Geology Reference
In-Depth Information
It is high summer, and in a small patch of English woodland near my house an oak
leaf far above the ground inhales a molecule of carbon dioxide. The molecule's heady
ten-year adventure through the atmosphere is over. Soon, it enters a bright green solar
reactor—a chloroplast—within one of the leaf's many photosynthetic cells. Here, en-
ergy skilfully harvested from sunlight is used to fuse the molecule's carbon atom to five
others, making a ring-shaped sugar molecule which stores a goodly amount of solar en-
ergy in the thrumming quantum entanglements of its chemical bonds.
Slowly, our freshly-forged sugar molecule glides into the tree's plumbing system,
joining an abundant stream of sugary, energy-rich sap that spreads out from the leaves to
feed the entire tree. Having flowed through a vein in the leaf, our sugar molecule passes
into a thicker tube in one of the tree's main branches, and then into one of the large con-
duits just below the bark of the main trunk that channel vast torrents of sap towards the
roots. Feeling itself drawn ever downwards, the sugar molecule travels in darkness, eas-
ing itself through microscopic sieve-like holes connecting specialised cells, flow-chan-
nels for the nutritious sappy juices. Incapable of thought, the sugar molecule can have
no inkling of where it is. But we who track it with such care know that it has reached
deep underground and that it flows unerringly towards the finest tip of one of the tree's
root hairs in the loamy soil some few metres from the towering trunk above.
If it could anticipate, which of course it cannot, our sugar molecule would know that
its journey, and indeed its life, will soon end in dismemberment within a root-tip cell
ready to extract the energy carried so dutifully from the leaves in the faraway world of
the sunlit, windswept canopy. But a different fate awaits. For instead of dissolution, the
sugar molecule experiences a peculiar change of scene as its sugary, energy-rich, ring-
shaped body is shunted from the pipes of the tree into those of a creature so alien to
our human imaginations that there is still much to learn about its peculiar subterranean
ways of life. The new being in which our sugar molecule now resides is of course a
fungus—a vital partner of the plant that combines with it to form a 'fungus root', or 'my-
corrhiza'. This fungal accessory to the root's adventure spreads itself secretively through
the soil by means of an extensive network of microscopic, protoplasm-filled tubes, some
of which, as we have seen, enter into deep and intimate contact with the plant cells.
No doubt, if it were aware of its circumstances, our sugar molecule would find this
change of plumbing rather disconcerting, for the tree's pipes were large and straight with
no branches, but those of the mycorrhiza are narrower and bend in all directions. Carried
far away from the tree's roots by the flow of fluid in the fungal pipeline, our sweet prot-
agonist passes through an endless labyrinth of interlinked hollow tubes just under the
surface of the soil leading seemingly nowhere in particular on a magical mystery tour
without destination.
