PLANTS WHICH PREY ON PLANTS
The kinds of cannibals--Bacteria--Spring flowers--Pale,
ghostly Wood-flowers--Their alliance with fungi--Gooseberries
growing on trees--Orchid-hunting--The life of an orchid--The
mistletoe--Balder the Beautiful--Druids--Mistletoe as a
remedy--Its parasitic roots--The trees it prefers--The _Cactus
Loranthus_--Yellow Rattle and Eyebright, or Milk-thief, and
their root-suckers--Broomrape and toothwort--Their colour and
tastes--The scales of the toothwort which catch animalcula--Sir
Stamford Raffles--A flower a yard across--The Dodder--Its
twining stem and sucker-roots--Parasites rare, degenerate and
dangerously situated.
The word _cannibal_ is often used in a very loose and unscientific
way. Amongst some savage tribes it is the custom to eat old people
and young children; but this is only in seasons of famine and
scarcity, when there is no other food available, and not because
they are specially fond of them. But amongst other tribes wars are
made for the special purpose of capturing fat young people to cook.
Sometimes they have become so accustomed to such delicacies that they
are unable to get their food in any other way. Of course, when tribes
become "pure cannibals" of this last type they have to be destroyed
like wild beasts.
Among plants we find all sorts of transitions and degrees of
cannibalism. There are plants which sometimes, and, as it were,
accidentally, attack others. But there are also real cannibal plants
which live entirely on the life-juices and sap of other plants,
and cannot exist by their own labours at all. Moreover, we can find
almost every conceivable state of transition. These can be clearly
and definitely traced from those plants which depend on the labour of
their own roots and leaves to others which have no leaves, and which
consist merely of one large flower and a large adhesive sucker fixed
on some one else's root.
The difficulty is very often to know where to draw the line. Probably
no flowering plant is quite independent of the labour and work of
its neighbours. As we have tried to show in another chapter, a long
preliminary cultivation by bacteria, lichens, and mosses is required
before flowering plants can develop on bare rock. That is also
necessary in all cases where the soil is mineral or _inorganic_,
without any _organic_ dust or fragments of vegetable or animal
matter. Bacteria must always begin the work by preparing nitrates and
other salts.
So that only those bacteria which weather rocks can be called really
free and independent. But other bacteria, such as those which cause
typhoid, anthrax, hydrophobia, etc., are the best possible examples
of pure cannibals, or, as they are usually described, parasites.
This last word is derived from a peculiar class of people in ancient
classical times, who used to appear whenever a meal was going to
begin, and received food without giving anything in return. They are
represented by our tramps or by the "sundowners" in Australia, who
appear as soon as the evening meal is ready and when there is no
possibility of going any further on their journey.
The way in which plants became parasites or cannibals is a very
interesting part of plant life, and we shall try to trace some of the
various stages.
To begin with, if one looks out for them in spring one is sure
to find a whole series of beautiful spring flowers. There is the
Primrose, with its bright, hardy, yellow flowers; the Violet, whose
strong perfume much annoys the huntsman, for it spoils the "scent"
and shows him that the end of winter has come; the delicate little
Moschatel, the Lesser Celandine, the Bluebell or Hyacinth, Dog's
Mercury, the Male and the Lady Fern, and many others.
Most of these begin to grow and are in flower early in the season.
That is because they are living on the _dead leaves_ of the last
year, or rather of two or three years ago. Their roots are breaking
up and devouring, with the help of worms, beetles, and insects, the
leaf-mould of past seasons.
They are quite dependent on the trees; they cannot exist except where
such leaf-mould is formed.
But it is very difficult to tell whether these humble little herbs
which live on the scraps that fall from the tall trees are either
parasites or clients, which last do some good in return for their
share.
Probably they are distinctly useful and good for the forest if this
is considered as a whole _establishment_. They use light which would
otherwise be wasted, and their own dead leaves increase the annual
deposit of leaf-mould.
There are other plants, such as the Bird's-nest (_Neottia_) and
Coralroot[143] Orchids, as well as Monotropa and others, which
also live on the rich, decaying leaf-mould of forests, but these
are generally pale in colour, for they possess but little green
chlorophyll. They are more directly dependent on the mould and have
ceased to do much work for themselves. Most of them in fact have
entered into an alliance with fungi, and use these fungi to get their
food material from the dead leaves.
[143] In the first, the entangled underground stems and roots
resemble a bird's nest; in the second, the peculiar red rhizomes
are rather like coral.
Such fungi are always abundant in good, well-grown forests (see p.
86). These Orchids and Monotropa have their roots and underground
stems covered and wrapped round by the fungus threads, which extend
from them in every direction, breaking up and decomposing the dead
leaves.
