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The Summer Woods and the Long Vacation: June
As every Boy Scout knows, the chapter on woodcraft is the longest in the Manual, and tells, among other things, how to know the trees and the birds and all the wild life of the woods; and about how to make a fire without matches, and how to make sure that no hidden sparks are left behind when you break camp.
The woods, indeed, may almost be said to have invented the Scouts; for the woods and woodland life are the very heart and centre of that wonderful boy organization. Boys who enjoy the woods -- and what boy ever lived who didn't enjoy them? -- have always found endless entertainment and instruction in the most delightful of Mother Nature's schools; particularly during the summer sessions, when the schools-within-the-walls have closed for the long vacation. I. THIS AND THAT AND A SHADY TREE Who but a boy would think of spending his time on a summer day lying on his back under a shady tree studying the different styles of dances among the leaf people?
One boy, I remember, who did that delighted an audience of several thousand people with the story of it when he got back to his pulpit in Brooklyn in the autumn, while the rest of the nation "listened in," as it were, through the newspaper reports of it; for this was the famous Henry Ward Beecher. Like most great men, he never wholly outgrew his boyishness nor his boyhood delight in the woods and their ways. This is what he said about the movements of leaves: Different species of trees move their leaves very differently. On the Tulip Tree, the Aspen, and all native Poplars each leaf moves to suit itself. Under the same wind, one is trilling up and down, another is whirling, another slowly vibrating right and left, while others are quieting themselves to sleep. Sometimes other trees have single frisky leaves, but usually the Oaks, Maples, and Beeches have community of interest; they are all active together or all alike still.
AND, SOMETIMES, TREES MOVE THEIR OWN LEAVES!
Of course we understand that trees can't move their leaves in the sense that we move our fingers or that Mr. Squirrel there is making inquiring gestures at us with his tail: "Well, just what are you fellows doing there under our family tree?" Yet they do say that while trees, as individuals, can't make their leaves dance, yet, by acting together, they often manage it; particularly on stewy days like some of those in June, when occasional little zephyrs are so welcome. It's this way: On hot days, open spaces in the woods and places like that meadow yonder, with the big, bare boulders on the hillside, become heated many times faster than the tree leaves; for 50 to 70 per cent of a leaf is water, and, as your study of physics will tell you, more heat is required to raise the temperature of a pound of water one degree than a pound of almost any other substance. So, while the rocks and the unshaded lands roundabout, and particularly slopes of barren land from which the fertile top soil has been washed away, are heating the air above them, the forest cover will still be absorbing heat and keeping the air below it cool. Leaves also cool the air by the evaporation of the water which is constantly coming up to them from the roots. In the same way the air is cooled by the evaporation from the surface of any running stream, or the pond; or, in a larger way, the sea. Moreover, trees require heat to "cook their meals"; that is to say, the heat of the sun is required to cause the soil water and its salts, and the carbonic-acid gas in the air which the leaves "breathe" in, to change into the sugars and starches on which trees grow.
Well, the upshot of it all is that this cooler air of the forest, being heavier than the warmer air of the sunny places, falls toward the ground and flows off toward the open country; while, from the open country, warmer currents flow in from above to take its place. After sunset the process is reversed because the open spaces, unshaded by the forest, cool quickest, and this heavier air flows into the forest while the warmer and lighter night air of the forest flows out.
The leaves "ketch `em acomin' and ketch `em agoin'" -- these home-made breezes -- and so set themselves to dancing, as it were!
OTHER "SELF-STARTERS" IN LEAFLAND Some leaves, when the sun is too hot to be agreeable, "move into the shade," by the simple process of assuming a vertical position. They thus avoid exposing so much surface to the sun's rays. Other trees, the Common Locust, for example, make the very same movement with their leaves at nightfall, just when those evening primroses over there will be opening their petals. If we're around this way this evening we'll notice that the primroses do this with a jerk, as if saying: "Mercy me, look what time it's getting to be! We've overslept!" Of course such "sleep movements," as they have been called, have nothing to do with either going to sleep or waking up; so say the scientists. "But then, Mr. Scientist, what are they up to, these leaves and petals, when they act like that?"
To which Mr. Scientist replies, in his large language, "The exact relation of these movements to external conditions is complex," and goes on to say that the upright position assumed by leaves in the hot sun -- as if taking a siesta or after-dinner nap -- may be to protect them from drying out, and that those that do the same thing at night are trying to retain as much of their day heat as possible and so escape dangerous chills or frost-bites. The scientist, one of the greatest in the history of science, who made the suggestion last mentioned kept clover leaves open on cold autumn nights and they were severely damaged, while those in the natural "sleep" position were not. You see, these boys who keep up their boyish prying into the why of things and thus become eminent men, "guess" at answers, as the rest of us boys are always doing, but with this difference: instead of guessing at random, they base their guesses on all the facts they can get hold of and then put their guesses to the test of experiment.
