DROSOPHYLLUM—RORIDULA—BYBLIS—GLANDULAR HAIRS OF OTHER PLANTS—CONCLUDING
REMARKS ON THE DROSERACEÆ.
Drosophyllum—Structure of leaves—Nature of the secretion—Manner of
catching insects— Power of absorption—Digestion of animal
substances—Summary on Drosophyllum—Roridula—Byblis—Glandular hairs of
other plants, their power of absorption—Saxifraga—Primula—
Pelargonium—Erica—Mirabilis—Nicotiana—Summary on glandular
hairs—Concluding remarks on the Droseraceae.
Drosophyllum lusitanicum.—This rare plant has been found only in
Portugal, and, as I hear from Dr. Hooker, in Morocco. I obtained living
specimens through the great kindness of Mr. W.C. Tait, and afterwards
from Mr. G. Maw and Dr. Moore. Mr. Tait informs me that it grows
plentifully on the sides of dry hills near Oporto, and that vast
numbers of flies adhere to the leaves. This latter fact is well-known
to the villagers, who call the plant the “fly-catcher,” and hang it up
in their cottages for this purpose. A plant in my hot-house caught so
many insects during the early part of April, although the weather was
cold and insects scarce, that it must have been in some manner strongly
attractive to them. On four leaves of a young and small plant, 8, 10,
14, and 16 minute insects, chiefly Diptera, were found in the autumn
adhering to them. I neglected to examine the roots, but I hear from Dr.
Hooker that they are very small, as in the case of the previously
mentioned members of the same family of the Droseraceae.
The leaves arise from an almost woody axis; they [page 333] are linear,
much attenuated towards their tips, and several inches in length. The
upper surface is concave, the lower convex, with a narrow channel down
the middle. Both surfaces, with the exception of the channel, are
covered with glands, supported on pedicels and arranged in irregular
longitudinal rows. These organs I shall call tentacles, from their
close resemblance to those of Drosera, though they have no power of
movement. Those on the same leaf differ much in length. The glands also
differ in size, and are of a bright pink or of a purple colour; their
upper surfaces are convex, and the lower flat or even concave, so that
they resemble miniature mushrooms in appearance. They are formed of two
(as I believe) layers of delicate angular cells, enclosing eight or ten
larger cells with thicker, zigzag walls. Within these larger cells
there are others marked by spiral lines, and apparently connected with
the spiral vessels which run up the green multi-cellular pedicels. The
glands secrete large drops of viscid secretion. Other glands, having
the same general appearance, are found on the flower-peduncles and
calyx.
FIG. 14. (Drosophyllum lusitanicum.) Part of leaf, enlarged seven
times, showing lower surface.
Besides the glands which are borne on longer or shorter pedicels, there
are numerous ones, both on the upper and lower surfaces of the leaves,
so small as to be scarcely visible to the naked eye. They are
colourless and almost sessile, either circular or oval in outline; the
latter occurring chiefly on the backs of the leaves (fig. 14).
Internally they have exactly the same structure as the larger glands
which are supported on pedicels; [page 334] and indeed the two sets
almost graduate into one another. But the sessile glands differ in one
important respect, for they never secrete spontaneously, as far as I
have seen, though I have examined them under a high power on a hot day,
whilst the glands on pedicels were secreting copiously. Nevertheless,
if little bits of damp albumen or fibrin are placed on these sessile
glands, they begin after a time to secrete, in the same manner as do
the glands of Dionaea when similarly treated. When they were merely
rubbed with a bit of raw meat, I believe that they likewise secreted.
Both the sessile glands and the taller ones on pedicels have the power
of rapidly absorbing nitrogenous matter.
The secretion from the taller glands differs in a remarkable manner
from that of Drosera, in being acid before the glands have been in any
way excited; and judging from the changed colour of litmus paper, more
strongly acid than that of Drosera. This fact was observed repeatedly;
on one occasion I chose a young leaf, which was not secreting freely,
and had never caught an insect, yet the secretion on all the glands
coloured litmus paper of a bright red. From the quickness with which
the glands are able to obtain animal matter from such substances as
well-washed fibrin and cartilage, I suspect that a small quantity of
the proper ferment must be present in the secretion before the glands
are excited, so that a little animal matter is quickly dissolved.
Owing to the nature of the secretion or to the shape of the glands, the
drops are removed from them with singular facility. It is even somewhat
difficult, by the aid of a finely pointed polished needle, slightly
damped with water, to place a minute particle of any kind on one of the
drops; for on withdrawing the [page 335] needle, the drop is generally
withdrawn; whereas with Drosera there is no such difficulty, though the
drops are occasionally withdrawn. From this peculiarity, when a small
insect alights on a leaf of Drosophyllum, the drops adhere to its
wings, feet, or body, and are drawn from the gland; the insect then
crawls onward and other drops adhere to it; so that at last, bathed by
the viscid secretion, it sinks down and dies, resting on the small
sessile glands with which the surface of the leaf is thickly covered.
In the case of Drosera, an insect sticking to one or more of the
exterior glands is carried by their movement to the centre of the leaf;
with Drosophyllum, this is effected by the crawling of the insect, as
from its wings being clogged by the secretion it cannot fly away.
There is another difference in function between the glands of these two
plants: we know that the glands of Drosera secrete more copiously when
properly excited. But when minute particles of carbonate of ammonia,
drops of a solution of this salt or of the nitrate of ammonia, saliva,
small insects, bits of raw or roast meat, albumen, fibrin or cartilage,
as well as inorganic particles, were placed on the glands of
Drosophyllum, the amount of secretion never appeared to be in the least
increased. As insects do not commonly adhere to the taller glands, but
withdraw the secretion, we can see that there would be little use in
their having acquired the habit of secreting copiously when stimulated;
whereas with Drosera this is of use, and the habit has been acquired.
Nevertheless, the glands of Drosophyllum, without being stimulated,
continually secrete, so as to replace the loss by evaporation. Thus
when a plant was placed under a small bell-glass with its inner surface
and support thoroughly wetted, there was no loss by evaporation, and so
much [page 336] secretion was accumulated in the course of a day that
it ran down the tentacles and covered large spaces of the leaves.
The glands to which the above named nitrogenous substances and liquids
were given did not, as just stated, secrete more copiously; on the
contrary, they absorbed their own drops of secretion with surprising
quickness. Bits of damp fibrin were placed on five glands, and when
they were looked at after an interval of 1 hr. 12 m., the fibrin was
almost dry, the secretion having been all absorbed. So it was with
three cubes of albumen after 1 hr. 19 m., and with four other cubes,
though these latter were not looked at until 2 hrs. 15 m. had elapsed.
The same result followed in between 1 hr. 15 m. and 1 hr. 30 m. when
particles both of cartilage and meat were placed on several glands.
Lastly, a minute drop (about 1/20 of a minim) of a solution of one part
of nitrate of ammonia to 146 of water was distributed between the
secretion surrounding three glands, so that the amount of fluid
surrounding each was slightly increased; yet when looked at after 2
hrs., all three were dry. On the other hand, seven particles of glass
and three of coal-cinders, of nearly the same size as those of the
above named organic substances, were placed on ten glands; some of them
being observed for 18 hrs., and others for two or three days; but there
was not the least sign of the secretion being absorbed. Hence, in the
former cases, the absorption of the secretion must have been due to the
presence of some nitrogenous matter, which was either already soluble
or was rendered so by the secretion. As the fibrin was pure, and had
been well washed in distilled water after being kept in glycerine, and
as the cartilage had been soaked in water, I suspect that these
substances must [page 337] have been slightly acted on and rendered
soluble within the above stated short periods.
