two quaint and rather uncommon flowers among the straggling brushwood.
One of them is growing like a creeper around the branches of this
overblown gorse-bush. It is the lesser dodder, a pretty clustering mass
of tiny pale pink convolvulus blossoms. The stem consists of a long red
thread, twining round and round the gorse, and bursting out here and
there into thick bundles of beautiful bell-shaped flowers. But where
are the leaves? You may trace the red threads through their
labyrinthine windings up and down the supporting gorse-branches all in
vain: there is not a leaf to be seen. As a matter of fact, the dodder
has none. It is one of the most thorough-going parasites in all nature.
Ordinary green-leaved plants live by making starches for themselves out
of the carbonic acid in the air, under the influence of sunlight; but
the dodder simply fastens itself on to another plant, sends down
rootlets or suckers into its veins, and drinks up sap stored with
ready-made starches or other foodstuffs, originally destined by its
host for the supply of its own growing leaves, branches, and blossoms.
It lives upon the gorse just as parasitically as the little green
aphides live upon our rose-bushes. The material which it uses up in
pushing forth its long thread-like stem and clustered bells is so much
dead loss to the unfortunate plant on which it has fixed itself.
Old-fashioned books tell us that the mistletoe is a perfect parasite,
while the dodder is an imperfect one; and I believe almost all
botanists will still repeat the foolish saying to the present day. But
it really shows considerable haziness as to what a true parasite is.
The mistletoe is a plant which has taken, it is true, to growing upon
other trees. Its very viscid berries are useful for attaching the seeds
to the trunk of the oak or the apple; and there it roots itself into
the body of its host. But it soon produces real green leaves of its
own, which contain the ordinary chlorophyll found in other leaves, and
help it to manufacture starch, under the influence of sunlight, on its
own account. It is not, therefore, a complete drag upon the tree which
it infests; for though it takes sap and mineral food from the host, it
supplies itself with carbon, which is after all the important thing for
plant-life. Dodder, however, is a parasite pure and simple. Its seeds
fall originally upon the ground, and there root themselves at first
like those of any other plant. But, as it grows, its long twining stem
begins to curl for support round some other and stouter stalk. If it
stopped there, and then produced leaves of its own, like the
honeysuckle and the clematis, there would be no great harm done: and
the dodder would be but another climbing plant the more in our flora.
However, it soon insidiously repays the support given it by sending
down little bud-like suckers, through which it draws up nourishment
from the gorse or clover on which it lives. Thus it has no need to
develop leaves of its own; and it accordingly employs all its stolen
material in sending forth matted thread-like stems and bunch after
bunch of bright flowers. As these increase and multiply, they at last
succeed in drawing away all the nutriment from the supporting plant,
which finally dies under the constant drain, just as a horse might die
under the attacks of a host of leeches. But this matters little to the
dodder, which has had time to be visited and fertilised by insects, and
to set and ripen its numerous seeds. One species, the greater dodder,
is thus parasitic upon hops and nettles; a second kind twines round
flax; and the third, which I have here under my eyes, mainly confines
its dangerous attentions to gorse, clover, and thyme. All of them are,
of course, deadly enemies to the plants they infest.
How the dodder acquired this curious mode of life it is not difficult
to see. By descent it is a bind-weed, or wild convolvulus, and its
blossoms are in the main miniature convolvulus blossoms still. Now, all
bind-weeds, as everybody knows, are climbing plants, which twine
themselves round stouter stems for mere physical support This is in
itself a half-parasitic habit, because it enables the plant to dispense
with the trouble of making a thick and solid stem for its own use. But
just suppose that any bind-weed, instead of merely twining, were to put
forth here and there little tendrils, something like those of the ivy,
which managed somehow to grow into the bark of the host, and so
naturally graft themselves to its tissues. In that case the plant would
derive nutriment from the stouter stem with no expense to itself, and
it might naturally be expected to grow strong and healthy, and hand
down its peculiarities to its descendants. As the leaves would thus be
rendered needless, they would first become very much reduced in size,
and would finally disappear altogether, according to the universal
custom of unnecessary organs. So we should get at length a leafless
plant, with numerous flowers and seeds, just like the dodder.
Parasites, in fact, whether animal or vegetable, always end by becoming
mere reproductive sacs, mechanisms for the simple elaboration of eggs
or seeds. This is just what has happened to the dodder before me.
