secretion of many glands on thirty leaves, which had not been in
any way excited, was tested with litmus-paper; and the secretion of
twenty-two of these leaves did not in the least affect the color,
whereas that of eight caused an exceedingly feeble and sometimes
doubtful tinge of red. Two other old leaves, however, which appeared
to have been inflected several times, acted much more decidedly on
the paper. Particles of clean glass were then placed on five of the
leaves, cubes of albumen on six, and bits of raw meat on three, on none
of which was the secretion at this time in the least acid. After an
interval of twenty-four hours, when almost all the tentacles on these
fourteen leaves had become more or less inflected, I again tested the
secretion, selecting glands which had not as yet reached the center or
touched any object, and it was now plainly acid. The degree of acidity
of the secretion varied somewhat on the glands of the same leaf. On
some leaves a few tentacles did not, from some unknown cause, become
inflected, as often happens; and in five instances their secretion
was found not to be in the least acid; while the secretion of the
adjoining and inflected tentacles on the same leaf was decidedly acid.
With leaves excited by particles of glass placed on the central glands,
the secretion which collects on the disk beneath them was much more
strongly acid than that poured forth from the exterior tentacles, which
were as yet only moderately inflected. When bits of albumen (and this
is naturally alkaline) or bits of meat were placed on the disk, the
secretion collected beneath them was likewise strongly acid. As raw
meat moistened with water is slightly acid, I compared its action on
litmus-paper before it was placed on the leaves, and afterward when
bathed in the secretion; and there could not be the least doubt that
the latter was very much more acid. I have indeed tried hundreds of
times the state of the secretion on the disks of leaves which were
inflected over various objects, and never failed to find it acid. We
may, therefore, conclude that the secretion from unexcited leaves,
though extremely viscid, is not acid or only slightly so, but that it
becomes acid, or much more strongly so, after the tentacles have begun
to bend over any inorganic or organic object; and still more strongly
acid after the tentacles have remained for some time closely clasped
over any object.
I may here remind the reader that the secretion appears to be to a
certain extent antiseptic, as it checks the appearance of mold and
infusoria, thus preventing for a time the discoloration and decay of
such substances as the white of an egg, cheese, etc. It therefore acts
like the gastric juice of the higher animals, which is known to arrest
putrefaction by destroying the microzymes.
* * * * *
[Page 98.]
Cubes of about one twentieth of an inch (1·27 millimetre) of moderately
roasted meat were placed on five leaves, which became in twelve hours
closely inflected. After forty-eight hours I gently opened one leaf,
and the meat now consisted of a minute central sphere, partially
digested, and surrounded by a thick envelope of transparent viscid
fluid. The whole, without being much disturbed, was removed and placed
under the microscope. In the central part the transverse striæ on the
muscular fibers were quite distinct; and it was interesting to observe
how gradually they disappeared, when the same fiber was traced into
the surrounding fluid. They disappeared by the striæ being replaced by
transverse lines formed of excessively minute dark points, which toward
the exterior could be seen only under a very high power; and ultimately
these points were lost.
* * * * *
[Page 134.]
Finally, the experiments recorded in this chapter show us that there
is a remarkable accordance in the power of digestion between the
gastric juice of animals, with its pepsin and hydrochloric acid, and
the secretion of _Drosera_ with its ferment and acid belonging to the
acetic series. We can, therefore, hardly doubt that the ferment in both
cases is closely similar.
DIVERSE MEANS BY WHICH PLANTS GAIN THEIR SUBSISTENCE.
[Insectivorous
Plants,
page 452.]
