PLANTS AND ANTS
Meaning of Plant Life--Captive and domesticated germs--Solomon's
observations denied by Buffon but confirmed by recent
writers--Ants as keepers and germinators of corn--Ant
fields--Ants growing mushrooms--Leaf-cutting ants--Plants
which are guarded by insects--The African bush--Ants boarded
by Acacias and by Imbauba trees--Ants kept in China and
Italy--Cockchafer _v._ ant--Scale insects--A fungus which
catches worms.
The world of plants supports all animal life, from the mite to the
elephant. There are most intricate relations between one form of life
and another. Thus a Rose tree attacked by an aphis or green-fly may
be succoured by the slim ichneumon, or other thin-waisted fly, which
lays its egg in that of the aphis. Another insect, say a spider,
catches the ichneumon. A starling may eat the spider, and be itself
eaten by an owl.
So that ichneumon and starling are friends to the Rose, whilst the
other insect, the spider, and the owl are enemies. Yet both the
starling and the spider are probably, almost certainly on the whole,
friends of the Rose, although they are unfriendly in this special
case.
With all other similar series or changes the final term is either a
bird or animal of prey or mankind.
Until we introduce the idea of man as the culminating point of the
series, the whole of it seems to be without any special meaning or
advantage.
But when we think of how man utilizes the work of plants and animals,
then the whole scheme becomes intelligible and complete; it is like a
well-rounded story with a worthy and adequate end.
Moreover, what man has done so far is only an instalment of what he
will probably succeed in doing. All who have brought up caterpillars
or bees know that their greatest difficulty arises from certain
minute insects or fungus enemies. We already know enough about these
latter to fight them with some chance of success, but there are
hundreds of other spores and germs floating in the atmosphere, and
coming to rest on animals, on clothing, or on the leaves or petals
of plants. These germs are now just as wild as, and infinitely more
dangerous than the furious aurochs, the disdainful wild asses, or the
ferocious wolves that our forefathers succeeded in domesticating.
Those bacteria, or germs, for instance, which are only one-thousandth
of a millimetre long, are only visible by the help of a microscope.
A row of three hundred thousand of them would be required to make
an inch in length! Yet one of these germs can be mature and divide
into two new germs in twenty minutes. In forty minutes there would be
four, in an hour eight, and so on. The number after twenty-four hours
is almost incredible.
These little germs stick to our clothes, fingers, lips, money,
newspapers, and anything that is often handled. They hover in the
air we breathe, permeate the food we eat, and inhabit water, and
especially milk, in enormous numbers. Some of them are deadly. One
might easily decimate a whole population, as indeed happened in the
South Sea Islands when smallpox was introduced. Others are harmless
and even necessary.
But to-day if you go into a bacteriological laboratory you will find
hundreds and thousands of little glass tubes all neatly labelled and
stoppered with cotton wool. If you read those labels you will see
that the bacteria of all sorts of horrible and loathsome diseases
have been captured and imprisoned. There is the deadly anthrax
bacillus peacefully discolouring gelatine; in another, possibly
the germs of hydrophobia may be undergoing a process of taming or
treatment.
Each of these colonies of germs is under perfect control, and in
many of them their natural wickedness has been so much alleviated
that they are now useful aids to the doctor, who gives his patient a
mild dose of the disease in order to accustom his system to resist
accidental infection by the original type.
Yet what has been done already is only an earnest of what will no
doubt be accomplished. Every farmer and ploughboy will in time sow
his own bacteria; every dairymaid will make all sorts of cheese, from
Camembert, Rochfort, to Gorgonzola, by sowing the right kind of germ
upon it.
Man will no doubt cultivate the whole earth in the way that he now
cultivates Europe and Great Britain, and will obtain mastery not only
over his domesticated plants and animals, but over fungi, bacteria,
and insects also.
Even if man had never risen above the state of the Banderlog of Mr.
Kipling, there are other animals which cultivate and even combine
together for warfare and conquest. In some respects they are better
disciplined even than man himself, and they can defy all sorts of
mankind except civilized man.
Possibly if man had not arisen on the scene, these insects might
have developed some sort of civilization like that imagined by Mr.
Wells in his story of the moon. We are only concerned with the
relations of these ants to plants. Those who are interested in their
conquests and civilization must consult the excellent account by Mr.
Selous in his _Romance of the Insect World_.
