The How and Why Library/Life/Plants-Section IV
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IV. Water Babies that Live on Land
Did you ever see a tad-pole? A tad-pole is a baby frog, you know, but it looks more like a baby fish. It breathes through feather-like gills. You can keep one in a gold fish bowl and watch it turn into a frog. Behind the gills a pair of little flippers pop out. They grow into legs. Then the gills go inside and become lungs. The hind legs come next. The tail grows shorter, and the animal broader. One day the tad-pole is gone. The frog jumps out on a rock and catches an insect for dinner. He sits blinking in the sun as if he wonders how he did it. The frog is a land animal that grew from a water baby. It likes to live on the edge of a pond where it can dive and swim whenever it feels like it.
There are plants something like that. They were born seaweeds or algae. You remember that some of the algae were so near the surface of the water that they turned green in the sunlight, and became able to make their own food? Many of them were lifted slowly, on rising seacoasts until, at last, they found themselves in the water part of the time, and part of the time out in the air. If these algae were to live, they had to jump from swimming tad-pole seaweeds into—not quite, but almost into froggy mosses with legs, and air-breathing lungs. They became those curious plants that we know as liver-worts. Like frogs they can live only in wet places; on tide-water faces of cliffs, on rocky river banks, around ponds and springs and even floating on patches of quiet water in marshes.
He-pat-i-cae is the book name of liver-worts. That is confusing, because there is a little pink flower that blooms in the woods and meadows in the spring, that is called the hepatica. We won't say he-pat-i-cae again, and we wouldn't say it even this once, if you didn't need to turn back to that word in Volume II, page 865, of this book, to see some pictures, and to read more about liverworts. There, a scientist who has made a study of life in plants and animals, says: "The liver-worts were probably derived from the algae and, in turn, have given rise to mosses and ferns."
Very likely this is the way the algae managed to turn into liver-worts. You remember the algae learned to cling to rocks, to spread into feathery fronds and to grow bud cells or spores. These are hints of roots and leaves and seeds. If you lift a frond of sea
weed from the water the hair-like cells will all fall together, so they look to be one narrow blade-like leaf. Every time the tide-water drops away and leaves seaweeds out in the air on rocks, the fronds fall and mat together.
After being left out a great many times, some of these frond cells learned to cling together, even when the waves washed back over them. The scattered strings of cells went into partnership and became a leaf. The clinging cells on the underside of the leaf grew into longer, stronger hairs to anchor the plant more firmly to the rock. They learned not only to cling, but to suck up water to feed the cells above. Then the little spore buds raised their heads, and tried to grow into something that would attend to the business of starting new plants better.
Liver-worts look much like very green lichens or very flat mosses, but they can easily be told from both. No lichen is so green and moist, and the smallest mosses have true sterns and roots. The liver-wort is just a mat of tiny, flat leaves. One leaf grows out of another, without a sign of a stem. The whole plant is just a thousand leafed mat. The upper side is the stem and leaf and flower, all in one. It is green, and can make food out of sunlight and rain and air. The lower side is white, and from it grows little thread-like, white hairs that act as anchor cables and water suckers. Every leaf that sprouts sends down its own little rootlets below, and grows spore cases on top.
Every part of this flat, mossy little liver-wort is so small that you will have to put it under a microscope to find out how wonderful and beautiful it is. You will find the leaves clearly marked; each a round, flat, green scale with curled up edges, and spotted with darker green. These spots are raised above the surface, and are of two different shapes; but you will never find both shapes on one leaf.
One of these spots looks like a tiny umbrella, upturned and fringed with spun-silk threads. Between the fringes peep little green cups or bottles with balls in them. They look as if they were waiting for something. They are. On the next leaf, perhaps, the raised spot looks like a toad stool with a star shaped top. It starts to grow from the under side of the leaf, curves around the edge, and suddenly stands up straight. On its flat top are little pocket holes.In each pocket is an egg floating in a bath smaller than the tiniest dewdrop. The egg is full of cells, that are scattered in the water when the egg bursts. Each of these cells has two little whip-lashes that thrash around in the water like wiggle tails. They seem to have minds of their own and to know just what to do. They make straight for the mouths of those bottles on the umbrella spots, whip themselves inside and find the little balls. The ball and the whip lash cell unite, and make spores.
The spores of the liver-wort are not exactly like flower seeds. They are really cases of spores, like the single spore grains on lichens and seaweeds. The plant has taken all that trouble just to be sure there will be enough spores to grow more plants. When the cases are ripe they burst, and the spores are carried on bundles of long threads that snap and scatter them. The liver-wort seems to like to use whips to drive itself along.
What wonderful changes from the simple-celled algae floating in the water, or waving from its rock anchor! The liver-wort has made a leaf; it has dreamed of a root, in its white hair-like suckers; and it has whipped and lashed itself into storing its spore cells into a sort of seed case.