The other queer plant here is a broomrape. It consists of a tall,
somewhat faded-looking stem, upright instead of climbing, and covered
with brown or purplish scales in the place of leaves. Its flowers
resemble the scales in colour, and the dead-nettle in shape. It is, in
fact, a parasitic dead-nettle, a trifle less degenerate as yet than the
dodder. This broomrape has acquired somewhat the same habits as the
other plant, only that it fixes itself on the roots of clover or broom,
from which it sucks nutriment by its own root, as the dodder does by
its stem-suckers. Of course it still retains in most particulars its
original characteristics as a dead-nettle; it grows with their upright
stem and their curiously shaped flowers, so specially adapted for
fertilisation by insect visitors. But it has naturally lost its leaves,
for which it has no further use, and it possesses no chlorophyll, as
the mistletoe does. Yet it has not probably been parasitic for as long
a time as the dodder, since it still retains a dwindling trace of its
leaves in the shape of dry purply scales, something like those of young
asparagus shoots. These leaves are now, in all likelihood, actually
undergoing a gradual atrophy, and we may fairly expect that in the
course of a few thousand years they will disappear altogether. At
present, however, they remain very conspicuous by their colour, which
is not green, owing to the absence of chlorophyll, but is due to the
same pigment as that of the blossoms. This generally happens with
parasites, or with that other curious sort of plants known as
saprophytes, which live upon decaying living matter in the mould of
forests. As they need no green leaves, but have often inherited leafy
structures of some sort, in a more or less degenerate condition, from
their self-supporting ancestors, they usually display most beautiful
colours in their stems and scales, and several of them rank amongst our
handsomest hot-house plants. Even the dodder has red stalks. Their only
work in life being to elaborate the materials stolen from their host
into the brilliant pigments used in the petals for attracting insect
fertilisers, they pour this same dye into the stems and scales, which
thus render them still more conspicuous to the insects' eyes. Moreover,
as they use their whole material in producing flowers, many of these
are very large and handsome; one huge Sumatran species has a blossom
which measures three feet across. On the other hand, their seeds are
usually small and very numerous. Thousands of seeds must fall on
unsuitable places, spring up, and waste all their tiny store of
nourishment, find no host at hand on which to fasten themselves, and so
die down for want of food. It is only by producing a few thousand young
plants for every one destined ultimately to survive that dodders and
broomrapes manage to preserve their types at all.
XIV.
_DOG'S MERCURY AND PLANTAIN._
The hedge and bank in Haye Lane are now a perfect tangled mass of
creeping plants, among which I have just picked out a queer little
three-cornered flower, hardly known even to village children, but
christened by our old herbalists 'dog's mercury.' It is an ancient
trick of language to call coarser or larger plants by the specific
title of some smaller or cultivated kind, with the addition of an
animal's name. Thus we have radish and horse-radish, chestnut and
horse-chestnut, rose and dog-rose, parsnip and cow-parsnip, thistle and
sow-thistle. On the same principle, a somewhat similar plant being
known as mercury, this perennial weed becomes dog's mercury. Both, of
course, go back to some imaginary medicinal virtue in the herb which
made it resemble the metal in the eyes of old-fashioned practitioners.
Dog's mercury is one of the oddest English flowers I know. Each blossom
has three small green petals, and either several stamens, or else a
pistil, in the centre. There is nothing particularly remarkable in the
flower being green, for thousands of other flowers are green and we
never notice them as in any way unusual. In fact, we never as a rule
notice green blossoms at all. Yet anybody who picked a piece of dog's
mercury could not fail to be struck by its curious appearance. It does
not in the least resemble the inconspicuous green flowers of the
stinging-nettle, or of most forest trees: it has a very distinct set of
petals which at once impress one with the idea that they ought to be
coloured. And so indeed they ought: for dog's mercury is a degenerate
plant which once possessed a brilliant corolla and was fertilised by
insects, but which has now fallen from its high estate and reverted to
the less advanced mode of fertilisation by the intermediation of the
wind. For some unknown reason or other this species and all its
relations have discovered that they get on better by the latter and
usually more wasteful plan than by the former and usually more
economical one. Hence they have given up producing large bright petals,
because they no longer need to attract the eyes of insects; and they
have also given up the manufacture of honey, which under their new
circumstances would be a mere waste of substance to them. But the dog's
mercury still retains a distinct mark of its earlier insect-attracting
habits in these three diminutive petals. Others of its relations have
lost even these, so that the original floral form is almost completely
obscured in their case. The spurges are familiar English roadside
examples, and their flowers are so completely degraded that even
botanists for a long time mistook their nature and analogies.