The most interesting points about them are as follows. They gather
a harvest and store it up for the winter. This habit of the ant was
well known to the ancients, and is mentioned by Solomon. At the
time of the French Encyclopædists, when the fashion of the times
was all for destruction and disbelief, the fact that ants do so
was ridiculed and flatly contradicted, and especially by the great
naturalist Buffon. They pointed out that ants hibernated during the
cold weather, and therefore required no food for the winter, so that
Solomon's story was absolutely ridiculous.
For nearly a hundred years people forgot that Palestine and those
other countries where the habits of ants had been reverently observed
possessed a climate much too warm and mild to make the ants hibernate.
After careful study it has been discovered that the ants thoroughly
understand the first stages of brewing!
The corn which they gather is not eaten by them in its hard winter
condition. When taken into the winter nest of the ants this corn
would very soon germinate and grow into a plant, but the ants manage
to prevent this by some method which is not yet understood. If such a
nest is left alone by the ants, the corn immediately begins to grow,
but it is not allowed to do so till it is required for food. Should
the store of corn get damp by heavy rain, or mould appear upon it,
then the careful ants bring up their store into the sunlight and dry
it there.
When it is required for food germination is permitted, but is soon
stopped: the ants nibble off the growing rootlet of the seed. Then
when the grain absorbs water and begins to change its starch into
sugar, the ants suck in the sugar and reap the reward of all this
labour and skill.
In the conduct of this germination of the grain they are, of course,
far in advance of all the savage races of mankind.
There are certain South American species which go at least one step
farther. They have their own fields--spaces three or four feet in
diameter--which are entirely occupied by one single grass, the
so-called Ant-rice (_Aristida stricta_). Dr. Lincecum states that the
ants "work" these plantations very carefully, removing every weed or
other plant that comes up, and sowing every year the new seed at the
proper season.[129]
[129] _Proceedings Linnean Society_, 1861. Dr. MacCook adds
nothing essential, and in no way disproves Dr. Lincecum's
statements.
These facts are sufficiently strange and startling, but there are
even, apparently, species still more intelligent, who not only sow
and reap, but actually prepare a soil and reap a crop of mushrooms,
or at least, if not of mushrooms, of fungi. These wonderful little
insects gather leaves and cut them into fragments of an appropriate
size; they are then collected together so as to form a bed, and the
fungus is introduced to this. The fungus is kept at a certain stage
of growth by very careful treatment; the fruit-bearing ends are
nibbled off, so that the young shoots come up indefinitely. The ants
feed upon these fungus shoots, and get a crop indefinitely prolonged.
This is, of course, a system of agriculture far beyond that employed
by any tribe of savages. Only man in a relatively advanced stage of
agriculture grows mushrooms for himself. These facts, startling as
they may seem, are apparently quite well authenticated and have not
been seriously questioned.
There are a great number of leaf-cutting ants who are, indeed,
amongst the most dangerous of the many insect pests in South America
and elsewhere. Wallace (_Revue Scientifique_, 179, p. 29), in
speaking of the Saauba or leaf-cutters, describes how he placed a
large heavy branch across the route of one of their columns.
The long line of laden ants was checked, and the greatest confusion
set in at the head of the column. Each ant, for several feet down the
column, then laid down its leaf, and all set to work to tunnel under
the obstacle. This was managed in about half an hour's time, and the
column then proceeded on its way.
Amongst other interesting and curious facts connected with these
extraordinary insects is that some kinds are actually kept up by
certain plants as a sort of standing army or police.
There are no less than 3030 species of plant which keep these
standing armies of ferocious ants, or if they do not keep them, at
any rate lay themselves out to attract them. The kinds which are
attracted live upon sugar, and are strong, active, and extremely
good fighters. When travelling through the bush in Africa, it is not
unusual in some places to touch inadvertently one of these protected
trees. In a moment one's hand and arm are covered by ants whose heads
are dug deep down into the skin, biting with all their strength.
It is of course impossible to describe all the plants which protect
themselves against injurious insects and even large animals in this
way, but two of them must be mentioned.
There are certain Acacias which are particularly interesting. Like
most of this order, they have large hollow spines instead of
stipules at the base of the leaf. It is inside these spines that the
troops of the police-insects live. These Acacias (Oxhorn Acacia,
as well as _A. sphærocephala_ and _A. spadicigera_) also produce
_sugar_, which is secreted by peculiar gland-like organs on the
stalks of the leaves, and even _albuminoids_, for at the tips of the
leaflets there are peculiar little bodies which contain albuminous
matter.