The colour of Monotropa is a pale waxen yellow, that of the others
is usually a ghostly pale, opalescent, steel-blue or coral-like hue,
which makes them very distinct in the dim, mysterious shades of the
forest.
These plants are undoubtedly of use, for they break up and decompose
the leaf-mould.
Another very interesting group are not well represented in this
country. Sometimes one may see on an old tree a Gooseberry bush in
full foliage quite high up the trunk in the fork of the branches.
In sheltered woody ravines, Polypody ferns are often established on
old moss-clad branches, where their green fronds hang over to catch
as much as they can of the sunlight. But Orchids, Bromeliads, and
Ferns which grow upon the branches of great trees are one of the
most conspicuous and beautiful features of tropical woods. It is for
these tree-orchids that the orchid-hunter braves the head-hunters of
Borneo or traverses the precipices and rugged forests of Guatemala
and Brazil. It is often necessary to cut down a tall tree in order
to get the orchids in its higher branches. Often, however, this is
unsuccessful, for the tree is so held up by creepers and other giants
of the forest that it never reaches the ground!
[Illustration: _Photo_ _Skeen and Co._
CINNAMON PEELING IN CEYLON]
Then, after being stripped from the branches, in some
out-of-the-way forest-clad range of Burma, Celebes, South America,
or Madagascar, these orchids are dried, put up in crates and packed
off to London, where they are carefully cultivated in hot-houses and
persuaded to flower. They may be worth sixpence or they may be worth
£500 each, but no one can tell until they have flowered in London.
But the romance of the orchid-hunter is not exactly what we have to
describe here. It is rather the romance of the life of the orchid
itself.
It is perched high up on the branches of the tallest trees in the
forest, exposed to sun, exposed to wind, and quite unable to gather
either salts or rain from the soil. How, then, does it manage to live?
These orchids, it must be remembered, are only found in
out-of-the-way and feverish, unhealthy places, where the aboriginal
savages still lurk and endure a dreadful existence of hunger and
starvation in dense tropical forests.
Now the word "dense" explains the whole story. Those forests are so
thick, so full of giant trees and exuberant growth, that civilized
man even to-day in 1906 can make nothing of them, and leaves them to
the savage. The reason why vegetation is so luxuriant is simply that
there are both plentiful moisture and a hot, tropical sun. That makes
the life of the orchid possible, and also ensures malaria for the
hunter.
It hangs out into the moist air long pendulous roots which act as so
many sponges absorbing and soaking in moisture. The tremendous energy
of growth covers bark and branches with creeping plants innumerable,
with a profusion of moss, liverworts, and ferns such as we cannot
imagine from our own experiences in this country. So the roots of our
orchid find on the branches rich leaf-mould, and it lives happily
and contentedly on the salts and moisture accumulated by the mosses
and other plants. Its leaves are fleshy and succulent, rather like
those of a desert plant, so that it can store up water against a
season of drought.
These plants which grow in this way on other plants, do not, as a
rule, greatly injure them, but many have not stopped at this stage.
Take, for instance, the Gooseberry growing in the fork of an old
tree. Some bird has been eating gooseberries and dropped the seed
there. The roots of the gooseberry will grow down into the rotten
part of the trunk. Earth and leaf-mould will accumulate there, and it
is quite probable that the whole inside of the tree will decay away.
The roots of the gooseberry will, if only indirectly, help in this
decay.
But it is far otherwise with another set of plants--the Mistletoe and
its allies. There is plenty of romance connected with the mistletoe.
Dr. M. T. Masters says as follows: "The origin of the modern custom
connected with mistletoe is not very clear. Like many other customs,
its original significance is only guessed at. If known, perhaps, the
innocent merriment now associated with the plant would be exchanged
for a feeling of stern disapproval, and the mistletoe would be
banished from our homes. In such a case ignorance is bliss."
It will be remembered that all the gods of Iceland were once gathered
together so that a general oath might be exacted of every plant
"that grew upon the earth," that they would do no harm to Balder the
Beautiful. The Mistletoe did not take the oath, because it does not
grow upon the earth but upon a tree. Then the enemy fashioned an
arrow out of the mistletoe, and killed Balder. There is a modern
idea that the story is a myth representing the death of Spring, for a
great many similar stories occur in widely distant places.
However, it seems pretty certain that the plant was a sacred one to
the Druids in the time of the Romans.
Ovid speaks of this in the line, "Ad Viscum Druidæ cantare solebant."
At their solemn meetings, which were held in remote sacred groves,
a Druid clad in white robes cut the mistletoe with a golden sickle.
Then, apparently, human sacrifices were offered and a general
festival took place.
Some remnant of this custom seems to have persisted in Herefordshire
until recent times, for the tune "Hey derry down, down down derry"
(which means _in a circle move we round the oak_) is supposed to be a
relic of the hymn chanted by the Druids when they had found mistletoe
on the oak.