The glands have not only the power of rapid absorption, but likewise of
secreting again quickly; and this latter habit has perhaps been gained,
inasmuch as insects, if they touch the glands, generally withdraw the
drops of secretion, which have to be restored. The exact period of
re-secretion was recorded in only a few cases. The glands on which bits
of meat were placed, and which were nearly dry after about 1 hr. 30 m.,
when looked at after 22 additional hours, were found secreting; so it
was after 24 hrs. with one gland on which a bit of albumen had been
placed. The three glands to which a minute drop of a solution of
nitrate of ammonia was distributed, and which became dry after 2 hrs.,
were beginning to re-secrete after only 12 additional hours.
Tentacles Incapable of Movement.—Many of the tall tentacles, with
insects adhering to them, were carefully observed; and fragments of
insects, bits of raw meat, albumen, &c., drops of a solution of two
salts of ammonia and of saliva, were placed on the glands of many
tentacles; but not a trace of movement could ever be detected. I also
repeatedly irritated the glands with a needle, and scratched and
pricked the blades, but neither the blade nor the tentacles became at
all inflected. We may therefore conclude that they are incapable of
movement.
On the Power of Absorption possessed by the Glands.—It has already been
indirectly shown that the glands on pedicels absorb animal matter; and
this is further shown by their changed colour, and by the aggregation
of their contents, after they have been left in contact with
nitrogenous substances or liquids. The following observations apply
both to the glands supported on [page 338] pedicels and to the minute
sessile ones. Before a gland has been in any way stimulated, the
exterior cells commonly contain only limpid purple fluid; the more
central ones including mulberry-like masses of purple granular matter.
A leaf was placed in a little solution of one part of carbonate of
ammonia to 146 of water (3 grs. to 1 oz.), and the glands were
instantly darkened and very soon became black; this change being due to
the strongly marked aggregation of their contents, more especially of
the inner cells. Another leaf was placed in a solution of the same
strength of nitrate of ammonia, and the glands were slightly darkened
in 25 m., more so in 50 m., and after 1 hr. 30 m. were of so dark a red
as to appear almost black. Other leaves were placed in a weak infusion
of raw meat and in human saliva, and the glands were much darkened in
25 m., and after 40 m. were so dark as almost to deserve to be called
black. Even immersion for a whole day in distilled water occasionally
induces some aggregation within the glands, so that they become of a
darker tint. In all these cases the glands are affected in exactly the
same manner as those of Drosera. Milk, however, which acts so
energetically on Drosera, seems rather less effective on Drosophyllum,
for the glands were only slightly darkened by an immersion of 1 hr. 20
m., but became decidedly darker after 3 hrs. Leaves which had been left
for 7 hrs. in an infusion of raw meat or in saliva were placed in the
solution of carbonate of ammonia, and the glands now became greenish;
whereas, if they had been first placed in the carbonate, they would
have become black. In this latter case, the ammonia probably combines
with the acid of the secretion, and therefore does not act on the
colouring matter; but when the glands are first subjected to an organic
[page 339] fluid, either the acid is consumed in the work of digestion
or the cell-walls are rendered more permeable, so that the undecomposed
carbonate enters and acts on the colouring matter. If a particle of the
dry carbonate is placed on a gland, the purple colour is quickly
discharged, owing probably to an excess of the salt. The gland,
moreover, is killed.
Turning now to the action of organic substances, the glands on which
bits of raw meat were placed became dark-coloured; and in 18 hrs. their
contents were conspicuously aggregated. Several glands with bits of
albumen and fibrin were darkened in between 2 hrs. and 3 hrs.; but in
one case the purple colour was completely discharged. Some glands which
had caught flies were compared with others close by; and though they
did not differ much in colour, there was a marked difference in their
state of aggregation. In some few instances, however, there was no such
difference, and this appeared to be due to the insects having been
caught long ago, so that the glands had recovered their pristine state.
In one case, a group of the sessile colourless glands, to which a small
fly adhered, presented a peculiar appearance; for they had become
purple, owing to purple granular matter coating the cell-walls. I may
here mention as a caution that, soon after some of my plants arrived in
the spring from Portugal, the glands were not plainly acted on by bits
of meat, or insects, or a solution of ammonia—a circumstance for which
I cannot account.
Digestion of Solid Animal Matter.—Whilst I was trying to place on two
of the taller glands little cubes of albumen, these slipped down, and,
besmeared with secretion, were left resting on some of the small
sessile glands. After 24 hrs. one of these cubes was found [page 340]
completely liquefied, but with a few white streaks still visible; the
other was much rounded, but not quite dissolved. Two other cubes were
left on tall glands for 2 hrs. 45 m., by which time all the secretion
was absorbed; but they were not perceptibly acted on, though no doubt
some slight amount of animal matter had been absorbed from them. They
were then placed on the small sessile glands, which being thus
stimulated secreted copiously in the course of 7 hrs. One of these
cubes was much liquefied within this short time; and both were
completely liquefied after 21 hrs. 15 m.; the little liquid masses,
however, still showing some white streaks. These streaks disappeared
after an additional period of 6 hrs. 30 m.; and by next morning (i.e.
48 hrs. from the time when the cubes were first placed on the glands)
the liquefied matter was wholly absorbed. A cube of albumen was left on
another tall gland, which first absorbed the secretion and after 24
hrs. poured forth a fresh supply. This cube, now surrounded by
secretion, was left on the gland for an additional 24 hrs., but was
very little, if at all, acted on. We may, therefore, conclude, either
that the secretion from the tall glands has little power of digestion,
though strongly acid, or that the amount poured forth from a single
gland is insufficient to dissolve a particle of albumen which within
the same time would have been dissolved by the secretion from several
of the small sessile glands. Owing to the death of my last plant, I was
unable to ascertain which of these alternatives is the true one.
Four minute shreds of pure fibrin were placed, each resting on one,
two, or three of the taller glands. In the course of 2 hrs. 30 m. the
secretion was all absorbed, and the shreds were left almost dry. They
[page 341] were then pushed on to the sessile glands. One shred, after
2 hrs. 30 m., seemed quite dissolved, but this may have been a mistake.
A second, when examined after 17 hrs. 25 m., was liquefied, but the
liquid as seen under the microscope still contained floating granules
of fibrin. The other two shreds were completely liquefied after 21 hrs.
30 m.; but in one of the drops a very few granules could still be
detected. These, however, were dissolved after an additional interval
of 6 hrs. 30 m.; and the surface of the leaf for some distance all
round was covered with limpid fluid. It thus appears that Drosophyllum
digests albumen and fibrin rather more quickly than Drosera can; and
this may perhaps be attributed to the acid, together probably with some
small amount of the ferment, being present in the secretion, before the
glands have been stimulated; so that digestion begins at once.
Concluding Remarks.—The linear leaves of Drosophyllum differ but
slightly from those of certain species of Drosera; the chief
differences being, firstly, the presence of minute, almost sessile,
glands, which, like those of Dionaea, do not secrete until they are
excited by the absorption of nitrogenous matter. But glands of this
kind are present on the leaves of Drosera binata, and appear to be
represented by the papillae on the leaves of Drosera rotundifolia.
