PLANTS ATTACKING ANIMALS
Brittle Star _v._ algæ--Fungus _v._ meal-worm--Stag-headed
caterpillars--Liverwort _v._ small insects--Natural
flower-pots--Watercups of Bromeliads--Sarracenia and inquiring
insects--An unfortunate centipede--Pitcher-plants: their
crafty contrivances--Blowflies defy them and spiders rob
them--Bladderwort's traps which catch small fry--Hairs and their
uses--Plants used as fly-papers--Butterwort _v._ midges--Its
use as rennet--Sundew and its sensitive tentacles--Pinning down
an insect--Suffocating and chloroforming the sundew--Venus'
fly-trap which acts like a rat-trap--Have plants a nervous
system?
On the whole the animal world preys upon the vegetable world, and is
in a way parasitic upon it. Indeed, the connexion between the two is
very intimate--that of the diner and his dinner. One can scarcely
imagine a more intimate connexion than this!
There are, however, a great many cases in which plants have turned
the tables on their enemies and deliberately laid themselves out to
catch and to destroy, to feed upon and to devour insects and small
animals. One finds a few examples in almost every group of plants.
Thus there are certain green seaweeds or algæ which are said to
attack and prey upon those peculiar sea-urchins known as Brittle
Stars. The fungus which forms loops, acting exactly like a poacher's
rabbit-snare, in order to catch mealworms, has been already mentioned.
Sometimes in the summer one may notice a little red club about two
to three inches long sticking out of short grass. If one carefully
pulls this up it is found to be growing out of a dead chrysalis or
grub. It is a fungus whose spores have attacked the caterpillar; they
have developed inside its body, and eventually, having completely
eaten up the insect, form the red club, which is producing hundreds
of thousands of spores intended to attack other caterpillars.
[Illustration: The branches like stag's horns are the fruit of
a fungus, Cordyceps Taylori, which lived inside and killed the
caterpillar.]
An allied fungus forms a peculiar branched fruit rather like a minute
stag's horn, and the caterpillar may be seen for some time crawling
about with this extraordinary fungus sticking out of its head. Of
course the bacteria are, some of them, by far the most dangerous foes
of animals (see page 328).
Then there is a small Liverwort, a little red, moss-like plant
(_Frullania tamarisci_), which may be found growing on the bark of
trees, which is said to catch animalcula in the small sack-like
leaves which are underneath the ordinary ones.
But it is amongst the higher flowering plants that one discovers the
most extraordinary and purposeful arrangements for capturing and
digesting insects and other creatures.
In the case of many of these insectivorous plants, traps or pitfalls
are prepared for the insect to fall into.
There are many plants in which the rain is intended to run in
one particular direction, and it is not at all uncommon to find
hollows at the base of the leaf where dust, dirt, and dead insects
accumulate. One very curious plant of this sort is _Dischidia
Rafflesiana_, in which the leaves have become quite like a pitcher,
and have been compared to "natural flower-pots" intended to hold rain
and leaf-mould.[146]
[146] Groom, _Ann. Bot._, 1903, p. 223.
Then there is the Bromelia or Pineapple family, which consists for
the most part of plants which live on the branches of trees. In very
many of these a small cup is formed in the middle of the rosette or
tuft of leaves, and water collects in this central cup.
The water smells abominably, and contains the bodies of dead insects,
and rubbish of all kinds (see also p. 298). The remnants of these
drowned insects are probably of use, because any valuable nitrogenous
or other material may be absorbed with the water by the plant and
help to nourish it, but in such a rough contrivance as this there
is nothing comparable to the Side-saddle plant, Pitcher plant, and
others.
The former, Sarracenia (or Side-saddle plant), is a common and rather
widespread North American plant, which is especially abundant in
Florida. It is cultivated in most botanical gardens, but can only
be grown in greenhouses. The leaves are about six inches to a foot
long, and are hollow, funnel-shaped tubes with a short, flat wing
along one edge. They may be an inch or two in diameter at the top or
wider end, where there is also a sort of half-open lid which keeps
rain from getting into the inside of the leaf. The colour of these
tube-like or vase-like leaves varies. It is often variegated with
brown, red, and yellow, and is conspicuous enough even at a distance.
Thus insects fly to these vases and alight on the little cap or lid,
where they find honey and enjoy themselves. Other insects crawl up
along the rim or wing of the vase, finding honey here and there along
their road. Having got to the lid, the insect, being of an inquiring
or inquisitive disposition, will look inside the tube and endeavour
to find more honey therein.
It reaches the rim of the vase and finds that there is honey inside;
it can easily crawl down, and fails to notice that the inside of
the vase is lined with long stiff points which all point downwards.
