observed on several occasions that the surface
of the leaf became slightly concave where bits of meat or large flies
had long lain, but this may have been due to injury from
over-stimulation.
The shortest time in which plainly marked movement was observed was 2
hrs. 17 m., and this occurred when either nitrogenous substances or
fluids were placed on the leaves; but I believe that in some cases
[page 377] there was a trace of movement in 1 hr. or 1 hr. 30 m. The
pressure from fragments of glass excites movement almost as quickly as
the absorption of nitrogenous matter, but the degree of incurvation
thus caused is much less. After a leaf has become well incurved and has
again expanded, it will not soon answer to a fresh stimulus. The margin
was affected longitudinally, upwards or downwards, for a distance of
.13 of an inch (3.302 mm.) from an excited point, but for a distance of
.46 of an inch between two excited points, and transversely for a
distance of .2 of an inch (5.08 mm.). The motor impulse is not
accompanied, as in the case of Drosera, by any influence causing
increased secretion; for when a single gland was strongly stimulated
and secreted copiously, the surrounding glands were not in the least
affected. The incurvation of the margin is independent of increased
secretion, for fragments of glass cause little or no secretion, and yet
excite movement; whereas a strong solution of carbonate of ammonia
quickly excites copious secretion, but no movement.
One of the most curious facts with respect to the movement of the
leaves is the short time during which they remain incurved, although
the exciting object is left on them. In the majority of cases there was
well-marked re-expansion within 24 hrs. from the time when even large
pieces of meat, &c., were placed on the leaves, and in all cases within
48 hrs. In one instance the margin of a leaf remained for 32 hrs.
closely inflected round thin fibres of meat; in another instance, when
a bit of sponge, soaked in a strong infusion of raw meat, had been
applied to a leaf, the margin began to unfold in 35 hrs. Fragments of
glass keep the margin incurved for a shorter time than do nitrogenous
bodies; for in the former case there was [page 378] complete
re-expansion in 16 hrs. 30 m. Nitrogenous fluids act for a shorter time
than nitrogenous substances; thus, when drops of an infusion of raw
meat were placed on the midrib of a leaf, the incurved margins began to
unfold in only 10 hrs. 37 m., and this was the quickest act of
re-expansion observed by me; but it may have been partly due to the
distance of the margins from the midrib where the drops lay.
We are naturally led to inquire what is the use of this movement which
lasts for so short a time? If very small objects, such as fibres of
meat, or moderately small objects, such as little flies or
cabbage-seeds, are placed close to the margin, they are either
completely or partially embraced by it. The glands of the overlapping
margin are thus brought into contact with such objects and pour forth
their secretion, afterwards absorbing the digested matter. But as the
incurvation lasts for so short a time, any such benefit can be of only
slight importance, yet perhaps greater than at first appears. The plant
lives in humid districts, and the insects which adhere to all parts of
the leaf are washed by every heavy shower of rain into the narrow
channel formed by the naturally incurved edges. For instance, my friend
in North Wales placed several insects on some leaves, and two days
afterwards (there having been heavy rain in the interval) found some of
them quite washed away, and many others safely tucked under the now
closely inflected margins, the glands of which all round the insects
were no doubt secreting. We can thus, also, understand how it is that
so many insects, and fragments of insects, are generally found lying
within the incurved margins of the leaves.
The incurvation of the margin, due to the presence of an exciting
object, must be serviceable in another [page 379] and probably more
important way. We have seen that when large bits of meat, or of sponge
soaked in the juice of meat, were placed on a leaf, the margin was not
able to embrace them, but, as it became incurved, pushed them very
slowly towards the middle of the leaf, to a distance from the outside
of fully .1 of an inch (2.54 mm.), that is, across between one-third
and one-fourth of the space between the edge and midrib. Any object,
such as a moderately sized insect, would thus be brought slowly into
contact with a far larger number of glands, inducing much more
secretion and absorption, than would otherwise have been the case. That
this would be highly serviceable to the plant, we may infer from the
fact that Drosera has acquired highly developed powers of movement,
merely for the sake of bringing all its glands into contact with
captured insects. So again, after a leaf of Dionaea has caught an
insect, the slow pressing together of the two lobes serves merely to
bring the glands on both sides into contact with it, causing also the
secretion charged with animal matter to spread by capillary attraction
over the whole surface. In the case of Pinguicula, as soon as an insect
has been pushed for some little distance towards the midrib, immediate
re-expansion would be beneficial, as the margins could not capture
fresh prey until they were unfolded. The service rendered by this
pushing action, as well as that from the marginal glands being brought
into contact for a short time with the upper surfaces of minute
captured insects, may perhaps account for the peculiar movements of the
leaves; otherwise, we must look at these movements as a remnant of a
more highly developed power formerly possessed by the progenitors of
the genus.
