Water evaporates from the surfaces of open bodies of water, and from the land. We have all seen “steam” rising from lakes and roadways, but the majority rises invisible, except as a component of “haze”. And vast amounts more stream skyward from the billions of open stomata of plants, which use water as both structure and conveyance.

Sessile organisms by human classification, plants still move, not just swayed by wind, but actually covering distance by growing, and by sending out roots or rhizomes in search of water or new territory. They can recreate themselves by the same processes, as well as by seeds, which are carried elsewhere by wind and animals, and yes, water. By movements both passive and self-actuated, plants spread outwards and upwards, going everywhere they can, dying on inhospitable substrates, but thriving where they meet the minimal requirements of soil (or that which they can convert to soil), sunlight and water. They take on every form from single-celled algae to huge, buoyant kelp beds in water, and from moss to massive firs on land.

Water or other plants may even supplant soil in many instances, leaving some plants dependent on just two elements, light and water. Plants actively seek the solar energy they absorb, and a plant’s use of water is neither passive nor static. Take water away, and plants wilt, their leaves, or even their whole structure sagging earthwards as internal pressures drop. Trees, “woody” plants, and many grasses have enough of a skeleton of lignin to stand aloft a time beyond death, but without water coursing through them, no plant remains “alive”, never mind upright. Crystal clear structural blood, with no heart to pump it, just the willing complicity of the properties of the blood itself.

“Stoma” means “mouth” in Greek, “stomata” small mouths. With a microcope one would see millions of them on the undersides of most leaves. Hormonally controlled pressures in the cells that thicken to form the “lips” of these tiny openings swell them shut, relax them open. And through these stomata a plant “breathes”. When open, water evaporates into the air, carrying both the fire of all carbon-based life, oxygen, as well as this crystal blood, an invisible vapor off to become cloud.

And a structural miracle keeps the cycle flowing. The cohesive forces of water rival that of steel in the right conditions, and those conditions are embodied in a plant’s vascular sytem, in particular the xylem. Thousands of towers of tiny tubes with closed end walls set at what a carpenter would call a scarf-joint angle extend root to leaftip. Those end walls are permeable to water, and they assure the chain links are not too long and never have an opportunity to break. In the reverse of the desktop ball-bearing swing toy, as one molecule leaves, the capillary force draws another up, and the cohesion of that chain extends to the roots, where water is drawn from that collected by the ambitious, patient root system. The pressure of this flow keeps many plants upright, and its release contributes mightily to the clouds through which I so recently raced.

This tense but graceful exploitation of the physics of water takes place in a war zone.  Outwardly trees embody patient peace to the humans who read poetry under their branches, photograph their fall colors.  But no Shakespearean plot that might be read under any tree’s cool shade ever had as many eager, rightful heirs held under such a delicately balanced tyranny as a tree.  It seems “right” that plants use gravity as a reference to balance their quest for the sky.  Sunflowers track the motion of the sun across the sky with shifts in water pressure and seem beatific in their serene, glacially-paced cycle of sun-worship.  But most plants -- and especially trees -- have many parts, all just as intent on success as their nearest siblings.  A slow-motion race is in progress, the combatants of similar ability, but over time unequally armed with chemical weaponry.




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