The How and Why Library/Life/Plants-Section VIII
VIII. Why Plants are Like Squirrels
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Did you ever see a squirrel gathering acorns and nuts in the autumn? All summer long squirrels eat their food as they find it, bring up their babies, grow fat themselves and play a great deal. But when the first frost sends the nuts rattling to the ground, they know winter is coming. So they lay away a good store of food, in some safe place, to last them through "hard times." Wise little brother of the tree tops! How busy he is, and how hard he works!
Plants are just like squirrels. They eat and grow all summer, feed their flower babies until they are ripe seeds, then they store food to last them until the Spring. It is not so easy to catch plants at work, as it is to catch squirrels, but if you have very sharp eyes and minds you can do it. All plants above the fungi, earn their own living. In all green plants the roots get food from the earth, and the leaves get food from the air. The two kinds of food come together in the leaves, and the sun mixes and changes them into plant cells.
In the story about water you learned that water never runs up hill. Then how does water get from far down in the ground to the top of a tree? It doesn't run up; it is pumped up. Get a basin of water. Hold your handkerchief so just the hem on one side of it is in the water. That becomes wet at once. Hold it there. See the water climb, thread by thread! In a little while the handkerchief is wet to the top. You know a wet piece of cloth dries rapidly in the sun. As the water in the handkerchief passes into the air as vapor, more water is drawn from the basin. After awhile it is all soaked up. The basin is empty, and soon afterwards the handkerchief is dry.
This drawing of water up by threads, is called capillary attraction. A lump of sugar has it. Hold a lump of sugar with one tiny corner of it just touching the top of a cup of coffee. Soon the whole lump is brown and wet. A plant is like a big handkerchief full of threads that run from the root hairs to the leaves. The sun draws the water, in vapor, from the leaves, and more water is pulled up just as long as the roots can find any in the earth. Those little wood-fibers that youfound in bundles in the stems of fern leaves, are not only bones, they are blood vessels, too.
Those little tubes are so small that they cannot carry anything but liquid food. The sap of trees looks like clear water, but it has a great many things melted in it. The sap of maple trees has sugar. Some saps are puckery, some spicy. In the earth are many things that melt in water. Water will take up and hold, salt, sugar, lime, iron and many minerals. When clear well water is boiled in a teakettle, it coats the inside of the kettle with lime. If you melt salt in water and then put it in the sun, the water will pass away as vapor, but the salt will be left in the glass.
Minerals will not burn. If you burn wood you have a little heap of ashes left. The ashes are the minerals that were in the wood. Plants do not like rain-water as they do well-water. They must have water that has gone down into the earth and taken up minerals. That is the reason why plants are so made that they get all their water through the roots.
You might think that plants drink the rain that falls on their leaves and stems. They don't. They merely use rain to wash their faces. They need to wash the dust out of their skin pores, just as you do. Ask them if this is not true. Leaves will talk, as they are supposed to do in fairy stories, if you know their language. This is one way to ask leaves if they drink through their leaf-pores, or through their roots.
Take a leafy branch. Lay it across the mouth of a jar of water, so some of the leaves dip into the water. In another jar put the stems of leaves in the water. These stay fresh several days, and drink the water, as you can see by the smaller amount in the jar. The others soon wilt and wither, and do not use the water in the jar.
The work of the leaves is to do the air-breathing for the plants. They do it just as you do, through lungs. Their lungs are more like the pores of your skin. There are little open mouths at the ends and crossings of the little tubes that come up from the roots. In net-veined leaves, like the rose and apple, they are scattered over the under surface. They open little mouths, breathe out the vapor of the water from the roots, and breathe in the air.
The leaves are little plant-food factories. In them they have water and minerals from the roots, and oxygen, nitrogen and carbon-dioxide from the air. Oxygen is a purifier. We use oxygen to purify the blood in our lungs. Carbon is the wood-fiber maker. Itis that solid part of a plant that makes a bright fire. Coal is nearly all carbon, and coal you know was made of plants pressed to a kind of stone. Nitrogen is a plant food. The roots get some of it out of the earth, and the leaves get some out of the air. Nitrogen is what is left when the oxygen is burned out of the air. (See Air.)
A leaf is very thin. The sun can shine right through it, for the cell walls or skin is as transparent as glass. In some way sunlight mixes with the water and minerals from the roots and the oxygen, from the air, and makes green plant cells. The clear, unused part of the water is drawn away in vapor, and most of the oxygen is given back to the air for animals to breathe. The carbon is laid away in the plant cells. The nitrogen is sent clear back to the roots to make nitrates, before the plant can use it. Clover draws a great deal of nitrogen from the air, to make this plant food.
The new plant food made in the leaves is sent back to all the growing parts of the plant where it is needed. Some of it stays in the leaves to build them larger. Some goes into flowers, fruit and seeds. In grasses and straight-veined plants without bark, the new plant food goes to every part. But in plants that grow by adding a new ring every year, the green cells form a layer between the old wood and the bark.
You can find this soft, green layer under the bark of a rose bush, or the twig of an orchard or nut tree, in the spring. It loosens the bark so it can easily be peeled off. That is why you can make a willow whistle in the spring, or peel a little switch. The new green layer makes the heart wood just that much larger, so the bark has to stretch to fit it. You know your skin stretches as you grow larger or become fatter, doesn't it? Maybe this stretching, year after year, is what makes the bark on hard wood trees crack in long deep ridges.
All summer a plant is busy feeding itself, growing and bringing up seed babies. But in the fall the leaves close their mouths and stop pulling up water. They know hard times are coming, when there will be little water to draw upon. So they must stop giving water to the air. They seem to rob themselves of the food they no longer need, and to send it all into ripening seeds, into the roots, and into next year's leaf-buds that form at the base of the leaf stalks. Food for the seeds, when they begin to grow, is stored in fruits and nuts, into thickened stems like lily bulbs, into tubers like potatoes, and into grains of corn and wheat. Everything is done to keep the plant alive over the winter, and to give it a new start in the spring.When you see bare, leafless trees blowing in the winter gale, and often loaded with snow, they look dead. But they are only asleep, like the squirrel and the bee, with their food safely stored away. On any bit of twig you can find little brown knobs and points, often smaller than wheat seeds. They are next year's leaf and branch and flower buds. They are rolled tight and wrapped in fur and spicy gums, to keep out the cold and water. In the first warm days, in February or March, these buds swell. If you break off some twigs of willow or lilac, and put them in a jar of water in a sunny window, you can watch them burst into green leaves and branches and flower buds.
Like the squirrel, the plant stores its food for winter, and it pops out of its hole and goes to work again, just as soon as earth and air and sunshine say:
"Wake up, children, spring is here."
And some trees, like the willow, alder and poplar, even whisk their saucy little catkin tails in the air, just like squirrels.