Ordinary plants of the higher classes procure the requisite inorganic
elements from the soil by means of their roots, and absorb carbonic
acid from the atmosphere by means of their leaves and stems. But we
have seen in a previous part of this work that there is a class of
plants which digest and afterward absorb animal matter, namely, all
the _Droseraceæ_, _Pinguicula_, and, as discovered by Dr. Hooker,
_Nepenthes_, and to this class other species will almost certainly soon
be added. These plants can dissolve matter out of certain vegetable
substances, such as pollen, seeds, and bits of leaves. No doubt their
glands likewise absorb the salts of ammonia brought to them by the
rain. It has also been shown that some other plants can absorb ammonia
by their glandular hairs; and these will profit by that brought to them
by the rain. There is a second class of plants which, as we have just
seen, can not digest, but absorb, the products of the decay of the
animals which they capture, namely, _Utricularia_ and its close allies;
and, from the excellent observations of Dr. Mellichamp and Dr. Canby,
there can scarcely be a doubt that _Sarracenia_ and _Darlingtonia_ may
be added to this class, though the fact can hardly be considered as yet
fully proved. There is a third class of plants which feed, as is now
generally admitted, on the products of the decay of vegetable matter,
such as the bird’s-nest orchid (_Neottia_), etc. Lastly, there is the
well-known fourth class of parasites (such as the mistletoe), which
are nourished by the juices of living plants. Most, however, of the
plants belonging to these four classes obtain part of their carbon,
like ordinary species, from the atmosphere. Such are the diversified
means, as far as at present known, by which higher plants gain their
subsistence.
HOW A PLANT PREYS UPON ANIMALS.
_The genus described is Genlisea ornata._
[Insectivorous
Plants,
page 446.]
The utricle is formed by a slight enlargement of the narrow blade of
the leaf. A hollow neck, no less than fifteen times as long as the
utricle itself, forms a passage from the transverse slit-like orifice
into the cavity of the utricle. A utricle which measured 1/36 of an
inch (·795 millimetre) in its longer diameter had a neck 15/36 (10·583
millimetres) in length, and 1/100 of an inch (·254 millimetre) in
breadth. On each side of the orifice there is a long spiral arm, or
tube; the structure of which will be best understood by the following
illustration: Take a narrow ribbon and wind it spirally round a thin
cylinder, so that the edges come into contact along its whole length;
then pinch up the two edges so as to form a little crest, which will,
of course, wind spirally round the cylinder, like a thread round a
screw. If the cylinder is now removed, we shall have a tube like one of
the spiral arms. The two projecting edges are not actually united, and
a needle can be pushed in easily between them. They are indeed in many
places a little separated, forming narrow entrances into the tube; but
this may be the result of the drying of the specimens. The lamina of
which the tube is formed seems to be a lateral prolongation of the lip
of the orifice; and the spiral line between the two projecting edges
is continuous with the corner of the orifice. If a fine bristle is
pushed down one of the arms, it passes into the top of the hollow neck.
Whether the arms are open or closed at their extremities could not be
determined, as all the specimens were broken; nor does it appear that
Dr. Warming ascertained this point.
So much for the external structure. Internally the lower part of
the utricle is covered with spherical papillæ, formed of four cells
(sometimes eight, according to Dr. Warming), which evidently answer
to the quadrifid processes within the bladders of _Utricularia_.
These papillæ extend a little way up the dorsal and ventral surfaces
of the utricle; and a few, according to Warming may be found in the
upper part. This upper region is covered by many transverse rows,
one above the other, of short, closely approximate hairs, pointing
downward. These hairs have broad bases, and their tips are formed by a
separate cell. They are absent in the lower part of the utricle where
the papillæ abound. The neck is likewise lined throughout its whole
length with transverse rows of long, thin, transparent hairs, having
broad bulbous bases, with similarly constructed sharp points. They
arise from little projecting ridges, formed of rectangular epidermic
cells. The hairs vary a little in length, but their points generally
extend down to the row next below; so that, if the neck is split open
and laid flat, the inner surface resembles a paper of pins--the hairs
representing the pins, and the little transverse ridges representing
the folds of paper through which the pins are thrust. These rows of
hairs are indicated in the previous figure by numerous transverse lines
crossing the neck. The inside of the neck is also studded with papillæ;
those in the lower part are spherical and formed of four cells, as
in the lower part of the utricle; those in the upper part are formed
of two cells, which are much elongated downward beneath their points
of attachment. These two-celled papillæ apparently correspond with
the bifid process in the upper part of the bladders of _Utricularia_.