Secondly, the presence of tentacles on the backs of the leaves; but we
have seen that a few tentacles, irregularly placed and tending towards
abortion, are retained on the backs of the leaves of Drosera binata.
There are greater differences in function between the two genera. The
most important one is that the tentacles of Drosophyllum have no power
of movement; this loss being partially replaced by the drops of viscid
[page 342] secretion being readily withdrawn from the glands; so that,
when an insect comes into contact with a drop, it is able to crawl
away, but soon touches other drops, and then, smothered by the
secretion, sinks down on the sessile glands and dies. Another
difference is, that the secretion from the tall glands, before they
have been in any way excited, is strongly acid, and perhaps contains a
small quantity of the proper ferment. Again, these glands do not
secrete more copiously from being excited by the absorption of
nitrogenous matter; on the contrary, they then absorb their own
secretion with extraordinary quickness. In a short time they begin to
secrete again. All these circumstances are probably connected with the
fact that insects do not commonly adhere to the glands with which they
first come into contact, though this does sometimes occur; and that it
is chiefly the secretion from the sessile glands which dissolves animal
matter out of their bodies.
RORIDULA.
Roridula dentata.—This plant, a native of the western parts of the Cape
of Good Hope, was sent to me in a dried state from Kew. It has an
almost woody stem and branches, and apparently grows to a height of
some feet. The leaves are linear, with their summits much attenuated.
Their upper and lower surfaces are concave, with a ridge in the middle,
and both are covered with tentacles, which differ greatly in length;
some being very long, especially those on the tips of the leaves, and
some very short. The glands also differ much in size and are somewhat
elongated. They are supported on multicellular pedicels.
This plant, therefore, agrees in several respects with [page 343]
Drosophyllum, but differs in the following points. I could detect no
sessile glands; nor would these have been of any use, as the upper
surface of the leaves is thickly clothed with pointed, unicellular
hairs directed upwards. The pedicels of the tentacles do not include
spiral vessels; nor are there any spiral cells within the glands. The
leaves often arise in tufts and are pinnatifid, the divisions
projecting at right angles to the main linear blade. These lateral
divisions are often very short and bear only a single terminal
tentacle, with one or two short ones on the sides. No distinct line of
demarcation can be drawn between the pedicels of the long terminal
tentacles and the much attenuated summits of the leaves. We may,
indeed, arbitrarily fix on the point to which the spiral vessels
proceeding from the blade extend; but there is no other distinction.
It was evident from the many particles of dirt sticking to the glands
that they secrete much viscid matter. A large number of insects of many
kinds also adhered to the leaves. I could nowhere discover any signs of
the tentacles having been inflected over the captured insects; and this
probably would have been seen even in the dried specimens, had they
possessed the power of movement. Hence, in this negative character,
Roridula resembles its northern representative, Drosophyllum.
BYBLIS.
Byblis gigantea (Western Australia).—A dried specimen, about 18 inches
in height, with a strong stem, was sent me from Kew. The leaves are
some inches in length, linear, slightly flattened, with a small
projecting rib on the lower surface. They are covered on all sides by
glands of two kinds [page 344] —sessile ones arranged in rows, and
others supported on moderately long pedicels. Towards the narrow
summits of the leaves the pedicels are longer than elsewhere, and here
equal the diameter of the leaf. The glands are purplish, much
flattened, and formed of a single layer of radiating cells, which in
the larger glands are from forty to fifty in number. The pedicels
consist of single elongated cells, with colourless, extremely delicate
walls, marked with the finest intersecting spiral lines. Whether these
lines are the result of contraction from the drying of the walls, I do
not know, but the whole pedicel was often spirally rolled up. These
glandular hairs are far more simple in structure than the so-called
tentacles of the preceding genera, and they do not differ essentially
from those borne by innumerable other plants. The flower-peduncles bear
similar glands. The most singular character about the leaves is that
the apex is enlarged into a little knob, covered with glands, and about
a third broader than the adjoining part of the attenuated leaf. In two
places dead flies adhered to the glands. As no instance is known of
unicellular structures having any power of movement,* Byblis, no doubt,
catches insects solely by the aid of its viscid secretion. These
probably sink down besmeared with the secretion and rest on the small
sessile glands, which, if we may judge by the analogy of Drosophyllum,
then pour forth their secretion and afterwards absorb the digested
matter.
Supplementary Observations on the Power of Absorption by the Glandular
Hairs of other Plants.—A few observations on this subject may be here
conveniently introduced. As the glands of many, probably of all,
* Sachs, ‘Traité de Bot.,’ 3rd edit. 1874, p. 1026. [page 345]
the species of Droseraceae absorb fluids or at least allow them readily
to enter,* it seemed desirable to ascertain how far the glands of other
plants which are not specially adapted for capturing insects, had the
same power. Plants were chosen for trial at hazard, with the exception
of two species of saxifrage, which were selected from belonging to a
family allied to the Droseraceae. Most of the experiments were made by
immersing the glands either in an infusion of raw meat or more commonly
in a solution of carbonate of ammonia, as this latter substance acts so
powerfully and rapidly on protoplasm. It seemed also particularly
desirable to ascertain whether ammonia was absorbed, as a small amount
is contained in rain-water. With the Droseraceae the secretion of a
viscid fluid by the glands does not prevent their absorbing; so that
the glands of other plants might excrete superfluous matter, or secrete
an odoriferous fluid as a protection against the attacks of insects, or
for any other purpose, and yet have the power of absorbing. I regret
that in the following cases I did not try whether the secretion could
digest or render soluble animal substances, but such experiments would
have been difficult on account of the small size of the glands and the
small amount of secretion. We shall see in the next chapter that the
secretion from the glandular hairs of Pinguicula certainly dissolves
animal matter.
[Saxifraga umbrosa.—The flower-peduncles and petioles of the leaves are
clothed with short hairs, bearing pink-coloured glands, formed of
several polygonal cells, with their pedicels divided by partitions into
distinct cells, which are generally colourless, but sometimes pink. The
glands secrete a yellowish viscid fluid, by
* The distinction between true absorption and mere permeation, or
imbibition, is by no means clearly understood: see Müller’s
‘Physiology,’ Eng. translat. 1838, vol. i. p. 280. [page 346]
which minute Diptera are sometimes, though not often, caught.* The
cells of the glands contain bright pink fluid, charged with granules or
with globular masses of pinkish pulpy matter. This matter must be
protoplasm, for it is seen to undergo slow but incessant changes of
form if a gland be placed in a drop of water and examined. Similar
movements were observed after glands had been immersed in water for 1,
3, 5, 18, and 27 hrs. Even after this latter period the glands retained
their bright pink colour; and the protoplasm within their cells did not
appear to have become more aggregated. The continually changing forms
of the little masses of protoplasm are not due to the absorption of
water, as they were seen in glands kept dry.
A flower-stem, still attached to a plant, was bent (May 29) so as to
remain immersed for 23 hrs. 30 m. in a strong infusion of raw meat. The
colour of the contents of the glands was slightly changed, being now of
a duller and more purple tint than before. The contents also appeared
more aggregated, for the spaces between the little masses of protoplasm
were wider; but this latter result did not follow in some other and
similar experiments. The masses seemed to change their forms more
rapidly than did those in water; so that the cells had a different
appearance every four or five minutes. Elongated masses became in the
course of one or two minutes spherical; and spherical ones drew
themselves out and united with others. Minute masses rapidly increased
in size, and three distinct ones were seen to unite. The movements
were, in short, exactly like those described in the case of Drosera.