In the four British species, and, as I hear from [page 380] Prof. Dyer,
in most or all the species of the genus, the edges of the leaves are in
some degree naturally and permanently incurved. This incurvation
serves, as already shown, to prevent insects from being washed away by
the rain; but it likewise serves for another end. When a number of
glands have been powerfully excited by bits of meat, insects, or any
other stimulus, the secretion often trickles down the leaf, and is
caught by the incurved edges, instead of rolling off and being lost. As
it runs down the channel, fresh glands are able to absorb the animal
matter held in solution. Moreover, the secretion often collects in
little pools within the channel, or in the spoon-like tips of the
leaves; and I ascertained that bits of albumen, fibrin, and gluten, are
here dissolved more quickly and completely than on the surface of the
leaf, where the secretion cannot accumulate; and so it would be with
naturally caught insects. The secretion was repeatedly seen thus to
collect on the leaves of plants protected from the rain; and with
exposed plants there would be still greater need of some provision to
prevent, as far as possible, the secretion, with its dissolved animal
matter, being wholly lost.
It has already been remarked that plants growing in a state of nature
have the margins of their leaves much more strongly incurved than those
grown in pots and prevented from catching many insects. We have seen
that insects washed down by the rain from all parts of the leaf often
lodge within the margins, which are thus excited to curl farther
inwards; and we may suspect that this action, many times repeated
during the life of the plant, leads to their permanent and well-marked
incurvation. I regret that this view did not occur to me in time to
test its truth.
It may here be added, though not immediately [page 381] bearing on our
subject, that when a plant is pulled up, the leaves immediately curl
downwards so as almost to conceal the roots,—a fact which has been
noticed by many persons. I suppose that this is due to the same
tendency which causes the outer and older leaves to lie flat on the
ground. It further appears that the flower-stalks are to a certain
extent irritable, for Dr. Johnson states that they “bend backwards if
rudely handled.”*
Secretion, Absorption, and Digestion.—I will first give my observations
and experiments, and then a summary of the results.
[The Effects of Objects containing Soluble Nitrogenous Matter.
(1) Flies were placed on many leaves, and excited the glands to secrete
copiously; the secretion always becoming acid, though not so before.
After a time these insects were rendered so tender that their limbs and
bodies could be separated by a mere touch, owing no doubt to the
digestion and disintegration of their muscles. The glands in contact
with a small fly continued to secrete for four days, and then became
almost dry. A narrow strip of this leaf was cut off, and the glands of
the longer and shorter hairs, which had lain in contact for the four
days with the fly, and those which had not touched it, were compared
under the microscope and presented a wonderful contrast. Those which
had been in contact were filled with brownish granular matter, the
others with homogeneous fluid. There could therefore be no doubt that
the former had absorbed matter from the fly.
(2) Small bits of roast meat, placed on a leaf, always caused much acid
secretion in the course of a few hours—in one case within 40 m. When
thin fibres of meat were laid along the margin of a leaf which stood
almost upright, the secretion ran down to the ground. Angular bits of
meat, placed in little pools of the secretion near the margin, were in
the course of
* ‘English Botany,’ by Sir J.E. Smith; with coloured figures by J.
Sowerby; edit. of 1832, tab. 24, 25, 26. [page 382]
two or three days much reduced in size, rounded, rendered more or less
colourless and transparent, and so much softened that they fell to
pieces on the slightest touch. In only one instance was a very minute
particle completely dissolved, and this occurred within 48 hrs. When
only a small amount of secretion was excited, this was generally
absorbed in from 24 hrs. to 48 hrs.; the glands being left dry. But
when the supply of secretion was copious, round either a single rather
large bit of meat, or round several small bits, the glands did not
become dry until six or seven days had elapsed. The most rapid case of
absorption observed by me was when a small drop of an infusion of raw
meat was placed on a leaf, for the glands here became almost dry in 3
hrs. 20 m. Glands excited by small particles of meat, and which have
quickly absorbed their own secretion, begin to secrete again in the
course of seven or eight days from the time when the meat was given
them.
