Look at Leaves - Donald Culross Peattie (1957)

Now once again the spring hangs out those lovely tapestries with which leaves furnish this, our planet home. It wouldn't be home without them; the love of them lies deep in the human heart.

It is an old trick of mine to think about leaves when I cannot sleep. I let my mind go first to the great oak outside my window, with its half-acre or so of leaf surface, all of it doing the tree, and me, a silent good. I listen to far-away foliage I have known, to the high seething of the silken needles of pines above a woodland cottage, or to the heavy rustle of a chestnut-tree. And I hear again the stiff rattle of palm leaves in the trade-wind on a tropic shore.

Take a leaf take any leaf and look at it closely. You will see that the two sides are unlike: the upper surface is darker, often glossy and waxen; the underside paler, sometimes with a pro-tective coating of down. Because it is thus two-faced a leaf can perform its two separate functions: respiration on the lower surface, work with the sun on the upper. Trees must breathe good sweet oxygen to keep alive. It's oxygen, entering into a man's blood, that kindles the fires of human energy. So with a leaf. It too must take in oxygen in order to release, from the sugars and starches packed away in it as stored food, the energy to expand upon the summer air and so to lift, by the power of growth, a sapling into a forest giant.

A leaf breathes through the pores on its shel-tered underside-so many and minute that they average about a hundred to an area the size of the loop in the letter P on this page. These pores are usually slit-shaped and respond to atmospheric changes. On hot, dry days, lest the leaf wilt by water loss through evaporation, its pores may almost close-but not completely or it would smother. When the pores open their widest, the leaf, and so the whole tree, breathes easier.

The pores of a leaf, even at the top of a tall tree, help to bring water up from the roots deep in earth. Evaporation at the pores causes a partial vacuum within the cells, and this suctionlike effect is communicated from cell to cell back through the leaf stalks and twigs, along the boughs, down the trunk. Boosted by root pressure from below, thread-fine columns of water are sucked up, like lemonade through dead against gravity, a straw. And this goes on, to the wind-tossed crown of a great oak or maple a hundred feet up and more,

Meanwhile, on the upper side of the leaf, the side exposed to the sunlight, world is going on. For aeons before this atomic a primal work of the age the green leaf has been using solar energy to power the greatest industrial plant on earth. No wheel turns in it, no smoke pollutes the air round it; instead, leaves purify the atmosphere. This foliage factory-which first, of course, serves the tree itself, thus giving us timber, pulp and plastics -uses for machinery the green stuff in the leaf called chlorophyll. And with it the leaf is able to capture part of that tremendous cascade of atomic energy which falls upon our planet from the sun.

As each tiny particle of solar energy (called a photon) collides with the green in the leaf, the energy leaps to the chlorophyll, setting it aglow. With this energy the chlorophyll smashes open the molecules of the water and carbon dioxide which the leaf has taken in through its pores from the air, and silently reassembles them into new patterns constituting sugars and starches, the basic foods in the leaf. Since it is with the energy from photons that the leaf synthesizes its foods, the whole elaborate but speedy process is called photosynthesis. Throughout the sunlit hours in every part of the world every leaf on every tree is doing this work.

No wonder chlorophyll has been called the green blood of the world! It is carried in minute green discs which, like the corpuscles in our own blood, can move about almost as if they led a life of their own. When the sunshine is too strong they can turn edge-on, or sink, or flee to the sides of the cells. When the skies grow grey, they may do a half-roll and turn broadside to make the most of the light, or rise to the top of the cell, like fish coming up in cloudy weather to bite.

And leaves help to provide us with the very breath of life. For when the leaf by photosynthesis breaks up those molecules of water and carbon dioxide into their elements, there is a lot of oxygen left over that the leaf itself doesn't use. This it breathes out through its pores, in such quantities that all our air is wonderfully freshened. When factory chimneys pour deadly gases into the sky, the oxygen exhaled by leaves purifies the polluted air. The winds of the world, for ever storming round our spinning globe, thoroughly mix and distribute the leaf-breath.

Without that gentle exhalation all animal life on earth would, like a candle lowered into a well full of carbon dioxide, long ago have flickered out. Thus the man who has a fine old shade tree over his roof lives under a sort of oxygen tent. More-over, the foliage not only tempers the wind and shuts out the glare but somewhat air-conditions his house. For the air round leaves is faintly cooled by the evaporation from them, just as a lake or river makes the neighbourhood cooler. You feel this sudden, delicious coolness when on a hot day you enter into a wood.

So, summer-long, a green, serene benediction is upon us. In autumn every leaf seems to have put on new colour. Not so; the reds and yellows are the natural pigments of certain foods stored by leaves which are merely masked by chlorophyll in the summer. We see orange in autumn foliage when red shines through yellow, and mauve when red begins to change chemically. Frost has no-thing to do with it. It is the leaves themselves which end their own lives in this blaze of glory.

Each leaf produces a growth of callous cells at the base of its stalk; this cuts off the water supply and makes a tear-line, like the perforations on a sheet of stamps, so that any breeze may pull the leaf off, or it may fall of its own weight. In the end, it will turn to mould, enriching earth; or, raked into some bonfire, may rise again in a last blue twirl of pungent smoke.