Refreshing moisture from leaves.
One use of leaves, as I told you in the last chapter, is to supply the air with water. In the hot weather the air would be very dry and uncomfortable to us if the leaves did not breathe out moisture from their pores. You can see how this is if in a hot day you walk across a sandy plain where there are no leaves except those of the scanty grass and weeds. Here no moisture is breathed out upon you, to lessen the heat that you suffer from the burning sun.
Another use of the leaves is this. They are pleasant and beautiful to the sight. I have told you about this use of them in the beginning of the seventeenth chapter.
Their shade.
Another use of leaves is to give shade. We know how refreshing this is to us in a hot day. When in a city we walk through streets where there are no trees, how delightful it is to come out of the blazing sun into a square that is full of trees! How comfortable are the cows in the pasture lying under the trees at mid-day, chewing the cud!
But the shade given by leaves does good not merely to man and animals. It does good to fruits, if there is not too much of it. The sun would very often be too hot for the fruits, if it shone full on them all the time. So the leaves partly shade them.
The grape-vine stripped of its leaves.
The chief use of leaves is to keep plants and trees alive andmake them grow. If you should strip off the leaves from a plant as fast as they came out, you would, after a while, kill it. Sometimes worms eat up the leaves on trees. If this is done year after year to a tree it dies. I knew a man to strip off all the leaves from a grape-vine. He thought that it would make the grapes grow finely. He had seen people take off some of the branches from grape-vines, to make the grapes grow large and full. So he thought that if he tookallthe leaves off, the sap would all go into the grapes and make them very large. He thought, too, that the sun would make them ripen fast. But he found that the grapes stopped growing, and wilted, and dropped off. There are two reasons for this. The sun was too hot for the grapes when all the leaves were gone. And besides, there were some leaves needed to keep the grapes alive.
Leaves are lungs to plants.
Leaves are the same thing to plants that lungs are to an animal. The air that goes into our lungs helps to keep us alive and make us grow. So the air that is all about the leaves of a plant or tree helps to keep it alive and to make it grow. How this is done you can not understand now. I explain it in another book, which you will be able to understand when you are a little older.
The barter between lungs and leaves.
There is one thing about this that you can understand, which is very curious. The air does not keep the plants alive in just the same way that it does animals. You know that by breathing air we make it bad; and so we must have all the time a supply of fresh air. Now what do you think becomes of the bad part of the air that we breathe out from the lungs? The leaves all around us take it in. It is good for them. It makes them andthe plants that they are on grow. They then, like our lungs, are all the time taking in air and giving out air. And leaves take what lungs give, and lungs take what leaves give. So lungs and leaves have a sort of trade together. They are always making this exchange with each other. And it is a good bargain for both. Both get what they want, and barter away what they do not want.
How it is carried on in winter.
But in winter, when the leaves are all gone except those on the evergreens, how is it with this trade between lungs and leaves? Lungs are all the time giving out bad air; but there are not leaves enough on the evergreens to take it all, and give back the good air. Well, what is to be done? A barter is carried on with the leaves a great way off in the southern countries. The air moves about so freely that this is easily done. The bad air goes there, and the leaves that take it into their pores give out the good air, which immediately spreads every where, even to us at the north. It is a free trade—as free as air, as we may say. There is not as much bad air made by lungs in winter as in summer, because many animals are either dead or torpid. But what is made is disposed of mostly in this way.
Questions.—How are leaves useful to us in giving out moisture to the air? What use of them is next mentioned? What is said of the shade made by leaves? Is this shade useful to fruits? What is the chief use of leaves? Tell about the man who stripped the leaves from his grape-vine. How are leaves like our lungs? What kind of barter is there between leaves and the lungs of animals? How is this barter carried on in winter?
Questions.—How are leaves useful to us in giving out moisture to the air? What use of them is next mentioned? What is said of the shade made by leaves? Is this shade useful to fruits? What is the chief use of leaves? Tell about the man who stripped the leaves from his grape-vine. How are leaves like our lungs? What kind of barter is there between leaves and the lungs of animals? How is this barter carried on in winter?