(3) Three minute cubes of tough cartilage from the leg-bone of a sheep
were laid on a leaf. After 10 hrs. 30 m. some acid secretion was
excited, but the cartilage appeared little or not at all affected.
After 24 hrs. the cubes were rounded and much reduced in size; after 32
hrs. they were softened to the centre, and one was quite liquefied;
after 35 hrs. mere traces of solid cartilage were left; and after 48
hrs. a trace could still be seen through a lens in only one of the
three. After 82 hrs. not only were all three cubes completely
liquefied, but all the secretion was absorbed and the glands left dry.
(4) Small cubes of albumen were placed on a leaf; in 8 hrs. feebly acid
secretion extended to a distance of nearly 1/10 of an inch round them,
and the angles of one cube were rounded. After 24 hrs. the angles of
all the cubes were rounded, and they were rendered throughout very
tender; after 30 hrs. the secretion began to decrease, and after 48
hrs. the glands were left dry; but very minute bits of albumen were
still left undissolved.
(5) Smaller cubes of albumen (about 1/50 or 1/60 of an inch, .508 or
.423 mm.) were placed on four glands; after 18 hrs. one cube was
completely dissolved, the others being much reduced in size, softened,
and transparent. After 24 hrs. two of the cubes were completely
dissolved, and already the secretion on these glands was almost wholly
absorbed. After 42 hrs. the two other cubes were completely dissolved.
These four glands began to secrete again after eight or nine days.
(6) Two large cubes of albumen (fully 1/20 of an inch, 1.27 mm.) were
placed, one near the midrib and the other near the margin [page 383] of
a leaf; in 6 hrs. there was much secretion, which after 48 hrs.
accumulated in a little pool round the cube near the margin. This cube
was much more dissolved than that on the blade of the leaf; so that
after three days it was greatly reduced in size, with all the angles
rounded, but it was too large to be wholly dissolved. The secretion was
partially absorbed after four days. The cube on the blade was much less
reduced, and the glands on which it rested began to dry after only two
days.
(7) Fibrin excites less secretion than does meat or albumen. Several
trials were made, but I will give only three of them. Two minute shreds
were placed on some glands, and in 3 hrs. 45 m. their secretion was
plainly increased. The smaller shred of the two was completely
liquefied in 6 hrs. 15 m., and the other in 24 hrs.; but even after 48
hrs. a few granules of fibrin could still be seen through a lens
floating in both drops of secretion. After 56 hrs. 30 m. these granules
were completely dissolved. A third shred was placed in a little pool of
secretion, within the margin of a leaf where a seed had been lying, and
this was completely dissolved in the course of 15 hrs. 30 m.
(8) Five very small bits of gluten were placed on a leaf, and they
excited so much secretion that one of the bits glided down into the
marginal furrow. After a day all five bits seemed much reduced in size,
but none were wholly dissolved. On the third day I pushed two of them,
which had begun to dry, on to fresh glands. On the fourth day
undissolved traces of three out of the five bits could still be
detected, the other two having quite disappeared; but I am doubtful
whether they had really been completely dissolved. Two fresh bits were
now placed, one near the middle and the other near the margin of
another leaf; both excited an extraordinary amount of secretion; that
near the margin had a little pool formed round it, and was much more
reduced in size than that on the blade, but after four days was not
completely dissolved. Gluten, therefore, excites the glands greatly,
but is dissolved with much difficulty, exactly as in the case of
Drosera. I regret that I did not try this substance after having been
immersed in weak hydrochloric acid, as it would then probably have been
quickly dissolved.
(9) A small square thin piece of pure gelatine, moistened with water,
was placed on a leaf, and excited very little secretion in 5 hrs. 30
m., but later in the day a greater amount. After 24 hrs. the whole
square was completely liquefied; and this would not have occurred had
it been left in water. The liquid was acid.
(10) Small particles of chemically prepared casein excited [page 384]
acid secretion, but were not quite dissolved after two days; and the
glands then began to dry. Nor could their complete dissolution have
been expected from what we have seen with Drosera.