The narrow transverse orifice is situated between the bases of the
two spiral arms. No valve could be detected here, nor was any such
structure seen by Dr. Warming. The lips of the orifice are armed with
many short, thick, sharply pointed, somewhat incurved hairs or teeth.
The two projecting edges of the spirally-wound lamina, forming the
arms, are provided with short incurved hairs or teeth, exactly like
those on the lips. These project inward at right angles to the spiral
line of junction between the two edges. The inner surface of the lamina
supports two-celled, elongated papillæ, resembling those in the upper
part of the neck, but differing slightly from them, according to
Warming, in their footstalks being formed by prolongations of large
epidermic cells; whereas the papillæ within the neck rest on small
cells sunk amid the larger ones. These spiral arms form a conspicuous
difference between the present genus and _Utricularia_.
Lastly, there is a bundle of spiral vessels which, running up the lower
part of the linear leaf, divides close beneath the utricle. One branch
extends up the dorsal and the other up the ventral side of both the
utricle and neck. Of these two branches, one enters one spiral arm, and
the other branch the other arm.
The utricles contained much _débris_, or dirty matter, which seemed
organic, though no distinct organisms could be recognized. It is,
indeed, scarcely possible that any object could enter the small orifice
and pass down the long, narrow neck, except a living creature. Within
the necks, however, of some specimens, a worm, with retracted horny
jaws, the abdomen of some articulate animal, and specks of dirt,
probably the remnants of other minute creatures, were found. Many of
the papillæ within both the utricles and necks were discolored, as if
they had absorbed matter.
From this description it is sufficiently obvious how genlisea
secures its prey. Small animals entering the narrow orifice--but
what induces them to enter is not known any more than in the case of
_Utricularia_--would find their egress rendered difficult by the sharp
incurved hairs on the lips, and, as soon as they passed some way down
the neck, it would be scarcely possible for them to return, owing to
the many transverse rows of long, straight, downward-pointing hairs,
together with the ridges from which these project. Such creatures
would, therefore, perish either within the neck or utricle; and the
quadrifid and bifid papillæ would absorb matter from their decayed
remains. The transverse rows of hairs are so numerous that they seem
superfluous merely for the sake of preventing the escape of prey,
and, as they are thin and delicate, they probably serve as additional
absorbents, in the same manner as the flexible bristles on the infolded
margins of the leaves of aldrovanda. The spiral arms, no doubt, act as
accessory traps. Until fresh leaves are examined, it can not be told
whether the line of junction of the spirally-wound lamina is a little
open along its whole course or only in parts, but a small creature
which forced its way into the tube at any point would be prevented from
escaping by the incurved hairs, and would find an open path down the
tube into the neck, and so into the utricle. If the creature perished
within the spiral arms, its decaying remains would be absorbed and
utilized by the bifid papillæ. We thus see that animals are captured
by genlisea, not by means of an elastic valve, as with the foregoing
species, but by a contrivance resembling an eel-trap, though more
complex.
II.
THE PART PLAYED BY WORMS IN THE HISTORY OF THIS PLANET.
[The Formation
of Vegetable
Mold through
the Action of
Earthworms,
page 305.]
Worms have played a more important part in the history of the world
than most persons would at first suppose. In almost all humid countries
they are extraordinarily numerous, and for their size possess great
muscular power. In many parts of England a weight of more than ten
tons (10,516 kilogrammes) of dry earth annually passes through their
bodies and is brought to the surface on each acre of land; so that the
whole superficial bed of vegetable mold passes through their bodies
in the course of every few years. From the collapsing of the old
burrows the mold is in constant though slow movement, and the particles
composing it are thus rubbed together. By these means fresh surfaces
are continually exposed to the action of the carbonic acid in the
soil, and of the humus-acids which appear to be still more efficient
in the decomposition of rocks. The generation of the humus-acids
is probably hastened during the digestion of the many half-decayed
leaves which worms consume. Thus the particles of earth, forming the
superficial mold, are subjected to conditions eminently favorable for
their decomposition and disintegration. Moreover, the particles of
the softer rocks suffer some amount of mechanical trituration in the
muscular gizzards of worms, in which small stones serve as mill-stones.
