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Fragile Foliage

In Chapter 10 of A Walk Through The Woods: Rediscovering America on the Appalachian Trail author Bill Bryson opens my eyes to something I've seen almost every day, but never observed—a tree:

For all its mass, a tree is a remarkably delicate thing. All of its internal life exists within three paper-thin layers of tissue—the phloem, xylem, and cambium—just beneath the bark, which together form a moist sleeve around the dead heartwood. However tall it grows, a tree is just a few pounds of living cells thinly spread between roots and leaves. These three diligent layers of cells perform all the intricate science and engineering needed to keep a tree alive, and the efficiency with which they do it is one of the wonders of life. Without noise or fuss, every tree in a forest lifts massive volumes of water—several hundred gallons in the case of a large tree on a hot day—from its roots to its leaves, where it is returned to the atmosphere. Imagine the din and commotion, the clutter of machinery, that would be needed for a fire department to raise a similar volume of water.

And lifting water is just one of the many jobs that the phloem, xylem, and cambium perform. They also manufacture lignin and cellulose; regulate the storage and production of tanning, sap, gum, oils, and resins; dole out minerals and nutrients; convert starches into sugars for future growth (which is where maple syrup comes into the picture); and goodness knows what else. But because all this is happening in such a thin layer, it also leaves the tree terribly vulnerable to invasive organisms. To combat this, trees have formed elaborate defense mechanisms. The reason a rubber tree seeps latex when cut is that this is its way of saying to insects and other organisms, "Not tasty. Nothing here for you. Go away." Trees can also deter destructive creatures like caterpillars by flooding their leaves with tannin, which makes the leaves less tasty and so inclines the caterpillars to look elsewhere. When infestations are particularly severe, some trees can even communicate the fact. Some species of oak release a chemical that tells other oaks in the vicinity that an attack is under way. In response, the neighboring oaks step up their tannin production the better to withstand the coming onslaught.

By such means, of course, does nature tick along. The problem arises when a tree encounters an attacker for which evolution has left it unprepared, ...

(cf. A Walk in the Woods (2008-08-17), ...) - ^z - 2008-08-25


(correlates: Joan Benoit Samuelson on the Marathon, Divine Hieroglyphics, Mooning Jane Eyre, ...)