(11) Minute drops of skimmed milk were placed on a leaf, and these
caused the glands to secrete freely. After 3 hrs. the milk was found
curdled, and after 23 hrs. the curds were dissolved. On placing the now
clear drops under the microscope, nothing could be detected except some
oil-globules. The secretion, therefore, dissolves fresh casein.
(12) Two fragments of a leaf were immersed for 17 hrs., each in a
drachm of a solution of carbonate of ammonia, of two strengths, namely
of one part to 437 and 218 of water. The glands of the longer and
shorter hairs were then examined, and their contents found aggregated
into granular matter of a brownish-green colour. These granular masses
were seen by my son slowly to change their forms, and no doubt
consisted of protoplasm. The aggregation was more strongly pronounced,
and the movements of the protoplasm more rapid, within the glands
subjected to the stronger solution than in the others. The experiment
was repeated with the same result; and on this occasion I observed that
the protoplasm had shrunk a little from the walls of the single
elongated cells forming the pedicels. In order to observe the process
of aggregation, a narrow strip of leaf was laid edgeways under the
microscope, and the glands were seen to be quite transparent; a little
of the stronger solution (viz. one part to 218 of water) was now added
under the covering glass; after an hour or two the glands contained
very fine granular matter, which slowly became coarsely granular and
slightly opaque; but even after 5 hrs. not as yet of a brownish tint.
By this time a few rather large, transparent, globular masses appeared
within the upper ends of the pedicels, and the protoplasm lining their
walls had shrunk a little. It is thus evident that the glands of
Pinguicula absorb carbonate of ammonia; but they do not absorb it, or
are not acted on by it, nearly so quickly as those of Drosera.
(13) Little masses of the orange-coloured pollen of the common pea,
placed on several leaves, excited the glands to secrete freely. Even a
very few grains which accidentally fell on a single gland caused the
drop surrounding it to increase so much in size, in 23 hrs., as to be
manifestly larger than the drops on the adjoining glands. Grains
subjected to the secretion for 48 hrs. did not emit their tubes; they
were quite discoloured, and seemed to contain less matter than before;
that [page 385] which was left being of a dirty colour, including
globules of oil. They thus differed in appearance from other grains
kept in water for the same length of time. The glands in contact with
the pollen-grains had evidently absorbed matter from them; for they had
lost their natural pale-green tint, and contained aggregated globular
masses of protoplasm.
(14) Square bits of the leaves of spinach, cabbage, and a saxifrage,
and the entire leaves of Erica tetralix, all excited the glands to
increased secretion. The spinach was the most effective, for it caused
the secretion evidently to increase in 1 hr. 40 m., and ultimately to
run some way down the leaf; but the glands soon began to dry, viz.
after 35 hrs. The leaves of Erica tetralix began to act in 7 hrs. 30
m., but never caused much secretion; nor did the bits of leaf of the
saxifrage, though in this case the glands continued to secrete for
seven days. Some leaves of Pinguicula were sent me from North Wales, to
which leaves of Erica tetralixand of an unknown plant adhered; and the
glands in contact with them had their contents plainly aggregated, as
if they had been in contact with insects; whilst the other glands on
the same leaves contained only clear homogeneous fluid.
(15) Seeds.—A considerable number of seeds or fruits selected by
hazard, some fresh and some a year old, some soaked for a short time in
water and some not soaked, were tried. The ten following kinds, namely
cabbage, radish, Anemone nemorosa, Rumex acetosa, Carex sylvatica,
mustard, turnip, cress, Ranunculus acris, and Avena pubescens, all
excited much secretion, which was in several cases tested and found
always acid. The five first-named seeds excited the glands more than
the others. The secretion was seldom copious until about 24 hrs. had
elapsed, no doubt owing to the coats of the seeds not being easily
permeable. Nevertheless, cabbage seeds excited some secretion in 4 hrs.
30 m.; and this increased so much in 18 hrs. as to run down the leaves.
The seeds or properly the fruits of Carex are much oftener found
adhering to leaves in a state of nature than those of any other genus;
and the fruits of Carex sylvatica excited so much secretion that in 15
hrs. it ran into the incurved edges; but the glands ceased to secrete
after 40 hrs. On the other hand, the glands on which the seeds of the
Rumex and Avena rested continued to secrete for nine days.