It was said in the Middle Ages to be a useful cure for apoplexy,
madness, and giddiness. That is not at present the general view.
Indeed, under present conditions it might conceivably promote the
last and even the second of these disorders, though in an agreeable
way!
The Mistletoe and its allies, Loranthus and Arceuthobium, grow upon
the branches of trees like the orchids and gooseberries already
mentioned, but they differ altogether in having a special kind of
absorbing root which sinks down into the bark until it reaches the
wood of the "host" tree. The sap running up the tree is then tapped
by this root, and goes to supply the mistletoe with water and salts
in solution. It has, however, its own green leaves. Thrushes eat the
berries of the mistletoe; they will be left upon a branch with the
_guano_; as the latter dries up, the seed is drawn to the underside
of the branch, and sticks in a crack or crevice; it then sends the
sinker-root mentioned above into the branch.
Every year afterwards new mistletoe "roots" are formed which grow
through the soft part of the bark and send down sinkers into the
wood. Cases of Mistletoes forty years old have been recorded. The
trees which they prefer are the Apple, and after that Black Poplar,
though mistletoe may be found on Silver Fir, various Pines, and
others. It is more difficult to get it to grow on the Oak than on any
other tree. Indeed, only seven cases of mistletoe growing on oak have
been recorded in this country.[144] It is quite a valuable crop in
some places, and is sent in tons to the London market.
[144] Dr. Bull, _Journal of Botany_, vol. 2, p. 273.
There are many species of Mistletoe, and at least one kind attacks,
and is parasitic upon, another species of Mistletoe.
Most Mistletoes and Loranthus have their own green leaves, and only
take from the plant to which they are attached sap and mineral salts.
But in Chile there is a beautiful Loranthus that has practically
no green leaves at all. Its blood-red flowers grow in dense masses
upon the giant Cactus, which is common on the drier hills, and
these are always mistaken for the Cactus's own flowers, which are
quite different. These almost leafless Loranthus, and the curious
Arceuthobium are more parasitic than ordinary mistletoes, for they
obviously take other food material (probably sugar and albuminoids)
from their "host."
Another series of parasites or cannibals are quite common in Great
Britain. One often sees in some meadow that the grasses are growing
in a scanty and unhealthy manner; one then notices amongst them
numbers of the Yellow Rattle or the Eyebright (which the Germans call
_Milk-thief_). These plants are not very remarkable in any way, but
if one examines them closely one sees that the leaves and stems are
more purplish-red than is at all usual with our ordinary flowering
plants. But if you dig up some specimens very carefully, then the
wickedness of the Yellow Rattle and Eyebright becomes apparent;
every here and there upon their roots are little whitish swellings
which are firmly attached to the roots of other plants (generally of
grasses). These two robber plants send from these swellings minute
sucker-roots which pierce into the grass-root and intercept the water
which the grass has been absorbing for itself.
They are therefore parasites, and indeed they may cause a
considerable loss of forage in a meadow.
A good many other British plants are root thieves. Besides these
two, there are the Cow-wheat, Red Rattles, Toadflax, Broomrapes, and
Toothwort.
A curious point about them is that they differ amongst themselves in
the degree in which they are dependent on the work of others. Some
are able to grow quite well without any such extraneous help, but the
Broomrape and Toothwort are entirely dependent on others' labours.
They have extremely little chlorophyll and very small leaves, and are
clearly parasites "pure and simple."
There are about 180 species of Broomrape (_Orobanche_). All of them
attack roots, and most confine their attentions to one particular
flowering plant. Their colours are generally very striking and
unusual. Our British species are reddish, flesh-coloured, or dirty
white, but some of the foreign kinds are blue or violet, yellow, or
yellowish to dark brown. Generally the seedling Broomrape worms its
way down into the earth till its root-tip touches the root of its
special favourite host, then the root of the Broomrape fixes itself
for life; its suckers grow into the host and absorb all the food
material which it requires. Those kinds which attack Tobacco and Hemp
are dangerous pests and do considerable damage.
The Toothwort (_Lathraea_) is so called because its scales have a
sort of resemblance to human teeth. With the curious superstition
which prevailed in medieval times, it was supposed that the
plant must be a remedy for toothache because it resembled teeth.
Unfortunately this is not the case.
It is, generally, quite like the Broomrape in its method of growth,
but it sends out long thread-like branching roots with suckers on the
ends, which become fastened on the Hazel roots. For several years
the plant remains underground and forms very odd-looking, white,
scaly branches. These scales are rolled back in such a way as to form
peculiar and irregular cavities which open to the outside near the
tip of the leaf. There is no doubt that animalcula of sorts get into
these cavities and probably die there. In that case, their remains
will form a useful supplement to the diet of the plant. The following
remarks, however, taken from Kerner have been disputed by other
botanists.