The male and female flowers of dog's mercury have taken to living upon
separate plants. Why is this? Well, there was no doubt a time when
every blossom had both stamens and pistil, as dog-roses and buttercups
always have. But when the plant took to wind fertilisation it underwent
a change of structure. The stamens on some blossoms became aborted,
while the pistil became aborted on others. This was necessary in order
to prevent self-fertilisation; for otherwise the pollen of each
blossom, hanging out as it does to the wind, would have been very
liable to fall upon its own pistil. But the present arrangement
obviates any such contingency, by making one plant bear all the male
flowers and another plant all the female ones. Why, again, are the
petals green? I think because dog's mercury would be positively injured
by the visits of insects. It has no honey to offer them, and if they
came to it at all, they would only eat up the pollen itself. Hence I
suspect that those flowers among the mercuries which showed any
tendency to retain the original coloured petals would soon get weeded
out, because insects would eat up all their pollen, thus preventing
them from fertilising others; while those which had green petals would
never be noticed and so would be permitted to fertilise one another
after their new fashion. In fact, when a blossom which has once
depended upon insects for its fertilisation is driven by circumstances
to depend upon the wind, it seems to derive a positive advantage from
losing all those attractive features by which its ancestors formerly
allured the eyes of bees or beetles.
Here, again, on the roadside is a bit of plantain. Everybody knows its
flat rosette of green leaves and its tall spike of grass-like blossom,
with long stamens hanging out to catch the breeze. Now plantain is a
case exactly analogous to dog's mercury. It is an example of a degraded
blossom. Once upon a time it was a sort of distant cousin to the
veronica, that pretty sky-blue speedwell which abounds among the
meadows in June and July. But these particular speedwells gave up
devoting themselves to insects and became adapted for fertilisation by
the wind instead. So you must look close at them to see at all that the
flowering spike is made up of a hundred separate little four-rayed
blossoms, whose pale and faded petals are tucked away out of sight flat
against the stem. Yet their shape and arrangement distinctly recall the
beautiful veronica, and leave one in little doubt as to the origin of
the plant. At the same time a curious device has sprung up which
answers just the same purpose as the separation of the male and female
flowers on the dog's mercury. Each plantain blossom has both stamens
and pistils, but the pistils come to maturity first, and are fertilised
by pollen blown to them from some neighbouring spike. Their feathery
plumes are admirably adapted for catching and utilising any stray
golden grain which happens to pass that way. After the pistils have
faded, the stamens ripen, and hang out at the end of long waving
filaments, so as to discharge all their pollen with effect. On each
spike of blossoms the lower flowerets open first; and so, if you pick a
half-blown spike, you will see that all the stamens are ripe below, and
all the pistils above. Were the opposite arrangement to occur, the
pollen would fall from the stamens to the lower flowers of the same
stalk; but as the pistils below have always been fertilised and
withered before the stamens ripen, there is no chance of any such
accident and its consequent evil results. Thus one can see clearly that
the plantain has become wholly adapted to wind-fertilisation, and as a
natural effect has all but lost its bright-coloured corolla.
Common groundsel is also a case of the same kind; but here the
degradation has not gone nearly so far. I venture to conjecture,
therefore, that groundsel has been embarked for a shorter time upon its
downward course. For evolution is not, as most people seem to fancy, a
thing which used once to take place; it is a process taking place
around us every day, and it must necessarily continue to take place to
the end of all time. By family the groundsel is a daisy; but it has
acquired the strange and somewhat abnormal habit of self-fertilisation,
which in all probability will ultimately lead to its total extinction.
Hence it does not need the assistance of insects; and it has
accordingly never developed or else got rid of the bright outer
ray-florets which may once have attracted them. Its tiny bell-shaped
blossoms still retain their dwarf yellow corollas; but they are almost
hidden by the green cup-like investment of the flower-head, and they
are not conspicuous enough to arrest the attention of the passing
flies. Here, then, we have an example of a plant just beginning to
start on the retrograde path already traversed by the plantain and the
spurges. If we could meet prophetically with a groundsel of some remote
future century, I have little doubt we should find its bell-shaped
petals as completely degraded as those of the plantain in our own day.
The general principle which these cases illustrate is that when flowers
have always been fertilised by the wind, they never have brilliant
corollas; when they acquire the habit of impregnating their kind by the
intervention of insects, they almost always acquire at the same time
alluring colours, perfumes, and honey; and when they have once been so
impregnated, and then revert once more to wind-fertilisation, or become
self-fertilisers, they generally retain some symptoms of their earlier
habits, in the presence of dwarfed and useless petals, sometimes green,
or if not green at least devoid of their former attractive colouring.
Thus every plant bears upon its very face the history of its whole
previous development.
XV.
