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About fourhundred and fifty million years ago, at the end of the Ordovician and in the beginning of the Silurian, the land was desolate and empty. Barren, hardly wethered rockgrounds, empty sand-, gravel- and clayplains, no green. Maybe some lichens. And at wet spots some algae with a couple of spider-like little creatures creeping around. In the neighbourhood of the mouth of rivers, where the water regularly flooded the land, it was probably green with algae. At such places, e.g. in Australia, traces of big seascorpions have been found. There was not much happening on the land. Life enacted itself nearly completely in the water.
The evolution from algae to land plants must have been a lengthy process. Many conditions had to be fulfilled before plants were able to maintain themselves on the land. There is at first the everlasting danger of desiccation. The remedy which developed is a thin waxy layer at the surface of the plant: the cuticle. Generally algae don't have a cuticle, nearly all land plants do. But a land plant also has to breath and it needs the possibility to assimilate carbondioxide from the air to generate its nutriments. Thus the isolation by the cuticle can not be absolute. That's why the stomata have evolved, which can be opened and closed if necessary.
Another important adaptation to landlife is the very tough wall that evolved around the spores. This provided the spores with an excellent protection against desiccation, fungi, and so on. In fact they became nearly invulnerable, for they have been fossilised very often. The first fossils of real land plants have been found in the Middle Silurian of Ireland. They are about 420 million years old. They consist of small bifurcations some centimeters in size. Only in the very last part of the Silurian fossils of land-plants become more common and also more complete. The best known plant from that time is called Cooksonia..
The compressed fossils of Devonian plants scarcely give any information. Determining these very old plants is extremely difficult as hardly anything shows on the fossils. Only stems, bifurcations and possibly spines. With luck you find sporangia. In that case it might be possible to detect the name of the plant. Sometimes the cuticle has stayed intact. After chemical treatment it is possible to make a microscope preparation of it in which e.g. cells and stomata are visible. The discovery of a silicified moor near the Scottish village of Rhynie, 40 km north-east of Aberdeen, has revealed a lot about the structure of very old plants. About 400 million years ago, in the beginning of the Devonian, there was a kind of Yellowstonepark with vulcanos and geysers at that location.
Some of the geysers spouted boiling, siliceous water with intervals of a couple of years which caused the vegetation, including the underlying peat layers, to silicify. The fossils in this so called Rhynie Chert are so perfect that the plants can be studied within the accuracy of cell structure level. It is possible to make very thin slices of the fossil bearing chert, which can be viewed under the microscope. The Rhynie Chert was discovered in 1912 by the gologist Dr. W. Mackie. The paleobotanists R.
Kidston and W.H. Lang have published a very extensive description of the plants and the fungi in the chert from 1917 to 1922. These primitive plants were the very beginning of the process of the assimilation of plants on the dry land. The importance of the change from water dependent life cycle to the one of the dry land was the most important contribution to the evolution of terrestrial plants. It made the existense of land animals possible. It was the beginning of the mass forest appearance, which, in its turn contributed to the chemical evolution of the atmosphere.
The conquest of dry land by animals had to follow the conquest of dry land by the plants, which had to prepare the soil for the first one. Due to the assimilation of plants on the dry land most of the minerals used by humanity came to existense. The evolution of terrestrial animals produced the mankind. But the starting point of all these things was the amphibious assault of the shoreline made by the primitive plants..
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