Certain of the cells lining these cavities "appear to send out
delicate filaments.
"When small animals penetrate into the labyrinthine chambers of a
Lathraea leaf and touch the organs just described, the protoplasmic
filaments are protruded and lay themselves upon the intruders.
They act as prehensile arms in holding the smaller prey, chiefly
Infusoria, and impede the motion of larger animals so as to cut off
their retreat. No special secretion has been observed to be exuded in
the foliar chambers of Lathraea. But seeing that some time after the
creatures have entered the chambers, the only remains of them that
one meets with are claws, legs, bristles, and little amorphous lumps,
their sarcode-flesh and blood having vanished and left no trace, we
must suppose that the absorption of nutriment from the dead prey here
ensues...."[145]
[145] Kerner and Oliver, _Natural History of Plants_, vol. 1, p.
136.
But strange as these Broomrapes and Toothworts may be, they are
quite inconspicuous as compared with the gigantic parasites found in
Sumatra and Java.
In 1818, when Sir Stamford Raffles was making a tour in the interior
of Sumatra, his party came across one of those extraordinary plants
which have been called after him.
Imagine a gigantic flower in shape resembling a very fleshy
forget-me-not, but more than a yard across! The colour is a livid,
fleshy tint, and the smell is like that of a charnel-house. This
extraordinary _Rafflesia Arnoldii_ is the biggest flower in the
world. It has no proper stems or leaves, but consists merely of this
huge flower-bud attached to the roots of Figs, etc., which traverse
the ground in these forests. It is said to be only found in places
frequented by elephants, which are supposed to carry its seeds on
their feet.
There are four other kinds known: all of them occur in Sumatra, Java,
and other neighbouring islands. _R. Padma_ for example, has a flower
about eighteen inches across. The central part is a dirty blood-red,
while the lobes have almost the colour of the human skin. This also
has a "cadaverous smell, anything but pleasant."
These weird Rafflesias seated on the roots "which wind about on the
dark forest ground" have impressed every observer.
Yet if one glances back, it is interesting to see how insensible
are the transitional steps which lead from independent life by the
plant's own exertions to these last "pure parasites," which are
entirely dependent on other plants for everything that they require.
The only other flowering plant which we shall mention in this chapter
is now fortunately very rare in Great Britain. This is the Dodder,
_Cuscuta_. It belongs to the Convolvulus or Bindweed order, but is
entirely different from the rest of the family. Some climbing plants
do throttle or choke the trunks of young trees if they twine round
them too closely, but the Dodder has an entirely special and peculiar
way of supporting itself to the detriment of others. It has no roots,
no leaves, and scarcely any green chlorophyll; the Dodder is just a
twining, thread-like, yellowish stem which carries here and there
small round clusters of little convolvulus-like flowers. Wherever the
Dodder thread twines round a hop or other plant, it puts out small
suckers which drive their way into the stem of the hop and take from
it all the food which the Dodder requires. When well developed it
forms dense yellowish tangles of intricately entwined threads, which
may cover whole bushes and entirely destroy the supporting plants.
The Flax, Clover, and Hop Dodders are perhaps the worst of them all.
There are some rather interesting points in the history of the tiny
dodder-seedling. It remains, quietly waiting, for about a month after
most other plants have germinated.
Then it begins to grow rapidly: its tip pierces the soil and becomes
fixed in it; then the rest of the little thread-like seedling begins
to curve round or revolve. If it touches a grass or even a nettle
stem, it twines itself or coils round it, drives in its suckers, and,
on the strength of the nourishment which it extracts, it goes on
revolving or turning until it forms the dense tangled masses referred
to.
Then an eruption of flowers appears, from which later on hundreds of
tiny seeds are let loose which will become Dodders in their turn.
The series of parasitic plants which have now been mentioned form a
very interesting set. It must be pointed out that those which live
merely on dead vegetable matter are "good" plants. They help on the
quick and thorough employment of worn-out material.
Nor can we say off-hand that other parasites are "bad." They do
kill other plants and do them harm, but then, are they not like a
cattle-breeder who sends his inferior cattle to the butcher, keeping
only those which are the very best of their kind? Perhaps these
plants, by destroying the weak and unhealthy kinds, are doing a great
deal of good.
Another interesting point about such parasites is that they are
generally _rare_. They must be less common than their "host." Yet
another is that they are all "degenerates." They show distinct traces
of decay and bad development in their flowers and seed. That is also
true in the case of parasitic animals.
Whether they do good or harm to the world of plants is doubtful, but
there is no doubt that they are doing harm to their own chances!