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TREES are indispensable to science. From physics to physiology, they
serve as metaphors, expressing in a word details that would otherwise occupy
a paragraph. They range from the momentous to the prosaic, from Charles
Darwin’s great Tree of Life to a layout for distributing cable television.
In science, the intellectual landscape is everywhere wooded and in places
a veritable thicket.

With their love of arboreal imagery, scientists are continuing a tradition
that reaches back to the religious beliefs of our remote forebears. In Norse
cosmology, Yggdrasil, a vast evergreen ash, had its roots in hell and touched
heaven with its branches. For North American Indians, a similar specimen
acted as a length of galactic dowel, pinning together the three layers of
the Universe. The ancient Egyptians considered the sky to be the canopy
of a tree and the stars its fruits.

Trees conjure up nature’s fecundity and are heavy with religious associations.
Small wonder then that they figure in many traditional tales about human
origins. A Bolivian myth, for example, asserts that mankind emerged from
a gash cut in a tree by the Creator. A North American story has it that
humans started out as trees but were transformed when a snake bit off their
roots. In Greek legend, Adonis’s mother was a myrtle – making Adonis a scion
of one of the most literal family trees on record.

In a separate strand of tradition, trees have always offered enlightenment
to the seeker after knowledge. Adam and Eve followed this course and were
evicted for their pains. The Buddha fared rather better, but then he sampled
only the shelter of the peepul tree. Zeus spoke to his disciples by rustling
the leaves of an oak tree, while in Celtic lore, a variety of hazelnut would
impart knowledge if ingested. In another Celtic tradition, rowan berries
offered a similar short cut to knowledge. Today’s version of the tale focuses
on Isaac Newton and his dramatic experience beneath the apple tree.

Newton’s wide intellectual interests would have made him familiar with
another symbolic use of trees – in alchemy. In that most esoteric of disciplines,
initiates summarised the methods and results of their work in elaborate
drawings, which can be as hard to penetrate as the corresponding sections
of a modern scientific paper. Trees were part and parcel of those images,
symbolising both the work as a whole and its alleged product, the philosopher’s
stone.

Against this historical background, it is only to be expected that the
image of the tree should be prominent in contemporary science. The finest
specimen is the biologist’s evolutionary tree – of which more later – but
saplings abound in every discipline. The alchemical legacy allows chemists,
for example, to talk of Saturn’s tree, which is a branched mass of lead
deposited from a solution of a lead salt. A similar formation of silver
is called Diana’s tree. Elsewhere, a tree is the name given to a mass of
crystals formed during electroplating.

Not to be outdone, engineers have cultivated groves of their own. Trees
of steel, or Christmas trees, are the assemblies built over oil wells to
regulate the flow of oil. Chemical engineers use a tree pump – named for
its suggestive shape – to transfer hot, corrosive fluids without the aid
of susceptible devices such as valves or pistons. The fluid travels along
the central trunk under the impetus of a second fluid blown into and sucked
from alternate branches.

In electrical circles, ‘treeing’ is a defect of cables, marked by the
appearance of hollow, ramifying channels in insulation. A ‘tree discharge’
is a sort of branching spark. Lightning develops as a tree before becoming
a bolt. Suggestively shaped circuits inside computers are sometimes called
trees. In telecommunications, a tree is a branching array of telephone lines,
with exchanges at the forks and individual handsets as the leaves. Whenever
we make a trunk call to a private branch exchange, we reinforce the imagery.

Tyro statisticians draw tree diagrams to portray all the possible routes
chance may take. Spinning a coin twice, for example, would yield a tree
along the following lines:

For geometers, trees belong to the theory of graphs. The following graph:

has 16 trees, a tree being any path through the graph that takes in
all the corners without creating loops. Examples are:

A collection of such trees is, sensibly enough, a ‘forest’.

Elsewhere in mathematics, realistic trees invariably decorate any discourse
on fractal geometry, the study of objects whose shape repeats itself on
different scales. Trees are prime examples, because sprigs, twigs, branches,
boughs and boles are progressively larger versions of the same basic shape.
Because of their repetitive structure, trees of surprising realism can be
drawn on a computer screen with a minimum of programming fuss. With credentials
like these, the tree looks set to become a symbol for the world of fractals.

Trees figure prominently in computer science. A classic case arises
when programmers instruct a computer to play games. A game tree begins with
the current state of play and depicts the various routes that the game can
take. The tree comes equipped with a root (the current position) and leaves
(the possible outcomes of the game). Programmers refer to ‘bushy trees’
in games such as chess, where there are a large number of options at each
stage. Computers cope with the arduousness of such games by discounting
pointless moves in advance. Pursuing the metaphor, programmers call this
‘t°ù±ð±ð-±è°ù³Ü²Ô¾±²Ô²µâ€™.