The Imbauba tree (_Cecropia spp._) also possesses a standing army of
these ants. It puts them up in the hollow pith in the centre of the
tree, which is divided into large roomy spaces and makes a convenient
nest. There is a minute opening by which they run in and out. On one
occasion a naturalist found that the ants had been benumbed by a
period of very cold weather, and in consequence had neglected their
duty, and the trees had been stripped of their leaves by leaf-cutting
kinds.[130]
[130] Belt, _Naturalist in Nicaragua_.
These last mentioned, the leaf-cutting ants, are especially dreaded
by owners of plantations. Foreign or introduced plants are not
specially guarded against their ravages by special secretions, as
is the case with the native flora, so that the coffee and cocoa
plantations are often severely injured. In some places man has copied
those Acacias and Imbaubas, for in the orange plantations of the
province of Canton, in China, ants' nests are collected and placed on
the trees. Moreover, the different trees are connected together by
bamboos, so that the ants can easily pass, as on a bridge, from one
tree to another.
Near Mantua, in Italy, the same system seems to be adopted, and
ants' nests are carefully placed near the fruit trees. Their use
can be quite well understood, for Forel, in his work on the Ants of
Switzerland, estimates that _one ants'_ _nest will require_ a supply
of 100,000 insects a day during the season.
It is quite common to find ants crawling about on the outside of the
large heads of the Garden Centaury and a few other Composites. If one
looks carefully, one finds that there are streaks of honey to be seen
coming from the scales. The honey is not produced in the flowers,
and seems at first sight to be of no use at all so far as the plant
is concerned, but that is very far from being the case. Here comes
a cockchafer or other destructive beetle, intent on absolutely
devouring and destroying the young flowers. At once the pugnacity and
wrath of the ants are aroused. They take up a menacing and ferocious
attitude, and the cockchafer passes to some other plant.[131]
[131] Kerner, _l.c._, vol. 2, fig. 264, p. 242.
Such honey-glands found on the leaves and not connected in any way
with the flowers, are more common than one would think. Even the
common Bracken produces curious honey-secreting hairs when it is in a
young condition. These attract ants which drive away caterpillars and
other dangerous insect foes.
Many very dangerous insects are too small for birds, and can only be
dealt with effectually by insects or fungi. Of these perhaps the most
dangerous are the "scale" insects. The best-known one is very like a
minute mussel shell. It is about one-quarter to one-third of an inch
long, and can be sometimes found in quantities on apples; they are
generally collected round the stalk. The mother insect has this scaly
back, and lies down and dies on the top of her eggs, so that her
scaly corpse forms a roof and a shield for her young ones. Like all
pests of this sort, these creatures increase very rapidly.
A certain scale insect was doing an immense amount of harm in the
orange plantations of Fiji, but it was destroyed by the introduction
of lady-birds, and of a certain parasitic fly. It is said that these
insects destroyed the "scale" in six months!
Experiments have also been tried with fungi. There are certain fungi
which attack the bodies of living insects. So far, however, it cannot
be said that the results have been at all satisfactory, for the
propagation and infection of the living insects by fungus spores is
not at all easy. There is also a certain feeling of doubt as to what
may happen. Those fungi, and particularly bacteria, might set up
dangerous epidemics.
Decaying meal contains hundreds of certain very curious worms called
_Nematodes_. They are short, about one-twenty-fifth of an inch in
length, and are smooth and very like minute eels. These creatures
are very active, wriggling or swaying to and fro in a characteristic
manner. Now in decaying meal there is a peculiar fungus. Like most
fungi, it consists of very minute transparent threads which contain
living matter or protoplasm. This particular fungus has branches,
but also forms curious loops or belts. When one of these eel-worms
is swaying about in the meal, it may quite well happen that its tail
slips into one of these loops. If that happens, the fate of the worm
is sealed, for the loop is elastic, and the more it wriggles the
farther it slips in and the stronger it is held. The fungus then
begins to grow, and forms a tube which grows _into the worm_ and
kills it. All the material in the worm's body goes to nourish the
fungus. This extraordinary fungus has been described and figured by
Professor Zopf, but seems to be a very unusual and rare form.