These points or hairs do not at all interfere with its passage down,
and it proceeds to the honey which forms a smooth, slippery coating.
Then, after greedily absorbing the honey, it tries to get out again.
But that is quite a different matter. Each one of these points or
hairs is facing it, and the whole inside is smooth and slippery. It
struggles, slips, and falls into a pool of water which fills the
lower part of the vase. That is what the plant has developed these
pitchers for. The body of the insect after a time decays away, and
only its empty shell remains. An extraordinary number of insects are
caught by these Sarracenia vases. Sometimes in one which is only ten
inches long, three or four inches will be full of the corpses of
blackbeetles and other drowned insects, and it is said that birds
occasionally visit these vases in order to pick them out. There is
probably some sort of secretion in the water. "A centipede 1-2/3
inches long having fallen into a vase of _Sarracenia purpurea_ in the
night was found only half-immersed in the water. The upper half of
the creature projected above the liquid, and made violent attempts to
escape; but the lower part had not only become motionless, but had
turned white from the effect of the surrounding liquid; it appeared
to be macerated, and exhibited alterations which are not produced in
so short a time in centipedes immersed in ordinary rainwater."[147]
[147] Kerner, _Natural History of Plants_. Many details are taken
from this work in the present chapter.
In some Sarracenias the vase is brought up into a sort of hood or
dome with the entrance at one side and below. There are thin patches
on this dome or cupola, and small insects, attracted by the light
which comes through these bare places, remain dashing themselves
against them or crawling over them just as flies do on a window-pane,
until they become tired and fall down into the water below.
There is something horrible in the cold and careful way in which this
plant arranges its baits for "confiding insects. The latter are fed
with honey, even on the very border of the assassin's den, but after
this farewell revel they generally slip upon the smooth edge, and are
hurled, like lost souls, down into the abyss."[148]
[148] Dennett.
In another plant, the Pitcher Plant (_Nepenthes_, so called from
the drug which produces the sleep of death), we find an even more
beautifully arranged pitcher which acts in very much the same way.
It is, however, only the end of a rather long leaf, or rather of its
midrib, that is turned up to act as a pitcher. There are similar
stiff hairs pointing downwards, and honey is plentifully secreted.
But, in Nepenthes, there is also a distinct secretion which digests
the bodies of the drowning insects. The ferment resembles the active
principle of the gastric and pancreatic juices of the human body,
and, as acids are also present, the insect's body becomes changed
into nutritious juices which readily diffuse into the plant.[149]
Dr. Macfarlane found that when the pitchers were stimulated by being
given insects, the liquid inside them could digest fibrin to jelly in
from three-quarters to one hour's time.[150] But certain insects have
somehow managed to educate their larvæ to resist the gastric juices
of Nepenthes.
[149] Green, _Vegetable Physiology_, p. 203.
[150] _Annals Botany_, vol. 3, p. 253, and vol. 6, p. 401.
Near Fort Dauphin, in Madagascar, I found great quantities of
_Nepenthes madagascariensis_. Almost every pitcher was one-third to
two-thirds full of corpses, but in some of them large, fat, white
maggots, of a very unprepossessing appearance, were quite alive and
apparently thriving. These must have been the larvæ of a blowfly
similar to that which has been mentioned by others as inhabiting
Sarracenia. At the same place a white spider was very often to be
seen. Its web was spun across the mouth of a pitcher, and its body
was quite invisible against the bleached remains inside.
It had suited its colour to the corpses within, in order that it
might steal from the Nepenthes the due reward of all its ingenious
contrivances!
A totally different arrangement is found in an inconspicuous and ugly
little marsh and ditch plant called Utricularia or Bladderwort. It is
very difficult to see, for unless it happens to be in flower it is
entirely submerged in the water. The flowers, which are purple, are
conspicuous and easily seen even at a distance. On these submerged
leaves there are hundreds of small bladders. They are about the
size of a pea, and are most ingeniously contrived to catch small
water-animalcula. The general idea of the bladderwort is exactly that
of the eel-pots so common in some parts of the Thames. There is a
small flap which acts as a trapdoor. Small creatures probably take
refuge in the bladders when pursued by the larger water-fleas, etc.,
for it must seem to them to be a safe and secure retreat.
But once within the door, they are imprisoned and cannot find their
way out again. They perish inside and their bodies are digested by
the plant; on the inside of the bladder there are gland hairs which
also secrete a digestive fluid.
The bladderwort is dangerous to fish, for the little fry, when quite
small, run their heads and gills into the bladders and are suffocated.
There are a great many kinds of Utricularia, and they occur in most
of the great floral regions.
