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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. |
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