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 ====VI. How the Fern Grew Bones and Babies==== In order to stand alone you must have a backbone. And you must have bones in your legs, too. Your bones are on the inside and are covered with muscles. A turtle's leg bones are on the inside, too, but it carries its backbone on the outside. The turtle's backbone is its shell. So you won't be surprised to learn that trees carry their backbones on the outside, in the form of bark. But it took a long time for plants to learn to make bones on the outside.

Did you ever find it hard to break the stem of a field daisy, wild aster or golden-rod? Such stems seem to be made of bundles of tough threads. They are called fibres. The fibres of the flax stem are so strong and fine they are woven into linen cloth. The stem of a fern leaf has so many of these tough fibres packed together that it is like wire. Fibres are strings of cells. Each little cell has a thin wall around it like the yeast cell, through which water can easily soak, and air pass. The plant used these strings of cells at first to soak up food and air, then to cling to rocks, then to make leaves and stems and roots and spores and seeds. Finally she bound them in bundles to help her stand alone.

The fern is nature's first effort to make bones in plants. The fern stem is very slender and bends easily, so easily that the single feathery leaf on its tip, sways in the wind like an algae frond in the water. We often say fern fronds because of this likeness. But they are not fronds. They are true leaves.

There was a time, ages and ages ago, when ferns were the highest kind of plants that grew. For a long time, nature just tried to see how many thousand varieties of mosses and ferns she could make. Most of them have disappeared as higher forms of life crowded them out, but there are still about four thousand kinds of ferns on earth. Some of them are rock ferns, almost as small as mosses; some are as big as trees. You can see tree ferns in the green houses of city parks, and there are whole forests of them in many hot countries.

When the seed of any plant begins to grow, the first little shoot is so soft that you can mash it to a green paste between your fingers. Even a sprouting oak or maple is as soft as that. It has no bark. The cells have not even hardened into fibres. The fern leaf starts