The cells of the pedicels were not affected by the infusion; nor were
they in the following experiment.
Another flower-stem was placed in the same manner and for the same
length of time in a solution of one part of nitrate of ammonia to 146
of water (or 3 grs. to 1 oz.), and the glands were discoloured in
exactly the same manner as by the infusion of raw meat.
Another flower-stem was immersed, as before, in a solution of one part
of carbonate of ammonia to 109 of water. The glands, after 1 hr. 30 m.,
were not discoloured, but after 3 hrs. 45 m. most of them had become
dull purple, some of them blackish-
* In the case of Saxifraga tridactylites, Mr. Druce says
(‘Pharmaceutical Journal,’ May 1875) that he examined some dozens of
plants, and in almost every instance remnants of insects adhered to the
leaves. So it is, as I hear from a friend, with this plant in Ireland.
[page 347]
green, a few being still unaffected. The little masses of protoplasm
within the cells were seen in movement. The cells of the pedicels were
unaltered. The experiment was repeated, and a fresh flower-stem was
left for 23 hrs. in the solution, and now a great effect was produced;
all the glands were much blackened, and the previously transparent
fluid in the cells of the pedicels, even down to their bases, contained
spherical masses of granular matter. By comparing many different hairs,
it was evident that the glands first absorb the carbonate, and that the
effect thus produced travels down the hairs from cell to cell. The
first change which could be observed is a cloudy appearance in the
fluid, due to the formation of very fine granules, which afterwards
aggregate into larger masses. Altogether, in the darkening of the
glands, and in the process of aggregation travelling down the cells of
the pedicels, there is the closest resemblance to what takes place when
a tentacle of Drosera is immersed in a weak solution of the same salt.
The glands, however, absorb very much more slowly than those of
Drosera. Besides the glandular hairs, there are star-shaped organs
which do not appear to secrete, and which were not in the least
affected by the above solutions.
Although in the case of uninjured flower-stems and leaves the carbonate
seems to be absorbed only by the glands, yet it enters a cut surface
much more quickly than a gland. Strips of the rind of a flower-stem
were torn off, and the cells of the pedicels were seen to contain only
colourless transparent fluid; those of the glands including as usual
some granular matter. These strips were then immersed in the same
solution as before (one part of the carbonate to 109 of water), and in
a few minutes granular matter appeared in the lowercells of all the
pedicels. The action invariably commenced (for I tried the experiment
repeatedly) in the lowest cells, and therefore close to the torn
surface, and then gradually travelled up the hairs until it reached the
glands, in a reversed direction to what occurs in uninjured specimens.
The glands then became discoloured, and the previously contained
granular matter was aggregated into larger masses. Two short bits of a
flower-stem were also left for 2 hrs. 40 m. in a weaker solution of one
part of the carbonate to 218 of water; and in both specimens the
pedicels of the hairs near the cut ends now contained much granular
matter; and the glands were completely discoloured.
Lastly, bits of meat were placed on some glands; these were examined
after 23 hrs., as were others, which had apparently not long before
caught minute flies; but they did not present any [page 348] difference
from the glands of other hairs. Perhaps there may not have been time
enough for absorption. I think so as some glands, on which dead flies
had evidently long lain, were of a pale dirty purple colour or even
almost colourless, and the granular matter within them presented an
unusual and somewhat peculiar appearance. That these glands had
absorbed animal matter from the flies, probably by exosmose into the
viscid secretion, we may infer, not only from their changed colour, but
because, when placed in a solution of carbonate of ammonia, some of the
cells in their pedicels become filled with granular matter; whereas the
cells of other hairs, which had not caught flies, after being treated
with the same solution for the same length of time, contained only a
small quantity of granular matter. But more evidence is necessary
before we fully admit that the glands of this saxifrage can absorb,
even with ample time allowed, animal matter from the minute insects
which they occasionally and accidentally capture.
Saxifraga rotundifolia (?).—The hairs on the flower-stems of this
species are longer than those just described, and bear pale brown
glands. Many were examined, and the cells of the pedicels were quite
transparent. A bent stem was immersed for 30 m. in a solution of one
part of carbonate of ammonia to 109 of water, and two or three of the
uppermost cells in the pedicels now contained granular or aggregated
matter; the glands having become of a bright yellowish-green. The
glands of this species therefore absorb the carbonate much more quickly
than do those of Saxifraga umbrosa, and the upper cells of the pedicels
are likewise affected much more quickly. Pieces of the stem were cut
off and immersed in the same solution; and now the process of
aggregation travelled up the hairs in a reversed direction; the cells
close to the cut surfaces being first affected.
Primula sinensis.—The flower-stems, the upper and lower surfaces of the
leaves and their footstalks, are all clothed with a multitude of longer
and shorter hairs. The pedicels of the longer hairs are divided by
transverse partitions into eight or nine cells. The enlarged terminal
cell is globular, forming a gland which secretes a variable amount of
thick, slightly viscid, not acid, brownish-yellow matter.
A piece of a young flower-stem was first immersed in distilled water
for 2 hrs. 30 m., and the glandular hairs were not at all affected.
Another piece, bearing twenty-five short and nine long hairs, was
carefully examined. The glands of the latter contained no solid or
semi-solid matter; and those of only two [page 349] of the twenty-five
short hairs contained some globules. This piece was then immersed for 2
hrs. in a solution of one part of carbonate of ammonia to 109 of water,
and now the glands of the twenty-five shorter hairs, with two or three
exceptions, contained either one large or from two to five smaller
spherical masses of semi-solid matter. Three of the glands of the nine
long hairs likewise included similar masses. In a few hairs there were
also globules in the cells immediately beneath the glands. Looking to
all thirty-four hairs, there could be no doubt that the glands had
absorbed some of the carbonate. Another piece was left for only 1 hr.
in the same solution, and aggregated matter appeared in all the glands.
My son Francis examined some glands of the longer hairs, which
contained little masses of matter, before they were immersed in any
solution; and these masses slowly changed their forms, so that no doubt
they consisted of protoplasm. He then irrigated these hairs for 1 hr.
15 m., whilst under the microscope, with a solution of one part of the
carbonate to 218 of water; the glands were not perceptibly affected,
nor could this have been expected, as their contents were already
aggregated. But in the cells of the pedicels numerous, almost
colourless, spheres of matter appeared, which changed their forms and
slowly coalesced; the appearance of the cells being thus totally
changed at successive intervals of time.