Students of linguistics must also acquire a talent for silviculture.
For them, a ‘phrase structure tree’ is a diagram explaining the anatomy
of a sentence. The tree grows progressively as they dissect the sentence
into its elements. Analysing the first sentence of this paragraph would
yield the following tree:

In anatomy proper, almost any branching structure qualifies as a tree.
Inside our bodies, blood courses through vascular trees, a biliary tree
delivers bile to the intestine and a spinal tree grows in the backbone.
We harbour a tree in the brain, the arbor vitae, or tree of life, which
is the pattern revealed by a vertical section of the cerebellum. Sea cucumbers
breathe by drawing oxygen-rich water into their respiratory trees. When
cells are producing an abundance of ribosomal RNA, their chromosomes take
on the look of Christmas trees bedecked with molecular tinsel. What with
dendrons, dendrites and dendritic ulcers, biology is thick with metaphorical
trees.

Nowhere is this more true than in the area of the subject dealing with
classification and evolution. The Tree ofPorphyry, an invention of the 3rd
century AD, is arguably themost ancient example. Porphyry was a Middle Eastern
philososopher who popularised the work of other writers and wrote several
books, including one about vegetarianism. His tree is a device for specifying
mankind’s place in the schemeof things. It works a little like the keys
that help biologiststo identify animals and plants. Starting at the top
of the tree,the first stage is to divide all of creation into corporealand
incorporeal. Subsequent divisions split the corporealinto animate and inanimate,
the animate into sensitiveand insensitive and, finally, the sensitive into
rational andirrational. Man – a rational, sensitive living body – then pops
out at the bottom of the tree.

The tree’s most illustrious role in biology, as a symbol of both natural
diversity and evolutionary change, emerged much later. Its emergence marked
a major shift in biological thinking, and an intriguing episode in the history
of science. Previously, biologists had been enamoured of ladders, chains
and other linear metaphors. A typical example was the ancient scale of nature
with its ascent from sponges to human beings along a single line of ‘increasing
perfection’. Again, it was a linear image, with every grade of life in its
proper place, that inspired Alexander Pope’s lines:

Vast chane of Being, which from God began, Natures aethereal, human,
angel, man, Beast, bird, fish, insect! what no eye can see, No glass can
reach! from Infinite to thee, From thee to Nothing! ….

Given this perspective, it would have been hard to imagine anything
more subversive than the tree, where forms fanned out in all directions
and could hold equal rank. Yet in the end the tree was triumphant because
it offered a better description of the natural world. Among the scholars
who have chronicled its rise are Edward Voss, who has traced the history
of evolutionary trees, and Dov Ospovat, the author of a book about the development
of Darwin’s theory. As their work makes clear, the branching tree was gaining
ground long before Darwin used it to such devastating effect in the Origin
of Species. A century earlier, it was appearing in the most unexpected quarters.

The work of Charles Bonnet, the 18th-century Swiss biologist, is a case
in point. Bonnet was a key proponent of the scale of nature, yet his writings
confirm that he toyed with the idea of a branching scale. ‘Does the scale
of nature become branched as it arises? Are the insects and the shell-fish
two parallel and lateral branches of this great trunk?’ he asked in 1764.
The power of the symbolic tree was already evident at what is widely considered
its first outing.

Jean Baptiste de Monet, Chevalier de Lamarck, went further out on a
limb. He illustrated his theory of evolution with a diagram showing the
origins of the major groups of animals. Despite being the wrong way up for
a tree, its true nature can scarcely be in dispute. ‘The table,’ he wrote
in 1809, ‘may facilitate the understanding of what I have said. It is there
shown that in my opinion the animal scale begins by at least two separate
branches, and that as it proceeds it appears to terminate in several twigs
in certain places.’

Studies on embryos reinforced the arboreal imagery. Zoologists such
as Karl von Baer and Henri Milne Edwards observed that related animals developed
along similar pathsas embryos before diverging to go their own separateways.
Again the analogy was with a branching tree. Because embryonic development
took this form, adultanimals could be arranged in a similar pattern. Accordingto
Milne Edwards, writing in 1844, this pattern was ‘a treewhich in rising
from the ground separates into several stemseach of which then divides into
secondary main branchesand terminates in innumerable little branches’.

An appreciation of the importance of embryology led Martin Barry, a
Scottish physician, to take to the trees in 1837. ‘Naturalists have begun,
just where they should have ended,’ he lamented. ‘Their attention has been
directed to the grouping of the twigs – as if thus they were to find their
natural connections, without even looking for assistance towards the branches,
or the trunk that gave them forth.’ By revealing similarities in development,
Barry argued, naturalists could start to work from the trunk upwards.

