Sangeetha Pulapaka

When mosses and liverworts first evolved, they dominated the terrestrial environment. But they were soon challenged by the more advanced tracheophytes. The ferns and "fern allies" formed the great planetary forests of the late Paleozoic. By the end of the Paleozoic, a new group of plants was challenging the 150 million-year domination of the ferns and fern allies. The seed plants protected the embryonic sporophyte from drying up by encasing it in a tough waterproof seed coat.

The evolution of the seed is as profound a step as the evolution of the shelled egg in reptiles. Just as the evolution of the amniotic egg enabled reptiles to become the first truly terrestrial vertebrates, to break that final link with their aquatic heritage, so did the evolution of the seed allow plants to escape the limitation of growing in very moist environments. These gymnosperms soon became the dominant plants. The Mesozoic is sometimes called the Age of Cycads.

But their success was short-lived. During the mid to late Mesozoic, the first flowering plants or angiosperms appeared. They rapidly dominated the more primitive gymnosperms, and are the dominant plants on Earth today. These waves of competition are typical of the history of life. The survivors are relegated to scattered populations in restricted habitats, where they live in the shadows of their successful competitors. Among the gymnosperms, only the conifers are major competitors with flowering plants. Having evolved in a dryer, cooler climate, conifers are better adapted to dry or cool habitats, and dominate forests in northern latitudes, at high elevations, and on sandy soils.