_BUTTERFLY PSYCHOLOGY._
A small red-and-black butterfly poises statuesque above the purple
blossom of this tall field-thistle. With its long sucker it probes
industriously floret after floret of the crowded head, and extracts
from each its wee drop of buried nectar. As it stands just at present,
the dull outer sides of its four wings are alone displayed, so that it
does not form a conspicuous mark for passing birds; but when it has
drunk up the last drop of honey from the thistle flower, and flits
joyously away to seek another purple mass of the same sort, it will
open its red-spotted vans in the sunlight, and will then show itself
off as one among the prettiest of our native insects. Each thistle-head
consists of some two hundred separate little bell-shaped blossoms,
crowded together for the sake of conspicuousness into a single group,
just as the blossoms of the lilac or the syringa are crowded into
larger though less dense clusters; and, as each separate floret has a
nectary of its own, the bee or butterfly who lights upon the compound
flower-group can busy himself for a minute or two in getting at the
various drops of honey without the necessity for any further change of
position than that of revolving upon his own axis. Hence these
composite flowers are great favourites with all insects whose suckers
are long enough to reach the bottom of their slender tubes.
The butterfly's view of life is doubtless on the whole a cheerful one.
Yet his existence must be something so nearly mechanical that we
probably overrate the amount of enjoyment which he derives from
flitting about so airily among the flowers, and passing his days in the
unbroken amusement of sucking liquid honey. Subjectively viewed, the
butterfly is not a high order of insect; his nervous system does not
show that provision for comparatively spontaneous thought and action
which we find in the more intelligent orders, like the flies, bees,
ants, and wasps. His nerves are all frittered away in little separate
ganglia distributed among the various segments of his body, instead of
being governed by a single great central organ, or brain, whose
business it always is to correlate and co-ordinate complex external
impressions. This shows that the butterfly's movements are almost all
automatic, or simply dependent upon immediate external stimulants: he
has not even that small capacity for deliberation and spontaneous
initiative which belongs to his relation the bee. The freedom of the
will is nothing to him, or extends at best to the amount claimed on
behalf of Buridan's ass: he can just choose which of two equidistant
flowers shall first have the benefit of his attention, and nothing
else. Whatever view we take on the abstract metaphysical question, it
is at least certain that the higher animals can do much more than this.
Their brain is able to correlate a vast number of external impressions,
and to bring them under the influence of endless ideas or experiences,
so as finally to evolve conduct which differs very widely with
different circumstances and different characters. Even though it be
true, as determinists believe (and I reckon myself among them), that
such conduct is the necessary result of a given character and given
circumstances--or, if you will, of a particular set of nervous
structures and a particular set of external stimuli--yet we all know
that it is capable of varying so indefinitely, owing to the complexity
of the structures, as to be practically incalculable. But it is not so
with the butterfly. His whole life is cut out for him beforehand; his
nervous connections are so simple, and correspond so directly with
external stimuli, that we can almost predict with certainty what line
of action he will pursue under any given circumstances. He is, as it
were, but a piece of half-conscious mechanism, answering immediately to
impulses from without, just as the thermometer answers to variations of
temperature, and as the telegraphic indicator answers to each making
and breaking of the electric current.
In early life the future butterfly emerges from the egg as a
caterpillar. At once his many legs begin to move, and the caterpillar
moves forward by their motion. But the mechanism which set them moving
was the nervous system, with its ganglia working the separate legs of
each segment. This movement is probably quite as automatic as the act
of sucking in the new-born infant. The caterpillar walks, it knows not
why, but simply because it has to walk. When it reaches a fit place for
feeding, which differs according to the nature of the particular larva,
it feeds automatically. Certain special external stimulants of sight,
smell, or touch set up the appropriate actions in the mandibles, just
as contact of the lips with an external body sets up sucking in the
infant. All these movements depend upon what we call instinct--that is
to say, organic habits registered in the nervous system of the race.
They have arisen by natural selection alone, because those insects
which duly performed them survived, and those which did not duly
perform them died out. After a considerable span of life spent in
feeding and walking about in search of more food, the caterpillar one
day found itself compelled by an inner monitor to alter its habits.
Why, it knew not; but, just as a tired child sinks to sleep, the gorged
and full-fed caterpillar sank peacefully into a dormant state. Then its
tissues melted one by one into a kind of organic pap, and its outer
skin hardened into a chrysalis. Within that solid case new limbs and
organs began to grow by hereditary impulses. At the same time the form
of the nervous system altered, to suit the higher and freer life for
which the insect was unconsciously preparing itself. Fewer and smaller
ganglia now appeared in the tail segments (since no legs would any
longer be needed there), while more important ones sprang up to govern
the motions of the four wings. But it was in the head that the greatest
changes took place. There, a rudimentary brain made its appearance,
with large optic centres, answering to the far more perfect and
important eyes of the future butterfly. For the flying insect will have
to steer its way through open space, instead of creeping over leaves
and stones; and it will have to suck the honey of flowers, as well as
to choose its fitting mate, all of which demands from it higher and
keener senses than those of the purblind caterpillar. At length one day
the chrysalis bursts asunder, and the insect emerges to view on a
summer morning as a full-fledged and beautiful butterfly.