In the same year, Darwin first sketched a simple evolutionary tree in
his notebook (left). ‘Organised beings represent a tree, irregularly branched,’
he wrote, ‘some branches far more branched – Hence Genera – As many terminal
buds dying, as new ones generated.’ In the following 20 years, Darwin skilfully
orchestrated the image, bringing it to the peak of perfection in Chapter
Four of the Origin of Species: ‘The affinities of all the beings of the
same class have sometimes been represented by a great tree. I believe this
simile largely speaks the truth. The green and budding twigs may represent
existing species; and those produced during each former year may represent
the long succession of extinct species.’ There follow 25 lines of sustained
metaphor, before the final flourish: ‘As buds give rise by growth to fresh
buds, and these, if vigorous, branch out and overtop on all sides many a
feebler branch, so by generation I believe it has been with the great Tree
of Life, which fills with its dead and broken branches the crust of the
earth, and covers the surface with its ever branching and beautiful ramifications.’

Darwin’s image was so apt, so effective and so elegant that its durability
was assured. It summarised the past in a single phrase, vividly portraying
how living things had diverged and adapted to a variety of habitats. At
the same time, the tree evoked all manner of associations, none of which
can have hindered its appeal. Darwin’s audience would already have been
familiar with genealogical trees. In church, they might have gazed on a
Jesse window, depicting in stained glass the genealogy of Christ as a great
tree ascending from Jesse’s loins. The capital letters in the phrase Tree
of Life must also have made it resonate with religious overtones, all the
way from the Book of Genesis to the pagan tradition.

Where Darwin led, others were quick to follow, soon providing an evolutionary
arboretum. The most notable was Ernst Haeckel, a vigorous Darwinian who
depicted his ideas on evolution with the help of some extraordinarily lifelike
trees. He executed one specimen with such artistry that we find ourselves
searching its boughs for a sleeping owl orplayful squirrel.

Biologists soon worked out ways to increase the scientific utility of
the tree. They drew their diagrams to scale so that the disposition of branches
revealed not only the ancestry of the various groups, but the time at which
they had diverged from common stock. More abstract layouts allowed further
refinements. In one design, the angle each branch made withthe vertical
was a measure of the rate of evolution withinthat lineage.

In more recent times, the tendency has been to shun naturalistic trees.
Today’s trees, or dendrograms, hide their vegetable origins behind sensible
straight lines, more like electronic circuitry than burgeoning foliage.
Yet symmetry and grace have a habit of re-emerging. A couple of years ago,
Rebecca Cann, Mark Stoneking and Allan Wilson at the University of California
at Berkeley, drew up an evolutionary tree based upon the chemical structure
of the DNA inside human mitochondria. The result is an immensely satisfying
design. Curving around like a Norman arch, it signals a triumphant return
to the tradition of elegant imagery.

At the same time, biologists are always looking for further refinements
to their most treasured metaphor. Derek Bryce, in a book entitled Evolution
and the New Phylogeny, argues that the conventional Tree of Life needs reappraisal.
The traditional oak or ash, with its all-too-definite forks, disguises our
ignorance about the actual course of evolution. Bryce contends that a ‘monkey
puzzle with the trunk and mostof the horizontal branches missing’ would
make a moresuitable image.

Whatever species suits us best, a tree remains the most apt way of arranging
most living organisms, because they really did evolve by diverging from
common ancestors. Yet the tree does have its limitations, according to John
Maynard Smith of the University of Sussex and Manfred Eigen and his colleagues
at the Max Planck Institute for Biophysical Chemistry in Gottingen. One
limitation arises with bacteria that exchange genes across the barriers
of species. In their case, separate evolutionary trees can coalesce into
an evolutionary thicket. One way of viewing the resulting tangle is to think
of it as a net, with the knots signalling points of genetic contact. Another
exception crops up with certain molecules – the various RNA species in living
cells, for example. These branched from common stock so long ago that we
can barely perceive their relationships with one another. Here, we are better
off thinking of a fan-shaped bundle of sticks, rather than a branching tree.
Eigen and his colleagues have devised mathematical ways of deciding which
image – tree, net or bundle – offers the best fit in any particular case.

Arresting as these ideas are, they offer no real challenge to the overall
supremacy of the evolutionary tree. The theory of evolution is unthinkable
without trees. Elsewhere within science, afforestation continues apace.
If trees did not exist, scientists would have to invent them.

Stephen Young is a science writer who lives in a well-wooded part of
Wales.