The nine following kinds of seeds excited only a slight amount of
secretion, namely, celery, parsnip, caraway, Linum grandiflorum,
Cassia, Trifolium pannonicum, Plantago, onion, [page 386] and Bromus.
Most of these seeds did not excite any secretion until 48 hrs. had
elapsed, and in the case of the Trifolium only one seed acted, and this
not until the third day. Although the seeds of the Plantago excited
very little secretion, the glands continued to secrete for six days.
Lastly, the five following kinds excited no secretion, though left on
the leaves for two or three days, namely lettuce, Erica tetralix,
Atriplex hortensis, Phalaris canariensis, and wheat. Nevertheless, when
the seeds of the lettuce, wheat, and Atriplex were split open and
applied to leaves, secretion was excited in considerable quantity in 10
hrs., and I believe that some was excited in six hours. In the case of
the Atriplex the secretion ran down to the margin, and after 24 hrs. I
speak of it in my notes “as immense in quantity and acid.” The split
seeds also of the Trifolium and celery acted powerfully and quickly,
though the whole seeds caused, as we have seen, very little secretion,
and only after a long interval of time. A slice of the common pea,
which however was not tried whole, caused secretion in 2 hrs. From
these facts we may conclude that the great difference in the degree and
rate at which various kinds of seeds excite secretion, is chiefly or
wholly due to the different permeability of their coats.
Some thin slices of the common pea, which had been previously soaked
for 1 hr. in water, were placed on a leaf, and quickly excited much
acid secretion. After 24 hrs. these slices were compared under a high
power with others left in water for the same time; the latter contained
so many fine granules of legumin that the slide was rendered muddy;
whereas the slices which had been subjected to the secretion were much
cleaner and more transparent, the granules of legumin apparently having
been dissolved. A cabbage seed which had lain for two days on a leaf
and had excited much acid secretion, was cut into slices, and these
were compared with those of a seed which had been left for the same
time in water. Those subjected to the secretion were of a paler colour;
their coats presenting the greatest differences, for they were of a
pale dirty tint instead of chestnut-brown. The glands on which the
cabbage seeds had rested, as well as those bathed by the surrounding
secretion, differed greatly in appearance from the other glands on the
same leaf, for they all contained brownish granular matter, proving
that they had absorbed matter from the seeds.
That the secretion acts on the seeds was also shown by some of them
being killed, or by the seedlings being injured. Fourteen cabbage seeds
were left for three days on leaves and excited [page 387] much
secretion; they were then placed on damp sand under conditions known to
be favourable for germination. Three never germinated, and this was a
far larger proportion of deaths than occurred with seeds of the same
lot, which had not been subjected to the secretion, but were otherwise
treated in the same manner. Of the eleven seedlings raised, three had
the edges of their cotyledons slightly browned, as if scorched; and the
cotyledons of one grew into a curious indented shape. Two mustard seeds
germinated; but their cotyledons were marked with brown patches and
their radicles deformed. Of two radish seeds, neither germinated;
whereas of many seeds of the same lot not subjected to the secretion,
all, excepting one, germinated. Of the two Rumex seeds, one died and
the other germinated; but its radicle was brown and soon withered. Both
seeds of the Avena germinated, one grew well, the other had its radicle
brown and withered. Of six seeds of the Erica none germinated, and when
cut open after having been left for five months on damp sand, one alone
seemed alive. Twenty-two seeds of various kinds were found adhering to
the leaves of plants growing in a state of nature; and of these, though
kept for five months on damp sand, none germinated, some being then
evidently dead.
The Effects of Objects not containing Soluble Nitrogenous Matter.
(16) It has already been shown that bits of glass, placed on leaves,
excite little or no secretion. The small amount which lay beneath the
fragments was tested and found not acid. A bit of wood excited no
secretion; nor did the several kinds of seeds of which the coats are
not permeable to the secretion, and which, therefore, acted like
inorganic bodies. Cubes of fat, left for two days on a leaf, produced
no effect.
