Botanists generally agree that the first truly land plants can be identified from the third Paleozoic period, the Silurian. These were leafless plants that had the vascular conducting system typical of the most advanced plant phylum. This phylum produced ferns and then all the seed plants, such as trees, shrubs, and the flowering plants, including grasses. Green plants also evolved in another direction to produce mosses and liverworts which lack a vascular structure. Botanists do not know whether there was a single plant migration ashore or whether there was more than one evolution of land plants from their aquatic ancestors, but they generally agree that the ancestral water plants that produced the first land plants were most likely green algae. Chemical studies provide the strongest evidence for this belief.The Carboniferous period, which is often subdivided into the Missippian and Pennsylvanian, produced the giant swamps and forests that eventually formed many of the Earth's coal deposits, including those in the Appalachian region of the United States. These forests were not like today's forests. The trees were giant club mosses, which are not true mosses but vascular plants, mammoth ferns and seed ferns and primitive conifers that were related to today's evergreen trees. These primitive seed ferns and conifers included some of the first-known seed plants. Another prominent plant during the Carboniferous was the giant horsetail which grew to heights of 30 to 40 feet. Today's horsetails, which grow only 1 to 3 feet high, are often referred to as "living fossils" because of their relationship and close resemblance to their Carboniferous ancestors.The Ginkgo tree is another so-called living fossil. The first ginkgos appeared in the Permian period, just before the end of the Paleozoic, and by the middle of the Mesozoic era they had spread around the world. Although they were once represented by several genera and species, today there is only one species, called the "maidenhair tree", which is common in the United States as an ornamental shade tree.
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