One of them has chosen a very extraordinary and curious situation.
It lives inside the little cups of water which, as we have already
mentioned, are formed by the leaves of some Bromeliads. The insects
in the water which ought to nourish the Bromeliad (_Tillandsia_)
are really used by the Utricularia. Other Utricularias live in damp
earth, moss, etc.
It is not only by traps and pitfalls that plants catch insects:
many have specially modified hairs which are quite efficient
insect-catchers.
Hairs are used by plants for many different purposes, and it is
rather interesting to see how quite a simple organ like a hair can be
altered. The stinging hair of the nettle has already been mentioned;
many grasses possess minute, rough, flinty hairs, which probably
prevent snails from eating them. That also is probably the reason
of the strong, rough, coarse hairs which cover the Borage and the
Comfrey.
Then on the Chickweed and the Bird's-eye Speedwell there are lines
of rather long, flexible hairs which at first sight appear to be of
no use at all. But if you take either of these plants, and, holding
it upright, place a large drop of water on the leaves, you will see
that these hairs are intended to carry the water down the stem.
The water runs along them. It is a very pretty little experiment,
especially if done in artificial light, so that these hairs are, like
the root hairs, intended to absorb or suck up water as it passes over
them. Then the Edelweiss and the Lammie's Lug (_Stachys lanata_) are
entirely covered with white cotton-woolly hairs: these are intended
to keep the water in the plant, and do so as effectually as a rough
woollen coat will keep out rain and mist. Silky hairs, downy hairs,
and others are found wrapping up the tiny baby leaves in the bud:
they probably keep them warm, and perplex and ward off objectionable
insects.
But, perhaps, the sticky or glutinous hairs are the most wonderful of
all. They are found on many plants, such as _Salvia glutinosa_,[151]
Plumbago, and Catchfly. One can see insects stuck on them and
vainly struggling to be free, and the hairs undoubtedly prevent
green-fly and other such pests from interfering with the honey of
the flower. In some of these cases it has been shown that the body
of the insect is actually used as food, but that is more obvious
with two interesting plants which specially devote themselves to
the capture of insect prey. One of these is very often kept in the
Boer farmhouses near Tulbagh, in South Africa, simply to attract
the flies, which are a perfect pest in those dry valleys. Another
Drosophyllum, the Fly-Catcher, grows on sandy and rocky ground in
Portugal and Morocco. This is also used by the peasants near Oporto
as a convenient fly-paper.
[151] Macchiati, _Botan. Centralblatt_, 41, 190.
In both of these plants large drops of a sticky, glistening liquid
are secreted by the hairs which cover the leaves. Any small insect
alighting on the latter is sure to get covered by the liquid, and
in trying to get away will become hopelessly involved in it. It is
probably soon suffocated, for the gummy matter will choke the small
air-holes by which it breathes. Both these plants are said to secrete
both an acid and a digestive secretion.
But we have two plants which are even more interesting in this
country.
Walking over the rough marshy pastures or moors of Scotland one is
sure to notice, generally on wet peaty and barren soil, a little
rosette of bright, yellow-green, glistening leaves. If it is the
right season there will be a handsome purple flower whose stalk
springs from them. This is the Butterwort (_Pinguicula_), and it is
not a bad name, for the leaves remind one of butter. The whole upper
surface of the leaves is covered with tiny glands secreting a sticky,
glistening matter. It is said that there will be as many as fifty
thousand of these glands on a square inch of the upper surface.
Now in such places every one knows that there are quantities of
midges, and also that these insects are always exceedingly thirsty.
They prefer blood, it is true, but when they see these bright
yellowish leaves they naturally go to them. When, however, the midge
touches the leaf, the sticky liquid clings to its wings and legs, and
it cannot escape.
So far this does not differ from the Fly Catchers mentioned above,
but another very curious action then begins. If the midge or fly is
near the margin of the leaf, the edge of the latter begins to curl
or roll inwards over it. It does so very slowly, and may not finish
rolling over the insect for some hours. Whilst this is going on acids
and "gastric juice," or ferments which act in the same way, are
being poured over the body of the midge, which is finally completely
digested. Next day, having finished the midge, the leaf majestically
unrolls itself again and waits for another.
The juice contains rennet, and is used by the Lapps in making a
horrible delicacy called Tätmiölk. It has also been used by the Swiss
shepherds for at least two hundred years, to cure sores on cows'
udders.
The other British plant is the Sundew (_Drosera_). Every one who
has been on peat-mosses and moors probably knows its little reddish
rosettes of small rounded or spoon-shaped leaves lying on bare peat
or wet mossy ground. Each leaf seems to be covered by hundreds of
glittering little dewdrops (whence the name).
