ABSTRACT
It is probably some 400 or 500 million years since the ancestors of all the land plants which now inhabit the earth emerged from the oceans. They must have evolved from simple marine organisms similar in some ways to some of the present-day Algae. Before this time, for many hundreds of millions of years, plant-life must have been evolving, but it had probably been almost exclusively marine. The direct ancestors of all the most primitive classes of plants—the Algae, Fungi and Bacteria—probably evolved in the primeval oceans. These relatively simple plants are usually grouped together as one major phylum of the plant kingdom known as the Thallophyta. It seems likely that some primitive Algae invaded the land some time during the Cambrian or Ordovician Periods and that from them evolved the original ancestors of all the ferns, horsetails and clubmosses (Fig. 2). These more complex, spore-bearing plants, known collectively as the Pteridophyta, gradually gained ascendancy until they dominated the vegetation of all the land areas. By the end of the Carboniferous Period great forests of tree ferns, giant horsetails and tall club-mosses covered vast areas (Plate Ia ). The coal seams of Carboniferous age are composed almost entirely of their remains. Some time during the Palaeozoic Era the original ancestors of all the mosses and liverworts—the Bryophyta—must also have sprung independently from thallophytic stock but the time and mode of their doing so is still quite obscure. Fossil remains of the Bryophyta are rare but sufficient have been found to show that both mosses and liverworts were in existence before the end of Carboniferous times. Unlike the pteridophytes, however, these plants appear never to have been important in the gross structure of most of the vegetation communities of the earth.