The glands on a young flower-stem, after having been left for 2 hrs. 45
m. in a strong solution of one part of the carbonate to 109 of water,
contained an abundance of aggregated masses, but whether generated by
the action of the salt, I do not know. This piece was again placed in
the solution, so that it was immersed altogether for 6 hrs. 15 m., and
now there was a great change; for almost all the spherical masses
within the gland-cells had disappeared, being replaced by granular
matter of a darker brown. The experiment was thrice repeated with
nearly the same result. On one occasion the piece was left immersed for
8 hrs. 30 m., and though almost all the spherical masses were changed
into the brown granular matter, a few still remained. If the spherical
masses of aggregated matter had been originally produced merely by some
chemical or physical action, it seems strange that a somewhat longer
immersion in the same solution should so completely alter their
character. But as the masses which slowly and spontaneously changed
their forms must have consisted of living protoplasm, there is nothing
surprising in its being injured or killed, and its appearance wholly
changed by long immersion in so strong a solution of the carbonate as
that [page 350] employed. A solution of this strength paralyses all
movement in Drosera, but does not kill the protoplasm; a still stronger
solution prevents the protoplasm from aggregating into the ordinary
full-sized globular masses, and these, though they do not disintegrate,
become granular and opaque. In nearly the same manner, too hot water
and certain solutions (for instance, of the salts of soda and potash)
cause at first an imperfect kind of aggregation in the cells of
Drosera; the little masses afterwards breaking up into granular or
pulpy brown matter. All the foregoing experiments were made on
flower-stems, but a piece of a leaf was immersed for 30 m. in a strong
solution of the carbonate (one part to 109 of water), and little
globular masses of matter appeared in all the glands, which before
contained only limpid fluid.
I made also several experiments on the action of the vapour of the
carbonate on the glands; but will give only a few cases. The cut end of
the footstalk of a young leaf was protected with sealing-wax, and was
then placed under a small bell-glass, with a large pinch of the
carbonate. After 10 m. the glands showed a considerable degree of
aggregation, and the protoplasm lining the cells of the pedicels was a
little separated from the walls. Another leaf was left for 50 m. with
the same result, excepting that the hairs became throughout their whole
length of a brownish colour. In a third leaf, which was exposed for 1
hr. 50 m., there was much aggregated matter in the glands; and some of
the masses showed signs of breaking up into brown granular matter. This
leaf was again placed in the vapour, so that it was exposed altogether
for 5 hrs. 30 m.; and now, though I examined a large number of glands,
aggregated masses were found in only two or three; in all the others,
the masses, which before had been globular, were converted into brown,
opaque, granular matter. We thus see that exposure to the vapour for a
considerable time produces the same effects as long immersion in a
strong solution. In both cases there could hardly be a doubt that the
salt had been absorbed chiefly or exclusively by the glands.
On another occasion bits of damp fibrin, drops of a weak infusion of
raw meat and of water, were left for 24 hrs. on some leaves; the hairs
were then examined, but to my surprise differed in no respect from
others which had not been touched by these fluids. Most of the cells,
however, included hyaline, motionless little spheres, which did not
seem to consist of protoplasm, but, I suppose, of some balsam or
essential oil.
Pelargonium zonale (var. edged with white).—The leaves [page 351] are
clothed with numerous multicellular hairs; some simply pointed; others
bearing glandular heads, and differing much in length. The glands on a
piece of leaf were examined and found to contain only limpid fluid;
most of the water was removed from beneath the covering glass, and a
minute drop of one part of carbonate of ammonia to 146 of water was
added; so that an extremely small dose was given. After an interval of
only 3 m. there were signs of aggregation within the glands of the
shorter hairs; and after 5 m. many small globules of a pale brown tint
appeared in all of them; similar globules, but larger, being found in
the large glands of the longer hairs. After the specimen had been left
for 1 hr. in the solution, many of the smaller globules had changed
their positions; and two or three vacuoles or small spheres (for I know
not which they were) of a rather darker tint appeared within some of
the larger globules. Little globules could now be seen in some of the
uppermost cells of the pedicels, and the protoplasmic lining was
slightly separated from the walls of the lower cells. After 2 hrs. 30
m. from the time of first immersion, the large globules within the
glands of the longer hairs were converted into masses of darker brown
granular matter. Hence from what we have seen with Primula sinensis,
there can be little doubt that these masses originally consisted of
living protoplasm.
A drop of a weak infusion of raw meat was placed on a leaf, and after 2
hrs. 30 m. many spheres could be seen within the glands. These spheres,
when looked at again after 30 m., had slightly changed their positions
and forms, and one had separated into two; but the changes were not
quite like those which the protoplasm of Drosera undergoes. These
hairs, moreover, had not been examined before immersion, and there were
similar spheres in some glands which had not been touched by the
infusion.
Erica tetralix.—A few long glandular hairs project from the margins of
the upper surfaces of the leaves. The pedicels are formed of several
rows of cells, and support rather large globular heads, secreting
viscid matter, by which minute insects are occasionally, though rarely,
caught. Some leaves were left for 23 hrs. in a weak infusion of raw
meat and in water, and the hairs were then compared, but they differed
very little or not at all. In both cases the contents of the cells
seemed rather more granular than they were before; but the granules did
not exhibit any movement. Other leaves were left for 23 hrs. in a
solution of one part of carbonate of ammonia to 218 of water, and here
again the granular matter appeared to have increased [page 352] in
amount; but one such mass retained exactly the same form as before
after an interval of 5 hrs., so that it could hardly have consisted of
living protoplasm. These glands seem to have very little or no power of
absorption, certainly much less than those of the foregoing plants.
Mirabilis longiflora.—The stems and both surfaces of the leaves bear
viscid hairs. young plants, from 12 to 18 inches in height in my
greenhouse, caught so many minute Diptera, Coleoptera, and larvæ, that
they were quite dusted with them. The hairs are short, of unequal
lengths, formed of a single row of cells, surmounted by an enlarged
cell which secretes viscid matter. These terminal cells or glands
contain granules and often globules of granular matter. Within a gland
which had caught a small insect, one such mass was observed to undergo
incessant changes of form, with the occasional appearance of vacuoles.
But I do not believe that this protoplasm had been generated by matter
absorbed from the dead insect; for, on comparing several glands which
had and had not caught insects, not a shade of difference could be
perceived between them, and they all contained fine granular matter. A
piece of leaf was immersed for 24 hrs. in a solution of one part of
carbonate of ammonia to 218 of water, but the hairs seemed very little
affected by it, excepting that perhaps the glands were rendered rather
more opaque. In the leaf itself, however, the grains of chlorophyll
near the cut surfaces had run together, or become aggregated. Nor were
the glands on another leaf, after an immersion for 24 hrs. in an
infusion of raw meat, in the least affected; but the protoplasm lining
the cells of the pedicels had shrunk greatly from the walls. This
latter effect may have been due to exosmose, as the infusion was
strong. We may, therefore, conclude that the glands of this plant
either have no power of absorption or that the protoplasm which they
contain is not acted on by a solution of carbonate of ammonia (and this
seems scarcely credible) or by an infusion of meat.
Nicotiana tabacum.—This plant is covered with innumerable hairs of
unequal lengths, which catch many minute insects. The pedicels of the
hairs are divided by transverse partitions, and the secreting glands
are formed of many cells, containing greenish matter with little
globules of some substance. Leaves were left in an infusion of raw meat
and in water for 26 hrs., but presented no difference. Some of these
same leaves were then left for above 2 hrs. in a solution of carbonate
of ammonia, but no effect was produced. I regret that other experiments
were not tried with more care, as M. Schloesing [page 353] has shown*
that tobacco plants supplied with the vapour of carbonate of ammonia
yield on analysis a greater amount of nitrogen than other plants not
thus treated; and, from what we have seen, it is probable that some of
the vapour may be absorbed by the glandular hairs.]