The hairs or tentacles which cover the leaf secrete this glistening,
sticky fluid. There must be about two hundred of them on a single
leaf.
An insect flying about near the Sundew is sure to be attracted by the
conspicuous glittering, reddish leaves, and probably alights upon it.
Then it finds itself caught and begins to struggle, but this simply
brings it against more tentacles.
Now happens the most wonderful part of the whole performance. All the
neighbouring tentacles, although they have not been touched, bend
over towards the struggling insect and pin it down in the middle of
the leaf. They do not bend over very quickly. In two or three minutes
they will bend over towards it through an angle of forty-five
degrees, and it takes them ten minutes to bend over ninety degrees.
There is something rather horrible in the sight of a large insect
struggling with these slow, remorseless, well-aimed tentacles; most
people free the insect unless, at least, it happens to be a midge.
The point which is so difficult to understand is to know how those
untouched tentacles know that the insect is there and exactly where
it is. There is no doubt that they do know, for they behave exactly
as if they were the arms of a spider.
If you put two insects on either side of the middle of the leaf, half
the tentacles will pin down one and the other half will deal with the
other insect.
At the same time acids and ferments are poured out which digest the
insect. It takes about two days for a leaf to finish off an insect,
and then the tentacles again unclose.
Moreover it is difficult to deceive those tentacles. They will bend
in for the tiniest piece of useful substance; for instance, a length
of one-seventy-fifth of an inch of woman's hair will make them
secrete digestive fluid. One millionth part of a pound of ammonium
phosphate will also produce secretion. But a shower of heavy rain,
grains of sand, or other useless material, will not cause any
secretion, and even if they do bend in a little, they soon discover
their mistake and stand out again. If you try the same experiment
under a bell-glass from which the oxygen has been withdrawn by an
air-pump, nothing happens; or if you chloroform the Sundew it will
pay no attention to small pieces of meat until it recovers from the
effects of the chloroform.
When these Droseras are taken to a greenhouse and experiments are
made on them, they run into very great danger. They are almost
certain to die of overfeeding or indigestion. It is impossible to
keep people from giving them too much to eat.
This wonderful little plant shows quite distinctly that there must
be some way of sending messages in its leaves. Somehow the message
travels from the tentacle which the fly has touched, down the stalk
into the leaf, and up into the other tentacles, and tells them that
there is something worth stooping for.
No one has explained this, and probably no one will ever do so.
The last, and in some ways the most interesting, of all these
carnivorous plants is Venus' Fly-trap (_Dionæa muscipula_), which
grows in North America from Rhode Island to Florida.
It is a quite small herb with a small circle of leaves which lie flat
on the ground. Each leaf ends in a nearly circular piece which is
divided by a very marked midrib. The two semicircular halves have a
series of teeth along their edges; these margin teeth are stiff and a
little bent upwards. In the centre of each half there are three small
hairs. On looking closely at these hairs one finds that each has a
joint near the base; all over the centre of these leaf halves there
are scattered glands which secrete ferments intended to digest any
animal matter.
The really interesting point is connected with these central jointed
or trigger hairs; they are extremely sensitive. But when they are
touched it is not they themselves that are affected, but the entire
circular end of the leaf!
Suppose an insect wanders on to the leaf and reaches one of these
semicircular halves, nothing happens until it touches one of these
hairs, but then _both_ halves suddenly close together, exactly like
an ordinary rat-trap! The teeth on the edges of the halves interlock
like the teeth of a trap, and the insect is caught and imprisoned.
Its body is slowly digested away and goes to nourish the plant. The
use of the joint in the sensitive hairs can be easily perceived, for
when the two halves shut up together, the hairs fold down exactly
like the funnel of a river steamboat when it passes under a bridge.
The closing of the two halves, which has been well compared to
shutting up a half-open book, is very quick, as it does not take more
than ten to thirty seconds. There is an abundant flow of "gastric
juice," but the leaf takes a long time to digest its food. It may
require three weeks to finish one insect. Moreover, if overfed, it
may turn a bilious or dyspeptic yellow colour, and wither or even
die. It only shuts for a short time if a grain of sand touches the
sensitive hair, and, like Drosera, is not deceived in its food.
The Dionæa, Drosera, the Sensitive Plant, Mimulus, Barberry, and
others, all show us clearly that plants somehow or other act as if
they were conscious of what they ought to do. In fact, in all these
cases, it is scarcely possible to help believing in some sort of
rudimentary nervous system. At any rate Wordsworth comes near this
belief, for he has written:--
"It is my faith, that every flower that blows
Enjoys the air it breathes."