(17) A particle of white sugar, placed on a leaf, formed in 1 hr. 10 m.
a large drop of fluid, which in the course of 2 additional hours ran
down into the naturally inflected margin. This fluid was not in the
least acid, and began to dry up, or more probably was absorbed, in 5
hrs. 30 m. The experiment was repeated; particles being placed on a
leaf, and others of the same size on a slip of glass in a moistened
state; both being covered by a bell-glass. This was done to see whether
the increased amount of fluid on the leaves could be due to mere
deliquescence; but this was proved not to be the case. The particle on
the leaf caused so much secretion that in the course of 4 hrs. it ran
down across two-thirds of the leaf. After 8 hrs. the leaf, which was
concave, was actually filled with very viscid [page 388] fluid; and it
particularly deserves notice that this, as on the former occasion, was
not in the least acid. This great amount of secretion may be attributed
to exosmose. The glands which had been covered for 24 hrs. by this
fluid did not differ, when examined under the microscope, from others
on the same leaf, which had not come into contact with it. This is an
interesting fact in contrast with the invariably aggregated condition
of glands which have been bathed by the secretion, when holding animal
matter in solution.
(18) Two particles of gum arabic were placed on a leaf, and they
certainly caused in 1 hr. 20 m. a slight increase of secretion. This
continued to increase for the next 5 hrs., that is for as long a time
as the leaf was observed.
(19) Six small particles of dry starch of commerce were placed on a
leaf, and one of these caused some secretion in 1 hr. 15 m., and the
others in from 8 hrs. to 9 hrs. The glands which had thus been excited
to secrete soon became dry, and did not begin to secrete again until
the sixth day. A larger bit of starch was then placed on a leaf, and no
secretion was excited in 5 hrs. 30 m.; but after 8 hrs. there was a
considerable supply, which increased so much in 24 hrs. as to run down
the leaf to the distance of 3/4 of an inch. This secretion, though so
abundant, was not in the least acid. As it was so copiously excited,
and as seeds not rarely adhere to the leaves of naturally growing
plants, it occurred to me that the glands might perhaps have the power
of secreting a ferment, like ptyaline, capable of dissolving starch; so
I carefully observed the above six small particles during several days,
but they did not seem in the least reduced in bulk. A particle was also
left for two days in a little pool of secretion, which had run down
from a piece of spinach leaf; but although the particle was so minute
no diminution was perceptible. We may therefore conclude that the
secretion cannot dissolve starch. The increase caused by this substance
may, I presume, be attributed to exosmose. But I am surprised that
starch acted so quickly and powerfully as it did, though in a less
degree than sugar. Colloids are known to possess some slight power of
dialysis; and on placing the leaves of a Primula in water, and others
in syrup and diffused starch, those in the starch became flaccid, but
to a less degree and at a much slower rate than the leaves in the
syrup; those in water remaining all the time crisp.]
From the foregoing experiments and observations we [page 389] see that
objects not containing soluble matter have little or no power of
exciting the glands to secrete. Non-nitrogenous fluids, if dense, cause
the glands to pour forth a large supply of viscid fluid, but this is
not in the least acid. On the other hand, the secretion from glands
excited by contact with nitrogenous solids or liquids is invariably
acid, and is so copious that it often runs down the leaves and collects
within the naturally incurved margins. The secretion in this state has
the power of quickly dissolving, that is of digesting, the muscles of
insects, meat, cartilage, albumen, fibrin, gelatine, and casein as it
exists in the curds of milk. The glands are strongly excited by
chemically prepared casein and gluten; but these substances (the latter
not having been soaked in weak hydrochloric acid) are only partially
dissolved, as was likewise the case with Drosera. The secretion, when
containing animal matter in solution, whether derived from solids or
from liquids, such as an infusion of raw meat, milk, or a weak solution
of carbonate of ammonia, is quickly absorbed; and the glands, which
were before limpid and of a greenish colour, become brownish and
contain masses of aggregated granular matter. This matter, from its
spontaneous movements, no doubt consists of protoplasm. No such effect
is produced by the action of non-nitrogenous fluids. After the glands
have been excited to secrete freely, they cease for a time to secrete,
but begin again in the course of a few days.
