The first land plants were small and inconspicuous, and generally confined to wet terrestrial environments. The evolution of vascular tissues, which could carry food and water, allowed plants to grow bigger, taller and to colonize a diversity of habitats. The earliest vascular plants, such as Cooksonia (see below), consisted of relatively simple small branched erect stems. From these psilopsids evolved four groups during the Devonian: ferns, club mosses and horsetails (which were spore producers) and seed-ferns (which were gymnosperms). These groups contributed to the Upper Carboniferous swamps, which when compressed under many thousands of feet of sediment produced coalfields. The gymnosperms reproduced by means of airborne male cells which fertilized the female ovule, and then the seed dropped to the ground and germinated. Flowering plants (angiosperms) evolved in the Cretaceous: these plants contain both male and female parts within the flower – an evolutionary development which is most advantageous to fertilisation.
Cooksonia – an early vascular plant from the Silurian
Cooksonia one of the very first vascular plants, evolved during the Silurian, approximately 430 million years ago. This somewhat unimpressive fragment of rock on the left is in fact is an important specimen from the Devilsbit Mountain, Co. Tipperary. At the time of discovery these were the oldest known examples of Cooksonia.
The Upper Carboniferous swamps were similar to mangrove swamps presently found in Florida, although the vegetation differed greatly. Horsetails, club mosses, tree ferns and seed ferns abounded, some growing to heights of 20 or 30 meters. Most of the British Isles was covered by swamps of this kind – when the plants died they became waterlogged and partially decayed. However, much plant material survived and when these thick deposits were compressed by overlying sediment coal was produced. These swamps have given rise to the coal measures in Castlecomer and Arigna here in Ireland, and to those in South Wales, Yorkshire and Lancashire amongst others in Britain.
Club moss – Lepidodendron
Lepidodendron, a type of club moss, was a major constituent of the swamps in the Carboniferous. Parts of the plants were given separate names because they were thought to belong to different species. Later it was discovered that they belonged to one plant. Lepidodendron (1) was the name given to the trunk, with rhombic leaf scar bases. Stigmaria (2), was given to the root with circular pits where rootlets occured. Lepidostrobus (3) were the cones, which produced spores of two types and Lepidophylloides (4) were the leaves, in a spiral, often surrounding the cones.
Horsetail – Calamites
Calamites is the fossil of the inner part of a horsetail stem. Horsetails evolved in the Devonian and there are still some representative species around today. However, today’s species are on a smaller scale than those that existed in the Carboniferous. Calamites is a common fossil in Carboniferous coal deposits.
Tree Fern – Psaronius (Pecopteris arborescens)
The tree ferns are true ferns, which reproduce using spores – unlike the ‘seed ferns’ (see below). Pecopteris is the name given to these fossil leaves, which are likely from the Psaronius tree.
Seed Fern – Neuropteris
The seed ferns, or pteridosperms, are an extinct group. Superficially, the leaves strongly resemble the true ferns. However, these plants reproduced with seeds and pollen.
The first gymnosperms (seed bearing plants) are Carboniferous or older, but they reach their greatest diversity in the Mesozoic. The gymnosperms include cycads, conifers and Ginkgos. The Ginkgo is one of the earliest gymnosperms; it first appeared during the Jurassic (201-145 million years ago), and is still extant today. Pictured below are a fossil Ginkgo leaf from the Mesozoic of India, and a leaf from a tree in College Park.
The flowering plants (Angiosperms) are a very diverse group, dominating most landscapes today, from rainforests to the tundra. Angiosperms first arose during the Cretaceous, and co-evolved with their pollinators – bees and wasps also radiate during the Cretaceous.