The finely levigated castings, when brought to the surface in a moist
condition, flow during rainy weather down any moderate slope; and the
smaller particles are washed far down even a gently inclined surface.
Castings when dry often crumble into small pellets, and these are apt
to roll down any sloping surface. Where the land is quite level and
is covered with herbage, and where the climate is humid so that much
dust can not be blown away, it appears at first sight impossible that
there should be any appreciable amount of subaërial denudation; but
worm-castings are blown, especially while moist and viscid, in one
uniform direction by the prevalent winds which are accompanied by
rain. By these several means the superficial mold is prevented from
accumulating to a great thickness; and a thick bed of mold checks in
many ways the disintegration of the underlying rocks and fragments of
rock.
The removal of worm-castings by the above means leads to results which
are far from insignificant. It has been shown that a layer of earth,
·2 of an inch in thickness, is in many places annually brought to the
surface per acre; and if a small part of this amount flows, or rolls,
or is washed, even for a short distance down every inclined surface, or
is repeatedly blown in one direction, a great effect will be produced
in the course of ages. It was found by measurements and calculations
that on a surface with a mean inclination of 9° 26’, 2·4 cubic inches
of earth which had been ejected by worms crossed, in the course of a
year, a horizontal line one yard in length; so that 240 cubic inches
would cross a line a hundred yards in length. This latter amount in a
damp state would weigh eleven and a half pounds. Thus a considerable
weight of earth is continually moving down each side of every valley,
and will in time reach its bed. Finally, this earth will be transported
by the streams flowing in the valleys into the ocean, the great
receptacle for all matter denuded from the land. It is known from the
amount of sediment annually delivered into the sea by the Mississippi,
that its enormous drainage-area must on an average be lowered ·00263 of
an inch each year; and this would suffice in four and a half million
years to lower the whole drainage-area to the level of the sea-shore.
So that, if a small fraction of the layer of fine earth, ·2 of an
inch in thickness, which is annually brought to the surface by worms,
is carried away, a great result can not fail to be produced within a
period which no geologist considers extremely long.
THEY PRESERVE VALUABLE RUINS.
[Page 308.]
Archæologists ought to be grateful to worms, as they protect and
preserve for an indefinitely long period every object, not liable to
decay, which is dropped on the surface of the land, by burying it
beneath their castings. Thus, also, many elegant and curious tesselated
pavements and other ancient remains have been preserved; though no
doubt the worms have in these cases been largely aided by earth washed
and blown from the adjoining land, especially when cultivated. The old
tesselated pavements have, however, often suffered by having subsided
unequally from being unequally undermined by the worms. Even old
massive walls may be undermined and subside; and no building is in this
respect safe, unless the foundations lie six or seven feet beneath the
surface, at a depth at which worms can not work. It is probable that
many monoliths and some old walls have fallen down from having been
undermined by worms.
THEY PREPARE THE GROUND FOR SEED.
[Page 309.]
Worms prepare the ground in an excellent manner for the growth of
fibrous-rooted plants and for seedlings of all kinds. They periodically
expose the mold to the air, and sift it so that no stones larger than
the particles which they can swallow are left in it. They mingle the
whole intimately together, like a gardener who prepares fine soil for
his choicest plants. In this state it is well fitted to retain moisture
and to absorb all soluble substances, as well as for the process of
nitrification. The bones of dead animals, the harder parts of insects,
the shells of land-mollusks, leaves, twigs, etc., are before long all
buried beneath the accumulating castings of worms, and are thus brought
in a more or less decayed state within reach of the roots of plants.
Worms likewise drag an infinite number of dead leaves and other parts
of plants into their burrows, partly for the sake of plugging them up
and partly as food.