Glands in contact with pollen, the leaves of other plants, and various
kinds of seeds, pour forth much acid secretion, and afterwards absorb
matter probably of an albuminous nature from them. Nor can the benefit
thus derived be insignificant, for a considerable [page 390] amount of
pollen must be blown from the many wind-fertilised carices, grasses,
&c., growing where Pinguicula lives, on to the leaves thickly covered
with viscid glands and forming large rosettes. Even a few grains of
pollen on a single gland causes it to secrete copiously. We have also
seen how frequently the small leaves of Erica tetralix and of other
plants, as well as various kinds of seeds and fruits, especially of
Carex, adhere to the leaves. One leaf of the Pinguicula had caught ten
of the little leaves of the Erica; and three leaves on the same plant
had each caught a seed. Seeds subjected to the action of the secretion
are sometimes killed, or the seedlings injured. We may, therefore,
conclude that Pinguicula vulgaris, with its small roots, is not only
supported to a large extent by the extraordinary number of insects
which it habitually captures, but likewise draws some nourishment from
the pollen, leaves, and seeds of other plants which often adhere to its
leaves. It is therefore partly a vegetable as well as an animal feeder.
PINGUICULA GRANDIFLORA.
This species is so closely allied to the last that it is ranked by Dr.
Hooker as a sub-species. It differs chiefly in the larger size of its
leaves, and in the glandular hairs near the basal part of the midrib
being longer. But it likewise differs in constitution; I hear from Mr.
Ralfs, who was so kind as to send me plants from Cornwall, that it
grows in rather different sites; and Dr. Moore, of the Glasnevin
Botanic Gardens, informs me that it is much more manageable under
culture, growing freely and flowering annually; whilst Pinguicula
vulgaris has to be renewed every year. Mr. Ralfs found numerous [page
391] insects and fragments of insects adhering to almost all the
leaves. These consisted chiefly of Diptera, with some Hymenoptera,
Homoptera, Coleoptera, and a moth. On one leaf there were nine dead
insects, besides a few still alive. He also observed a few fruits of
Carex pulicaris, as well as the seeds of this same Pinguicula, adhering
to the leaves. I tried only two experiments with this species; firstly,
a fly was placed near the margin of a leaf, and after 16 hrs. this was
found well inflected. Secondly, several small flies were placed in a
row along one margin of another leaf, and by the next morning this
whole margin was curled inwards, exactly as in the case of Pinguicula
vulgaris.
PINGUICULA LUSITANICA.
This species, of which living specimens were sent me by Mr. Ralfs from
Cornwall, is very distinct from the two foregoing ones. The leaves are
rather smaller, much more transparent, and are marked with purple
branching veins. The margins of the leaves are much more involuted;
those of the older ones extending over a third of the space between the
midrib and the outside. As in the two other species, the glandular
hairs consist of longer and shorter ones, and have the same structure;
but the glands differ in being purple, and in often containing granular
matter before they have been excited. In the lower part of the leaf,
almost half the space on each side between the midrib and margin is
destitute of glands; these being replaced by long, rather stiff,
multicellular hairs, which intercross over the midrib. These hairs
perhaps serve to prevent insects from settling on this part of the
leaf, where there are no viscid glands by which they could be caught;
but it is hardly probable that they were developed for this purpose.
The spiral vessels pro- [page 392] ceeding from the midrib terminate at
the extreme margin of the leaf in spiral cells; but these are not so
well developed as in the two preceding species. The flower-peduncles,
sepals, and petals, are studded with glandular hairs, like those on the
leaves.
The leaves catch many small insects, which are found chiefly beneath
the involuted margins, probably washed there by the rain. The colour of
the glands on which insects have long lain is changed, being either
brownish or pale purple, with their contents coarsely granular; so that
they evidently absorb matter from their prey. Leaves of the Erica
tetralix, flowers of a Galium, scales of grasses, &c. likewise adhered
to some of the leaves. Several of the experiments which were tried on
Pinguicula vulgaris were repeated on Pinguicula lusitanica, and these
will now be given.
[(1) A moderately sized and angular bit of albumen was placed on one
side of a leaf, halfway between the midrib and the naturally involuted
margin. In 2 hrs. 15 m. the glands poured forth much secretion, and
this side became more infolded than the opposite one. The inflection
increased, and in 3 hrs. 30 m. extended up almost to the apex. After 24
hrs. the margin was rolled into a cylinder, the outer surface of which
touched the blade of the leaf and reached to within the 1/20 of an inch
of the midrib. After 48 hrs. it began to unfold, and in 72 hrs. was
completely unfolded. The cube was rounded and greatly reduced in size;
the remainder being in a semi-liquefied state.