THE LITTLE AUTOMATIC PUMPS UNDER THE GROUND As a schoolboy I always envied that engaging youth who used to wander through the woods, as we've been doing in this chapter, and then about lunch-time stretch himself out in a good, shady spot like this: "There at the foot of yonder nodding beech That wreathes its old, fantastic roots so high, His listless length at noontide he would stretch And pore upon the brook that babbled by."
Apparently he didn't have anything else to do but wander about and pore on brooks and things -- being a poet born. Not being so born I've had to be content to enjoy poetry, instead of writing it, and I have found that I get more out of it in proportion as I mix it with science; for poetry is essentially science, after all -- a searching into the heart of things. As an example, take "those old fantastic roots so high." Science tells us why they come above ground, under such conditions as the poet describes -- to buttress the big tree against the tugging of the winds; (This has been said to be especially true of the Beeches, on account of the great mass of their leaves and branches offering so much purchase to the wind; but you can see it, more or less, in any big tree, and particularly in trees in exposed positions.) and also why, having done this patriotic duty, the roots dive back into the ground as quickly as they can, fairly hugging their way around other roots or boulders or whatever may happen to be in the way! This, as we know, is because their real business is to help supply the rest of the tree with water, and they are trying to get back on their regular job as soon as possible! But the active work, the work which the huge pumps at the city water-works do, is performed by the tiny root-hairs that never come above ground under any circumstances. The big roots are chiefly water-mains; although they do absorb water to a limited extent.
These root-hairs have their self-starting machinery, too, and the exact nature of its operation is fully as mysterious as that of the "sleep movements" of the leaves. In some way these roots act as little pumps, in the sense that they take in water from the soil and pass it on for the service of the rest of the tree, only keeping, as I have already said, enough to supply their own modest needs.
And the more the tree needs water, after a long dry spell, the faster the little pumps pump! They immediately rise to the situation; although not so literally as their big brothers rose above ground to help the trunk and branches wrestling with the winds. As the water passes from the root-hairs into the "main," it creates some kind of suction force, as to the nature of which the authorities are not agreed. But this force, whatever it is, does the business -- it draws in the water; and this water, passing on, creates more suction. The drier the soil, the more rapidly what little water there is in it must be collected and passed on. This automatically sets the "pumps" to going faster and faster until, if the dryness keeps up, they can't pump any more, their cell walls have shrunk so. Then they die, die at their posts, as it were, and with them the whole tree.
DO TREES INCREASE RAINFALL?
Now if we could find that forests increase rainfall, as has frequently been asserted, we'd have an automatic and endless chain reforesting service for a fact! More trees to make more rain to make more trees to make more rain! It wouldn't take long to restore all the lost millions of acres of goodly trees that we now need so badly. But, unfortunately for the larger claims in this direction, we know, from the pages in the front part of our geographies, that the causes of rain are, for the most part, wholly beyond the influence of the forest. These causes are the great currents of warm and cold water in the ocean, the presence or absence of mountain ranges, and the direction of the prevailing winds.
Nevertheless, careful observation and records show that forests do affect rainfall to some extent, owing to their colder and moister air and the resistance which they offer to the movement of the winds. Forest air, when sufficiently moist and sufficiently cool, will precipitate its moisture as rain, or coming in contact with warmer, moisture-laden winds will cause this moisture to fall as rain. The effect of a forest in checking moisture-laden winds is something like that of mountain ranges, although, in the case of the mountain ranges, there is an additional element. Warm, moist winds striking a mountain are forced up the incline and, owing to the fact that air expands as it rises, are thereby cooled and so cause rainfall.
THE WOODS AS WEATHER-MAKERS But when we come to speak of weather in broader terms, that is when we speak of the climate of a given region, the case for the trees is much stronger. What we call "climate" is the average condition of the weather; and this depends, not only on the distance of the region from the equator, its elevation above the sea, the distribution of land and water, the relief of the land -- as flat or hilly or mountainous -- and the direction of the prevailing winds, but, to an important degree, to the extent of its woodlands. We have already seen the several ways in which the woods not only cool the air within their own borders but actually create breezes. Woods also, as we saw in the March chapter, ward off bad weather when it comes riding in on the winds -- the biting blasts of winter and the hot, dry blasts of summer. The trees are all the more successful in this because they are such skilful wrestlers. They don't oppose the winds too stiffly, but "give in" a little, and "sidestep" their onrushes. They do this by the elastic swaying of the twigs and branches and, in the case of younger trees, the swaying of the trunks also. Whatever limitations there are in the abilities of the trees to produce rain, there are none when it comes to taking care of it after it has fallen. What with the leaves and the mould and the undergrowth that make up the forest floor, forests have been known to hold a rainfall of five inches; and most of this rainfall, after meeting the "current expenses" of the trees and the requirements of a conservative "reserve system," in the little water reservoir cells, is returned to the springs and "the brooks that babble by," and so passes on to the rivers and the sea, where it again being its endless round of service.
II. THE OLD HOME TREES AND THE FAMILY DOCTOR In this and a hundred other ways you can count on it that the trees will repay any kindness you do to them -- provided you don't wait too long before doing it!