Plants

Members of the Plant Kingdom grow from an embryo into a multicellular organism with cellulose in their walls. They photosynthesise (employing the sun’s energy to make sugars from water and carbon dioxide) using chlorophyll (the green coloration). A vascular plant is one that contains special cells that allow water to move through it. Non-vascular plants lack these special cells.

The first land plants were probably relatives of the mosses, liverworts and hornworts of today (non-vascular bryophytes). Fossils of these very fragile plants are rare. The oldest known bryophytes lived on wet mudflats during the early Devonian, but it is likely that they colonised the land during the late Silurian. Vascular plants, which emerged during the late Silurian resemble green algae from which they evolved. Cooksonia was the first to evolve. It was a small plant (six centimetres high) that lacked leaves, but had spherical spore sacs at the ends of its branched stems. A little later Zosterophllophytina (the ancestor of the clubmosses) and Rhyniophytina (ancestral to leaf-bearing Trimerophytina and all the other vascular plants we see today) appeared.

During the Carboniferous many new groups of plants evolved and great forests grew in the tropical swamps and deltas. Trees were not like those we know today, but mainly clubmosses and horsetails, and the earliest gymnosperms (seed-bearing plants) like conifers and seed ferns also developed. Conifers grew mainly on the high ground and are rarely fossilised, although cones are occasionally found. Seed ferns, the only major plant group to have become extinct, were usually small, but Medullosa grew to about five metres tall.

Gymnosperms dominated the plant kingdom during the Jurassic and Cretaceous. Conifers continued to thrive and cycads and ginkgos spread throughout the world. By the Jurassic, cycads and cycad-like plants (cycadeoids) were abundant and widespread. Some cycadeoids had what look like flowers, but these were in fact petal-like scales for seed dispersal rather than pollination. Ginkgos, or maidenhairs, grew throughout the northern hemisphere until the end of the Cretaceous, when they began to disappear, so that only a single species, in the forests of western China, is left today.

Flowering plants (angiosperms) are the most successful plants to have evolved. Of the 300 000 species of plants in the world today, about 250 000 are angiosperms. Their diversification started in tropical regions during the Cretaceous and by the Palaeogene they had spread globally. The oldest known fossil angiosperms, were found in Lower Cretaceous rocks in America and Russia. They are known from their leaves, pollen, seeds, branches, etc, but not their blooms. Flowers, are short-lived, delicate and rarely fossilised. The oldest known petals come from Middle Cretaceous rocks in North America and are related to Magnolia.

By about 50 million years ago, more plants were similar to those of today. This laurel leaf comes from Palaeogene rocks on the Isle of Wight, southern England.

The last major group of gymnosperms to evolve, the Gnetophytes, is little known and comprises only three living genera: Gnetum, Ephedra and Welwitschia. Pollen of this group has been found in Palaeogene and Cretaceous rocks, but its earlier distribution and origins are obscure. Welwitschia is an unusual plant as it lives in the Namibian desert where its only source of water is from the fog that condenses on its leaves.

Plants have grown in Britain for over 420 million years and fossils can be found with careful search. Some of the earliest plants are found in Silurian and Devonian rocks of Wales and Scotland, and many fossils are found in the Carboniferous coalfields. Jurassic and Lower Cretaceous rocks of eastern and southern England may contain fossils of plants as well as the dinosaurs that ate them. Although flowers have not been found in Britain, leaves, seeds and pollen of angiosperms occur in Cretaceous and Palaeogene rocks.