And because I have told it to you in the shape of a fairy story, you must not think it is not true. It is indeed true. Everywhere in the sunshiny woods and fields of summer, the story of Leaf Green and Sunbeam is being lived. But when the day is cloudy or the sun sets, then there is no Sunbeam to help the Princess, and then no carbon is stolen from the air. PLANT OR ANIMAL? Did you ever stop to ask yourself, “What is the difference between a plant and an animal?” because this is the place where that question should be answered. “Why, an animal is _altogether_ different from a plant,” you answer, perhaps a little scornfully. “I have no trouble in telling which is which.” It is very natural that you should feel this way. A cow or a horse, for example, is not at all like a tree; and when you think of animals, you think of the ones you know best, and likewise of plants. But wise men have discovered plants that look and act so much like animals, and animals that look and act so much like plants, that at one time they say, “Now, these are animals, surely,” and a little later exclaim, “No, after all, these are plants;” and they take a long time to make up their minds as to whether certain objects are plants or animals. And already even you children have discovered that the plants you know best belong to families, and have children, and care for them in a very motherly fashion; that they drink earth food with their roots, and eat carbon food with their leaves; and soon you will find that they do many other things which once upon a time you would have thought it a great joke to be told a plant could do. You remember my telling you of one little plant cell that could swim; and there are some animals, you know, that are rooted to one spot as we usually think only a plant is rooted. What, then, _is_ the difference between a plant and an animal? Leaf Green and Sunbeam between them put life into what had no life before; and the living plant matter, which they help to make, is that which animals cannot make themselves, yet which they cannot live without, for this living matter is absolutely necessary to them as food. And the one real difference between a plant and an animal is this,—a plant can make out of certain dead substances the living matter that all animals must have for food; an animal cannot do this. HOW WE ARE HELPED BY LEAF GREEN AND SUNBEAM The cell in which Leaf Green lives has no little mouths such as we saw in the picture some time ago. Its walls are so delicate that the carbonic-acid gas passes through them quite easily,—as easily as the gas escaping from an unlighted jet in the schoolroom could pass to your nose even if you wore a veil, or as easily as water would pass through a piece of muslin.