(2) A moderately sized bit of albumen was placed near the apex of a
leaf, under the naturally incurved margin. In 2 hrs. 30 m. much
secretion was excited, and next morning the margin on this side was
more incurved than the opposite one, but not to so great a degree as in
the last case. The margin unfolded at the same rate as before. A large
proportion of the albumen was dissolved, a remnant being still left.
(3) Large bits of albumen were laid in a row on the midribs of two
leaves, but produced in the course of 24 hrs. no effect; [page 393] nor
could this have been expected, for even had glands existed here, the
long bristles would have prevented the albumen from coming in contact
with them. On both leaves the bits were now pushed close to one margin,
and in 3 hrs. 30 m. this became so greatly inflected that the outer
surface touched the blade; the opposite margin not being in the least
affected. After three days the margins of both leaves with the albumen
were still as much inflected as ever, and the glands were still
secreting copiously. With Pinguicula vulgaris I have never seen
inflection lasting so long.
(4) Two cabbage seeds, after being soaked for an hour in water, were
placed near the margin of a leaf, and caused in 3 hrs. 20 m. increased
secretion and incurvation. After 24 hrs. the leaf was partially
unfolded, but the glands were still secreting freely. These began to
dry in 48 hrs., and after 72 hrs. were almost dry. The two seeds were
then placed on damp sand under favourable conditions for growth; but
they never germinated, and after a time were found rotten. They had no
doubt been killed by the secretion.
(5) Small bits of a spinach leaf caused in 1 hr. 20 m. increased
secretion; and after 3 hrs. 20 m. plain incurvation of the margin. The
margin was well inflected after 9 hrs. 15 m., but after 24 hrs. was
almost fully re-expanded. The glands in contact with the spinach became
dry in 72 hrs. Bits of albumen had been placed the day before on the
opposite margin of this same leaf, as well as on that of a leaf with
cabbage seeds, and these margins remained closely inflected for 72
hrs., showing how much more enduring is the effect of albumen than of
spinach leaves or cabbage seeds .
(6) A row of small fragments of glass was laid along one margin of a
leaf; no effect was produced in 2 hrs. 10 m., but after 3 hrs. 25 m.
there seemed to be a trace of inflection, and this was distinct, though
not strongly marked, after 6 hrs. The glands in contact with the
fragments now secreted more freely than before; so that they appear to
be more easily excited by the pressure of inorganic objects than are
the glands of Pinguicula vulgaris. The above slight inflection of the
margin had not increased after 24 hrs., and the glands were now
beginning to dry. The surface of a leaf, near the midrib and towards
the base, was rubbed and scratched for some time, but no movement
ensued. The long hairs which are situated here were treated in the same
manner, with no effect. This latter trial was made because I thought
that the hairs might perhaps be sensitive to a touch, like the
filaments of Dionaea. [page 394]
(7) The flower-peduncles, sepals and petals, bear glands in general
appearance like those on the leaves. A piece of a flower-peduncle was
therefore left for 1 hr. in a solution of one part of carbonate of
ammonia to 437 of water, and this caused the glands to change from
bright pink to a dull purple colour; but their contents exhibited no
distinct aggregation. After 8 hrs. 30 m. they became colourless. Two
minute cubes of albumen were placed on the glands of a flower-peduncle,
and another cube on the glands of a sepal; but they were not excited to
increased secretion, and the albumen after two days was not in the
least softened. Hence these glands apparently differ greatly in
function from those on the leaves.]
From the foregoing observations on Pinguicula lusitanica we see that
the naturally much incurved margins of the leaves are excited to curve
still farther inwards by contact with organic and inorganic bodies;
that albumen, cabbage seeds, bits of spinach leaves, and fragments of
glass, cause the glands to secrete more freely;—that albumen is
dissolved by the secretion, and cabbage seeds killed by it;—and lastly
that matter is absorbed by the glands from the insects which are caught
in large numbers by the viscid secretion. The glands on the
flower-peduncles seem to have no such power. This species differs from
Pinguicula vulgarisand grandiflora in the margins of the leaves, when
excited by organic bodies, being inflected to a greater degree, and in
the inflection lasting for a longer time. The glands, also, seem to be
more easily excited to increased secretion by bodies not yielding
soluble nitrogenous matter. In other respects, as far as my
observations serve, all three species agree in their functional powers.
[page 395]