Long, long ago, the dry land was bare of green plants. There were no trees, no violets and no grassy meadows. This was before the old Appalachian Mountains had humped up their backs and in place of the proud Rockies there was a long shallow trough of muddy water. The rocks of the dry land were naked, for the only living plants and animals were in the sea.
This is how the earth was 360 million summers ago. In the shallow pools there were water plants, worms and crusty little animals known as trilobites. The plants were algae. For countless ages they had been single celled plants, rather like the green algae which cloud our fish bowls. Now some of them had learned to live together in colonies. Lots of single cells joined together and became a simple seaweed.
These plants had to live near the surface of the water, for plants must have sunlight. They thrived in shallow beach pools and the tides tossed them this way and that. Some of them developed holdfasts. A few cells at the bottom of a long green colony would cling to perhaps a pebble on the ocean floor.
Here and there, a trailing seaweed fixed itself to a pebble above the tide level. For a short while, twice each day when the tide was low, this wet little water dweller was left high and dry in the air. Most likely, this was the beginning of plant life on the earth.
Sometimes the pattern of the sea changed and a watery area was left high and dry. The water weeds in the deserted pool had to learn to live on dry land, or perish. Of course, some of them did learn: They developed roots and dug them down into the moist powdery rocks and managed to save enough water from one rain shower to the next.
Some of the earliest plants to live on the earth were the mosses. Though they never developed proper stems, they had the kind of foliage that could trap moisture from rain and dew and hold it for a long time. Other early land plants were the lichens and a lichen is actually a partnership between two plants. The main part of the plant is a spongy fungus, able to hold onto moisture, but unable to make plant food from air, water and sunlight. Living among the damp spongy threads of the fungus are countless small alga plants. These little plants can make plant food from air and water. They use the moisture in the spongy fungus and make enough plant food for both the partners.
As they lived and died, these first simple plants poured oxygen into the air. This made it possible for the first animals to come out of the sea. They also enriched the soil and made it possible for more advanced plants to develop.