Precursors of Spring


The day began wet and gray and dreary, and now a snowfall is coating the shrubs and lawn and outlining the bare branches of trees. It looks like any other day in winter.

But there's a difference.

Unnoticed buds of the sweet gum trees are swelling and pushing out the dangling seed pod balls that have hung brown and motionless since last fall. The ground beneath these trees is strewn with the balls, like dark, sculptured Christmas tree ornaments.

Buds of the beech saplings are stretching too, detaching a few parchment-thin leaves and sending them downward to meld their chemicals back into the earth. These leaves have been hanging on for months, becoming paler and more translucent in their passage through winter. Lower branches of the great oaks now also are relinquishing their weathered leaves. Unlike most deciduous trees that have dropped their seeds and leaves by late fall, these three species of trees cling all winter long to a few shriveled brown remnants of last year.

Other trees seem to look the same way they've looked all winter long -- somber, motionless, silent studies of gray and brown trunks and branches. But inside boils a turmoil of activity: The sap is rising!

In a wondrously complex universe inside the tree, sugary juices are beginning to creep up through columns of specialized cells in a region called phloem around the tree's heartwood. From deep inside the earth, water from the moist soil is absorbed into thousands of tiny root hairs and, pushed by root pressure, begins rising up the trunk against the force of gravity.

How does the tree do this? Where is its pump to lift sap some 50 to 60 feet high? Several phenomena work together. Capillary action, for one. Water tends to stick to most surfaces, and in a small tube or narrow cell, it creeps up on the sides.

Cohesion is another physiological process: Water molecules are naturally attracted to one another, stick together, and pull one another through the continuous column of cells from the roots to the tips of branches. Osmosis is at work here too, moving the water across membranes from cell to cell, from roots through the trunk, branches and twigs, to the top.

Thus, silently, effortlessly, with no moving parts, the tree is an efficient machine that pumps, pushes and pulls the sugar-saturated liquid through tiny pipes of the living wood, centimeter by centimeter. Could human beings, with all our knowledge and computer technology, invent such a remarkable system of bringing renewed life to a plant?

Meanwhile, squirrels take advantage of this silent, invisible activity and gnaw at the burgeoning buds to drink the sweet sap. Last week, after a grape arbor was trimmed, the sap oozed out from the clipped branches, dripping sweet clear water like white blood from a severed finger. A cardinal stopped by to drink. Farther north, sugar maples are being hung with buckets to tap the rising syrupy sap.

Through the branches, up and over the limbs, in spirals up and down the trunks, squirrels chase each other in their pre-spring caper of courtship. Starlings are poking around beneath the eaves, throwing leaves from the gutter, and flying back and forth carrying twigs for nesting materials. Their frenetic activity is accompanied by coarse squawks that suddenly crack and break into a clear whistle -- precursors of sweet spring songs from birds now winging up from the tropics.

The day may seem like a winter day -- but it is the end of winter.

Barbara Tufty is a science and nature writer and editor.

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