A Summary of the Observations on Glandular Hairs.—From the foregoing
observations, few as they are, we see that the glands of two species of
Saxifraga, of a Primula and Pelargonium, have the power of rapid
absorption; whereas the glands of an Erica, Mirabilis, and Nicotiana,
either have no such power, or the contents of the cells are not
affected by the fluids employed, namely a solution of carbonate of
ammonia and an infusion of raw meat. As the glands of the Mirabilis
contain protoplasm, which did not become aggregated from exposure to
the fluids just named, though the contents of the cells in the blade of
the leaf were greatly affected by carbonate of ammonia, we may infer
that they cannot absorb. We may further infer that the innumerable
insects caught by this plant are of no more service to it than are
those which adhere to the deciduous and sticky scales of the leaf-buds
of the horse-chestnut.
The most interesting case for us is that of the two species of
Saxifraga, as this genus is distantly allied to Drosera. Their glands
absorb matter from an infusion of raw meat, from solutions of the
nitrate and carbonate of ammonia, and apparently from decayed insects.
This was shown by the changed dull purple colour of the protoplasm
within the cells of the glands, by its state of aggregation, and
apparently by its more rapid spontaneous movements.
* ‘Comptes rendus,’ June 15, 1874. A good abstract of this paper is
given in the ‘Gardener’s Chronicle,’ July 11, 1874. [page 354]
The aggregating process spreads from the glands down the pedicels of
the hairs; and we may assume that any matter which is absorbed
ultimately reaches the tissues of the plant. On the other hand, the
process travels up the hairs whenever a surface is cut and exposed to a
solution of the carbonate of ammonia.
The glands on the flower-stalks and leaves of Primula sinensis quickly
absorb a solution of the carbonate of ammonia, and the protoplasm which
they contain becomes aggregated. The process was seen in some cases to
travel from the glands into the upper cells of the pedicels. Exposure
for 10 m. to the vapour of this salt likewise induced aggregation. When
leaves were left from 6 hrs. to 7 hrs. in a strong solution, or were
long exposed to the vapour, the little masses of protoplasm became
disintegrated, brown, and granular, and were apparently killed. An
infusion of raw meat produced no effect on the glands.
The limpid contents of the glands of Pelargonium zonale became cloudy
and granular in from 3 m. to 5 m. when they were immersed in a weak
solution of the carbonate of ammonia; and in the course of 1 hr.
granules appeared in the upper cells of the pedicels. As the aggregated
masses slowly changed their forms, and as they suffered disintegration
when left for a considerable time in a strong solution, there can be
little doubt that they consisted of protoplasm. It is doubtful whether
an infusion of raw meat produced any effect.
The glandular hairs of ordinary plants have generally been considered
by physiologists to serve only as secreting or excreting organs, but we
now know that they have the power, at least in some cases, of absorbing
both a solution and the vapour of ammonia. As rain-water contains a
small percentage of ammonia, and the atmosphere a minute quantity of
the carbonate, this [page 355] power can hardly fail to be beneficial.
Nor can the benefit be quite so insignificant as it might at first be
thought, for a moderately fine plant of Primula sinensis bears the
astonishing number of above two millions and a half of glandular
hairs,* all of which are able to absorb ammonia brought to them by the
rain. It is moreover probable that the glands of some of the above
named plants obtain animal matter from the insects which are
occasionally entangled by the viscid secretion.
CONCLUDING REMARKS ON THE DROSERACEÆ.
The six known genera composing this family have now been described in
relation to our present subject, as far as my means have permitted.
They all capture insects. This is effected by Drosophyllum, Roridula,
and Byblis, solely by the viscid fluid secreted from their glands; by
Drosera, through the same means, together with the movements of the
tentacles; by Dionaea and Aldrovanda, through the closing of the blades
of the leaf. In these two last genera rapid
* My son Francis counted the hairs on a space measured by means of a
micrometer, and found that there were 35,336 on a square inch of the
upper surface of a leaf, and 30,035 on the lower surface; that is, in
about the proportion of 100 on the upper to 85 on the lower surface. On
a square inch of both surfaces there were 65,371 hairs. A moderately
fine plant bearing twelve leaves (the larger ones being a little more
than 2 inches in diameter) was now selected, and the area of all the
leaves, together with their foot-stalks (the flower-stems not being
included), was found by a planimeter to be 39.285 square inches; so
that the area of both surfaces was 78.57 square inches. Thus the plant
(excluding the flower-stems) must have borne the astonishing number of
2,568,099 glandular hairs. The hairs were counted late in the autumn,
and by the following spring (May) the leaves of some other plants of
the same lot were found to be from one-third to one-fourth broader and
longer than they were before; so that no doubt the glandular hairs had
increased in number, and probably now much exceeded three millions.
[page 356]
movement makes up for the loss of viscid secretion. In every case it is
some part of the leaf which moves. In Aldrovanda it appears to be the
basal parts alone which contract and carry with them the broad, thin
margins of the lobes. In Dionaea the whole lobe, with the exception of
the marginal prolongations or spikes, curves inwards, though the chief
seat of movement is near the midrib. In Drosera the chief seat is in
the lower part of the tentacles, which, homologically, may be
considered as prolongations of the leaf; but the whole blade often
curls inwards, converting the leaf into a temporary stomach.
There can hardly be a doubt that all the plants belonging to these six
genera have the power of dissolving animal matter by the aid of their
secretion, which contains an acid, together with a ferment almost
identical in nature with pepsin; and that they afterwards absorb the
matter thus digested. This is certainly the case with Drosera,
Drosophyllum, and Dionaea; almost certainly with Aldrovanda; and, from
analogy, very probable with Roridula and Byblis. We can thus understand
how it is that the three first-named genera are provided with such
small roots, and that Aldrovanda is quite rootless; about the roots of
the two other genera nothing is known. It is, no doubt, a surprising
fact that a whole group of plants (and, as we shall presently see, some
other plants not allied to the Droseraceae) should subsist partly by
digesting animal matter, and partly by decomposing carbonic acid,
instead of exclusively by this latter means, together with the
absorption of matter from the soil by the aid of roots. We have,
however, an equally anomalous case in the animal kingdom; the
rhizocephalous crustaceans do not feed like other animals by their
mouths, for they are destitute of an [page 357] alimentary canal; but
they live by absorbing through root-like processes the juices of the
animals on which they are parasitic.*
Of the six genera, Drosera has been incomparably the most successful in
the battle for life; and a large part of its success may be attributed
to its manner of catching insects. It is a dominant form, for it is
believed to include about 100 species,** which range in the Old World
from the Arctic regions to Southern India, to the Cape of Good Hope,
Madagascar, and Australia; and in the New World from Canada to Tierra
del Fuego. In this respect it presents a marked contrast with the five
other genera, which appear to be failing groups. Dionaea includes only
a single species, which is confined to one district in Carolina. The
three varieties or closely allied species of Aldrovanda, like so many
water-plants, have a wide range from Central Europe to Bengal and
Australia. Drosophyllum includes only one species, limited to Portugal
and Morocco. Roridula and Byblis each have (as I
* Fritz Müller, ‘Facts for Darwin,’ Eng. trans. 1869, p. 139. The
rhizocephalous crustaceans are allied to the cirripedes. It is hardly
possible to imagine a greater difference than that between an animal
with prehensile limbs, a well-constructed mouth and alimentary canal,
and one destitute of all these organs and feeding by absorption through
branching root-like processes. If one rare cirripede, the Anelasma
squalicola, had become extinct, it would have been very difficult to
conjecture how so enormous a change could have been gradually effected.
