The How and Why Library/Life/Plants-Section VI
VI. How the Fern Grew Bones and Babies
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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 startsas a little spiral, like a green snail. It does not grow from an upright stem like the moss leaf. It is just as if nature said: "I cannot make a stem strong enough yet to stand up and bear many leaves. So I will just flatten and bury the stem, and make a broader crown. From that a stronger root can burrow into the ground, and many leaves rise into the air."
The stem of the little curled up snail-like leaf is soft and green, but it is quite thick. As the leaf uncurls, this stem seems to stretch, and grow more slender. Slowly it stiffens. The strings of cells that were simply water pipes and lungs, also become bones to hold it up. These bone tubes run right out to the tips of the leaves, growing smaller and smaller, as they have less to support. Then, from the sides, grow ribs, just as your ribs grow from your backbone. The stem of the fern leaf is a hint of a coming tree trunk; the veins in the leaf are hints of branches on the trunks. The fern leaf is really the far-away promise of a forest tree. Most fern-leaves are deeply parted between the ribs, clear down to the main stem, making branching leaflets. And on the very ends of the smallest leaflet veins, around the edges, the fern leaf bears its fruit, or spore discs, just as the most perfect rose tree bears its flowers and seed pods.
Find a fern leaf that is fully grown, in some shady spot in the woods. All around the edges of the underside, you will find little rusty spots. These are very regularly spaced, so you can easily guess there is some plan. If you look at them through a microscope you will see that they all grow from the ends of the veins, and are connected with the mid-ribs of the leaflet, and those mid-ribs with the main stem. (See Filicales, Volume II, page 661.) So all those little rusty spots are fed from the root in the ground. The rust spots are only tiny, pin-head grains, that look like a brown powder. But you will find them fastened tight. The microscope shows them to be little brown cases, filled with still smaller grains. These are spores, like those on the liver-wort and moss. Around each spore case is hung a necklace of crystal beads. These draw together, tighter and tighter, until they force the spore case to burst open, and shoot out the ripe spores.You remember moss first made seeds, and then used the seeds to grow spores, to grow new plants. The fern turns right around. It grows spores first. Then it uses the spores to grow seeds to grow plants. You can watch it do this if you have a great deal of patience, and a good magnifying glass. Take a bit of fern leaf and lay it on a pot full of soft, rich earth. Keep the earth moist and in a dim light. As the fern leaf decays and the spore cases burst, you may see the little scattered spores swell. They are soaking up water. One of them is sure to crack open and show a little white tip. That is a plant cell. Like the yeast cell it has a thin skin, and it is filled with jelly protoplasm. One cell grows out of another, like a honey comb. The cells spread and spread, until a heart-shaped leaf is formed.
The leaf lies on the ground, and it acts exactly like the leaf of a liver-wort. The upper side turns green, while from the lower side little hair-like suckers go down. Then little dots lift up their heads. These are cups and bottles just as on the liver-wort and mosses, too. In one kind of cup is a ball or egg, and in the other kind is the whip lash. The egg is called an ovule. It takes an ovule a certain time to become ripe, just as the egg in a chicken takes time to make a yolk and a white and to put a shell around it. The whips seem to know when the ovules are ripe, for they thrash around in the water in their cups, and finally flash across into the ovule cups. They cling together with a gelatin on the ovule, and grow together into——seed!
The fern seed use that heart-shaped leaf grown on the ground by the spore for food, and begin to grow right away. They sprout just like all true seeds, sending cells down for roots and up for leaves. The crown of the root is the buried stem of the plant. In the leaf stems the cell walls lengthen and stiffen into woody fibres. In some way that no one understands, the air and water pipes of the plants learn also to do the work of bones strong enough to support long feathery leaves. Besides, the fern is the first plant that grew from seeds. The mosses make seeds and then go back to spores to make new plants, but the ferns make spores first, and then seeds to grow plants. After the ferns, plants had no further use for spores. Having learned how to make seeds they all make them directly. Seeds are embryo, or unborn baby plants. The fern took two steps forward. It made leg and arm bones, and it made seed babies. (See Ferns, Filicales, Seeds.)