The leaves which are dragged into the burrows as food, after being torn
into the finest shreds, partially digested, and saturated with the
intestinal and urinary secretions, are commingled with much earth. This
earth forms the dark-colored, rich humus which almost everywhere covers
the surface of the land with a fairly well-defined layer or mantle.
Von Hensen placed two worms in a vessel eighteen inches in diameter,
which was filled with sand, on which fallen leaves were strewed; and
these were soon dragged into their burrows to a depth of three inches.
After about six weeks an almost uniform layer of sand, a centimetre
(·4 inch) in thickness, was converted into humus by having passed
through the alimentary canals of these two worms. It is believed by
some persons that worm-burrows, which often penetrate the ground almost
perpendicularly to a depth of five or six feet, materially aid in its
drainage; notwithstanding that the viscid castings piled over the
mouths of the burrows prevent or check the rain-water directly entering
them. They allow the air to penetrate deeply into the ground. They also
greatly facilitate the downward passage of roots of moderate size; and
these will be nourished by the humus with which the burrows are lined.
Many seeds owe their germination to having been covered by castings;
and others buried to a considerable depth beneath accumulated castings
lie dormant, until at some future time they are accidentally uncovered
and germinate.
* * * * *
[Page 313.]
When we behold a wide, turf-covered expanse, we should remember that
its smoothness, on which so much of its beauty depends, is mainly due
to all the inequalities having been slowly leveled by worms. It is
a marvelous reflection that the whole of the superficial mold over
any such expanse has passed, and will again pass, every few years
through the bodies of worms. The plow is one of the most ancient and
most valuable of man’s inventions; but long before he existed the
land was in fact regularly plowed, and still continues to be thus
plowed, by earth-worms. It may be doubted whether there are many other
animals which have played so important a part in the history of the
world as have these lowly organized creatures. Some other animals,
however, still more lowly organized, namely corals, have done far more
conspicuous work in having constructed innumerable reefs and islands in
the great oceans; but these are almost confined to the tropical zones.
INTELLIGENCE OF WORMS.
[Page 91.]
We can hardly escape from the conclusion that worms show some degree
of intelligence in their manner of plugging up their burrows. Each
particular object is seized in too uniform a manner, and from causes
which we can generally understand, for the result to be attributed to
mere chance. That every object has not been drawn in by its pointed
end, may be accounted for by labor having been saved through some being
inserted by their broader or thicker ends. No doubt worms are led by
instinct to plug up their burrows; and it might have been expected that
they would have been led by instinct how best to act in each particular
case, independently of intelligence. We see how difficult it is to
judge whether intelligence comes into play, for even plants might
sometimes be thought to be thus directed; for instance, when displaced
leaves redirect their upper surfaces toward the light by extremely
complicated movements and by the shortest course. With animals, actions
appearing due to intelligence may be performed through inherited habit
without any intelligence, although aboriginally thus acquired. Or the
habit may have been acquired through the preservation and inheritance
of beneficial variations of some other habit; and in this case the
new habit will have been acquired independently of intelligence
throughout the whole course of its development. There is no _a priori_
improbability in worms having acquired special instincts through
either of these two latter means. Nevertheless, it is incredible that
instincts should have been developed in reference to objects, such
as the leaves or petioles of foreign plants, wholly unknown to the
progenitors of the worms which act in the described manner. Nor are
their actions so unvarying or inevitable as are most true instincts.
As worms are not guided by special instincts in each particular case,
though possessing a general instinct to plug up their burrows, and, as
chance is excluded, the next most probable conclusion seems to be that
they try in many different ways to draw in objects, and at last succeed
in some one way. But it is surprising that an animal so low in the
scale as a worm should have the capacity for acting in this manner, as
many higher animals have no such capacity.
* * * * *
[Page 95.]
Mr. Romanes, who has specially studied the minds of animals, believes
that we can safely infer intelligence only when we see an individual
profiting by its own experience. Now, if worms try to drag objects into
their burrows first in one way and then in another, until they at last
succeed, they profit at least in each particular instance by experience.
* * * * *
[Page 98.]