But, as Fritz Müller remarks, we have in Anelasma an animal in an
almost exactly intermediate condition, for it has root-like processes
embedded in the skin of the shark on which it is parasitic, and its
prehensile cirri and mouth (as described in my monograph on the
Lepadidae, ‘Ray Soc.’ 1851, p. 169) are in a most feeble and almost
rudimentary condition. Dr. R. Kossmann has given a very interesting
discussion on this subject in his ‘Suctoria and Lepadidae,’ 1873. See
also, Dr. Dohrn, ‘Der Ursprung der Wirbelthiere,’ 1875, p. 77.
** Bentham and Hooker, ‘Genera Plantarum.’ Australia is the metropolis
of the genus, forty-one species having been described from this
country, as Prof. Oliver informs me. [page 358]
hear from Prof. Oliver) two species; the former confined to the western
parts of the Cape of Good Hope, and the latter to Australia. It is a
strange fact that Dionaea, which is one of the most beautifully adapted
plants in the vegetable kingdom, should apparently be on the high-road
to extinction. This is all the more strange as the organs of Dionaea
are more highly differentiated than those of Drosera; its filaments
serve exclusively as organs of touch, the lobes for capturing insects,
and the glands, when excited, for secretion as well as for absorption;
whereas with Drosera the glands serve all these purposes, and secrete
without being excited.
By comparing the structure of the leaves, their degree of complication,
and their rudimentary parts in the six genera, we are led to infer that
their common parent form partook of the characters of Drosophyllum,
Roridula, and Byblis. The leaves of this ancient form were almost
certainly linear, perhaps divided, and bore on their upper and lower
surfaces glands which had the power of secreting and absorbing. Some of
these glands were mounted on pedicels, and others were almost sessile;
the latter secreting only when stimulated by the absorption of
nitrogenous matter. In Byblis the glands consist of a single layer of
cells, supported on a unicellular pedicel; in Roridula they have a more
complex structure, and are supported on pedicels formed of several rows
of cells; in Drosophyllum they further include spiral cells, and the
pedicels include a bundle of spiral vessels. But in these three genera
these organs do not possess any power of movement, and there is no
reason to doubt that they are of the nature of hairs or trichomes.
Although in innumerable instances foliar organs move when excited, no
case is known of a trichome having such [page 359] power.* We are thus
led to inquire how the so-called tentacles of Drosera, which are
manifestly of the same general nature as the glandular hairs of the
above three genera, could have acquired the power of moving. Many
botanists maintain that these tentacles consist of prolongations of the
leaf, because they include vascular tissue, but this can no longer be
considered as a trustworthy distinction.** The possession of the power
of movement on excitement would have been safer evidence. But when we
consider the vast number of the tentacles on both surfaces of the
leaves of Drosophyllum, and on the upper surface of the leaves of
Drosera, it seems scarcely possible that each tentacle could have
aboriginally existed as a prolongation of the leaf. Roridula, perhaps,
shows us how we may reconcile these difficulties with respect to the
homological nature of the tentacles. The lateral divisions of the
leaves of this plant terminate in long tentacles; and these include
spiral vessels which extend for only a short distance up them, with no
line of demarcation between what is plainly the prolongation of the
leaf and the pedicel of a glandular hair. Therefore there would be
nothing anomalous or unusual in the basal parts of these tentacles,
which correspond with the marginal ones of Drosera, acquiring the power
of movement; and we know that in Drosera it is only the lower part
which becomes inflected. But in order to understand how in this latter
genus not only the marginal but all the inner tentacles have become
capable of movement, we must further assume, either that through the
principle of correlated development this
* Sachs, ‘Traité de Botanique’ 3rd edit. 1874, p. 1026.
** Dr. Warming ‘Sur la Diffrence entres les Trichomes,’ Copenhague,
1873, p. 6. ‘Extrait des Videnskabelige Meddelelser de la Soc. d’Hist.
nat. de Copenhague,’ Nos. 10-12, 1872. [page 360]
power was transferred to the basal parts of the hairs, or that the
surface of the leaf has been prolonged upwards at numerous points, so
as to unite with the hairs, thus forming the bases of the inner
tentacles.
The above named three genera, namely Drosophyllum, Roridula, and
Byblis, which appear to have retained a primordial condition, still
bear glandular hairs on both surfaces of their leaves; but those on the
lower surface have since disappeared in the more highly developed
genera, with the partial exception of one species, Drosera binata. The
small sessile glands have also disappeared in some of the genera, being
replaced in Roridula by hairs, and in most species of Drosera by
absorbent papillae. Drosera binata, with its linear and bifurcating
leaves, is in an intermediate condition. It still bears some sessile
glands on both surfaces of the leaves, and on the lower surface a few
irregularly placed tentacles, which are incapable of movement. A
further slight change would convert the linear leaves of this latter
species into the oblong leaves of Drosera anglica, and these might
easily pass into orbicular ones with footstalks, like those of Drosera
rotundifolia. The footstalks of this latter species bear multicellular
hairs, which we have good reason to believe represent aborted
tentacles.
The parent form of Dionaea and Aldrovanda seems to have been closely
allied to Drosera, and to have had rounded leaves, supported on
distinct footstalks, and furnished with tentacles all round the
circumference, with other tentacles and sessile glands on the upper
surface. I think so because the marginal spikes of Dionaea apparently
represent the extreme marginal tentacles of Drosera, the six (sometimes
eight) sensitive filaments on the upper surface, as well as the more
numerous ones in Aldrovanda, representing the central [page 361]
tentacles of Drosera, with their glands aborted, but their
sensitiveness retained. Under this point of view we should bear in mind
that the summits of the tentacles of Drosera, close beneath the glands,
are sensitive.
The three most remarkable characters possessed by the several members
of the Droseraceae consist in the leaves of some having the power of
movement when excited, in their glands secreting a fluid which digests
animal matter, and in their absorption of the digested matter. Can any
light be thrown on the steps by which these remarkable powers were
gradually acquired?
As the walls of the cells are necessarily permeable to fluids, in order
to allow the glands to secrete, it is not surprising that they should
readily allow fluids to pass inwards; and this inward passage would
deserve to be called an act of absorption, if the fluids combined with
the contents of the glands. Judging from the evidence above given, the
secreting glands of many other plants can absorb salts of ammonia, of
which they must receive small quantities from the rain. This is the
case with two species of Saxifraga, and the glands of one of them
apparently absorb matter from captured insects, and certainly from an
infusion of raw meat. There is, therefore, nothing anomalous in the
Droseraceae having acquired the power of absorption in a much more
highly developed degree.
It is a far more remarkable problem how the members of this family, and
Pinguicula, and, as Dr. Hooker has recently shown, Nepenthes, could all
have acquired the power of secreting a fluid which dissolves or digests
animal matter. The six genera of the Droseraceae very probably
inherited this power from a common progenitor, but this cannot apply to
[page 362] Pinguicula or Nepenthes, for these plants are not at all
closely related to the Droceraceae. But the difficulty is not nearly so
great as it at first appears. Firstly, the juices of many plants
contain an acid, and, apparently, any acid serves for digestion.
Secondly, as Dr. Hooker has remarked in relation to the present subject
in his address at Belfast (1874), and as Sachs repeatedly insists,* the
embryos of some plants secrete a fluid which dissolves albuminous
substances out of the endosperm; although the endosperm is not actually
united with, only in contact with, the embryo. All plants, moreover,
have the power of dissolving albuminous or proteid substances, such as
protoplasm, chlorophyll, gluten, aleurone, and of carrying them from
one part to other parts of their tissues. This must be effected by a
solvent, probably consisting of a ferment together with an acid.** Now,
in the case of plants which are able to absorb already soluble matter
from captured insects, though not capable of true digestion, the
solvent just referred to, which must be occasionally present in the
glands, would be apt to exude from the glands together with the viscid
secretion, inasmuch as endosmose is accompanied by exosmose. If such
exudation did ever occur, the solvent would act on the animal matter
contained within the captured insects, and this would be an act of true
digestion. As it cannot be doubted that this process would be of high
service to plants
* ‘Traité de Botanique’ 3rd edit. 1874, p. 844. See also for following
facts pp. 64, 76, 828, 831.
** Since this sentence was written, I have received a paper by
Gorup-Besanez (‘Berichte der Deutschen Chem. Gesellschaft,’ Berlin,
1874, p. 1478), who, with the aid of Dr. H. Will, has actually made the
discovery that the seeds of the vetch contain a ferment, which, when
extracted by glycerine, dissolves albuminous substances, such as
fibrin, and converts them into true peptones. [page 363]
growing in very poor soil, it would tend to be perfected through
natural selection. Therefore, any ordinary plant having viscid glands,
which occasionally caught insects, might thus be converted under
favourable circumstances into a species capable of true digestion. It
ceases, therefore, to be any great mystery how several genera of
plants, in no way closely related together, have independently acquired
this same power.
As there exist several plants the glands of which cannot, as far as is
known, digest animal matter, yet can absorb salts of ammonia and animal
fluids, it is probable that this latter power forms the first stage
towards that of digestion. It might, however, happen, under certain
conditions, that a plant, after having acquired the power of digestion,
should degenerate into one capable only of absorbing animal matter in
solution, or in a state of decay, or the final products of decay,
namely the salts of ammonia. It would appear that this has actually
occurred to a partial extent with the leaves of Aldrovanda; the outer
parts of which possess absorbent organs, but no glands fitted for the
secretion of any digestive fluid, these being confined to the inner
parts.
Little light can be thrown on the gradual acquirement of the third
remarkable character possessed by the more highly developed genera of
the Droseraceae, namely the power of movement when excited. It should,
however, be borne in mind that leaves and their homologues, as well as
flower-peduncles, have gained this power, in innumerable instances,
independently of inheritance from any common parent form; for instance,
in tendril-bearers and leaf-climbers (i.e. plants with their leaves,
petioles and flower-peduncles, &c., modified for prehension) belonging
to a large [page 364] number of the most widely distinct orders,—in the
leaves of the many plants which go to sleep at night, or move when
shaken,—and in the irritable stamens and pistils of not a few species.
We may therefore infer that the power of movement can be by some means
readily acquired. Such movements imply irritability or sensitiveness,
but, as Cohn has remarked,* the tissues of the plants thus endowed do
not differ in any uniform manner from those of ordinary plants; it is
therefore probable that all leaves are to a slight degree irritable.
Even if an insect alights on a leaf, a slight molecular change is
probably transmitted to some distance across its tissue, with the sole
difference that no perceptible effect is produced. We have some
evidence in favour of this belief, for we know that a single touch on
the glands of Drosera does not excite inflection; yet it must produce
some effect, for if the glands have been immersed in a solution of
camphor, inflection follows within a shorter time than would have
followed from the effects of camphor alone. So again with Dionaea, the
blades in their ordinary state may be roughly touched without their
closing; yet some effect must be thus caused and transmitted across the
whole leaf, for if the glands have recently absorbed animal matter,
even a delicate touch causes them to close instantly. On the whole we
may conclude that the acquirement of a high degree of sensitiveness and
of the power of movement by certain genera of the Droseraceae presents
no greater difficulty than that presented by the similar but feebler
powers of a multitude of other plants.
* See the abstract of his memoir on the contractile tissues of plants,
in the ‘Annals and Mag. of Nat. Hist.’ 3rd series, vol. xi. p. 188.)
[page 365]
The specialised nature of the sensitiveness possessed by Drosera and
Dionaea, and by certain other plants, well deserves attention. A gland
of Drosera may be forcibly hit once, twice, or even thrice, without any
effect being produced, whilst the continued pressure of an extremely
minute particle excites movement. On the other hand, a particle many
times heavier may be gently laid on one of the filaments of Dionaea
with no effect; but if touched only once by the slow movement of a
delicate hair, the lobes close; and this difference in the nature of
the sensitiveness of these two plants stands in manifest adaptation to
their manner of capturing insects. So does the fact, that when the
central glands of Drosera absorb nitrogenous matter, they transmit a
motor impulse to the exterior tentacles much more quickly than when
they are mechanically irritated; whilst with Dionaea the absorption of
nitrogenous matter causes the lobes to press together with extreme
slowness, whilst a touch excites rapid movement. Somewhat analogous
cases may be observed, as I have shown in another work, with the
tendrils of various plants; some being most excited by contact with
fine fibres, others by contact with bristles, others with a flat or a
creviced surface. The sensitive organs of Drosera and Dionaea are also
specialised, so as not to be uselessly affected by the weight or impact
of drops of rain, or by blasts of air. This may be accounted for by
supposing that these plants and their progenitors have grown accustomed
to the repeated action of rain and wind, so that no molecular change is
thus induced; whilst they have been rendered more sensitive by means of
natural selection to the rarer impact or pressure of solid bodies.
Although the absorption by the glands of Drosera of various fluids
excites move- [page 366] ment, there is a great difference in the
action of allied fluids; for instance, between certain vegetable acids,
and between citrate and phosphate of ammonia. The specialised nature
and perfection of the sensitiveness in these two plants is all the more
astonishing as no one supposes that they possess nerves; and by testing
Drosera with several substances which act powerfully on the nervous
system of animals, it does not appear that they include any diffused
matter analogous to nerve-tissue.
Although the cells of Drosera and Dionaea are quite as sensitive to
certain stimulants as are the tissues which surround the terminations
of the nerves in the higher animals, yet these plants are inferior even
to animals low down in the scale, in not being affected except by
stimulants in contact with their sensitive parts. They would, however,
probably be affected by radiant heat; for warm water excites energetic
movement. When a gland of Drosera, or one of the filaments of Dionaea,
is excited, the motor impulse radiates in all directions, and is not,
as in the case of animals, directed towards special points or organs.
This holds good even in the case of Drosera when some exciting
substance has been placed at two points on the disc, and when the
tentacles all round are inflected with marvellous precision towards the
two points. The rate at which the motor impulse is transmitted, though
rapid in Dionaea, is much slower than in most or all animals. This
fact, as well as that of the motor impulse not being specially directed
to certain points, are both no doubt due to the absence of nerves.
Nevertheless we perhaps see the prefigurement of the formation of
nerves in animals in the transmission of the motor impulse being so
much more rapid down the confined space within the tentacles of Drosera
than [page 367] elsewhere, and somewhat more rapid in a longitudinal
than in a transverse direction across the disc. These plants exhibit
still more plainly their inferiority to animals in the absence of any
reflex action, except in so far as the glands of Drosera, when excited
from a distance, send back some influence which causes the contents of
the cells to become aggregated down to the bases of the tentacles. But
the greatest inferiority of all is the absence of a central organ, able
to receive impressions from all points, to transmit their effects in
any definite direction, to store them up and reproduce them. [page 368]