and the second was their establishment on land, then the third crucial stage was the development of their ability to grow from their limp origins, to become tall and tough, and to gain reproductive success over rivals. But from the origins of the early bryophytes some 450 million years ago, plants had to overcome two major obstacles, in order for them to diversify into the shapes and forms that we see today – how to get water and nutrients to all those parts that are not in contact with the soil, and how to support these parts without the buoyancy of water. The solution was found in the tracheophytes, or vascular plants as they are more commonly known.
Silurian landscape
The Silurian period, 444 to 416 million years ago, saw the evolution of the first vascular plants.
© dieKleinert/SuperStock
The evolution of a vascular system was crucial to the plant world. Vascular plants have been the basis of all terrestrial ecology since their arrival on land. Among the first were a group of branching, 10-centimetre-tall plants called Cooksonia, which have been found in the layers of sediment that were laid down during the Silurian period, 444 to 416 million years ago, and are found most commonly today in the fossil fields of the Welsh Borders. Cooksonia had a simple structure, with no leaves or roots, but an internal system of tubes allowing them to move water from beneath the ground up to their photosynthetic structures, and to evenly distribute fuel throughout their branching arms. This hollow internal channel was created by open-ended cells along the length of their stems. In some cells photosynthetic ability was traded in order to take on a purely structural role in the plant. These cells lost their nucleus and their life-giving organs, and instead their walls became thickened with structural sugars, such as cellulose and a tough material called lignin. These vascular plants began to fortify their walls with woody lignin, which gives plants the structure and strength to sprout upwards, unsupported except by their own woody tissues, a key characteristic that separated them from aquatic plants. Over the next 350 million years, the vascular plants would eventually give rise to cycads, ginkgos, ferns, conifers, and ultimately all flowering plants.
By the time vascular plants began to make their mark on land, the story of the origin of plants had already spanned a vast timescale of over 2.5 billion years. Over this period the world had shifted during its fiery volcanic youth, deep in the Archaean eon, its skies became filled with life-giving gases during the Great Oxygenation of the Proterozoic eon, and it had played host to the first endeavours by plants to colonise the land in the Devonian period. By now the world was warm and humid and its surface was dominated by the ancient landmasses of Gondwana and Laurasia. The seas continued to support an increasing array of marine animal life, dominated by filter-feeding bryozoa and a diversity of prehistoric fish, and for the first time animals began to follow in the tracks of plants, and make their way out of the lakes and oceans and onto the land.
With the spread of plants onto land, their shapes and sizes soon began to diversify. By possessing tough lignin-enforced stems that allowed them to counter the force of gravity, vascular plants soon evolved into an array of new and fascinating forms. From the vegetation of the early Devonian, which consisted of small plants no more than a metre high, plants soon began to use their rigid stems to reach new heights. Fossils which have been dated between 407 and 397 million years old show evidence of plants which produced thickened body parts completely separate from their water-carrying internal tubes. These additional structures were the first examples of plants producing bark. As well as the emergence of woody body parts like bark, fossils from the Devonian reveal a whole host of novel structures that emerged at that time, giving this period its name of the ‘Devonian Explosion’. Fossils from this time include plants such as Archaeopteris, which had frond-like leaves, and plants like Drepanophycus, which had metre-long roots that could reach nutrients deep in the soil. These first tree-like plants grew in vast numbers alongside rivers and estuaries and began to give height to the first primitive forests, some growing up to 20 metres tall. Other woody plants from the Devonian include Rhacophyton, which is suggested to be the precursor of the ferns, an 8-metre tree with a large crown called Eospermatopteris, and a plant called Moresnetia which is thought to have been the forerunner to seed plants. The Devonian Explosion also gave rise to many familiar plant species. The 12,000 or so species of ferns that still thrive throughout our Earth’s tropical and temperate zones today bear testament to their early success in the Devonian.
Ginkgo tree
These plants are living fossils, dating back to the Permian period, some 270 million years ago.
© RBG Kew
Animal life was also taking new shape in the warm Devonian climate. Along the damp forest floor millipedes scuttled through the organic mulch, and the first predatory animals, such as trigonotarbids, thought to be relatives of modern spiders, crawled through the undergrowth searching for a meal. In the seas great armoured fish equipped with powerful slicing jaws were quickly increasing in a variety of shapes and sizes, as they evolved to fill the expanding niches of the marine world at the time. In the Late Devonian, around 360 million years ago, the first of these fish made tracks onto land, giving rise to four-legged, air-breathing amphibians, such as Hynerpeton.
The Devonian flora had a fundamental impact on the very nature and substance of the land itself. As the tough, tall forest trees put down their networks of anchoring roots, they began to transform the hard and rocky substrate beneath them into hospitable and nutrient-rich soil. Prior to the plant development of the Devonian, the land surface of the Late Silurian was largely exposed bedrock, near-impenetrable to early root systems. The spread of the land plants of the Early Devonian aided the chemical weathering of rocks, helping break them down and release their mineral nutrients. The plants supplied organic acids from the fungi which colonised their roots, and together with acids given off by the decomposition of plant matter on the top of the substrate, these leached into the rock. This leaching softened the rock, enabling the roots to penetrate further into it, gradually breaking it up into smaller sediments. Over time, as organic matter from the surface was drawn deeper down into the ground, the soil depth progressively increased, allowing it to accommodate longer roots below the ground, and in turn larger trees above ground. As the soils of the Late Devonian and Early Carboniferous progressively deepened and became more developed, plant growth was greatly enhanced.
Archaeopteris
Archaeopteris was one of the first tree-like plants to appear in the Late Devonian.
© Parc national de Miguasha/Steve Deschenes
A world of colour
With the evolution of flowers, the face of the Earth was transformed forever.
© Rob Hollingworth
All of the early plants up until the Middle Devonian possessed male and female gametes which required water, in some form, for their fertilisation. As the original aquatic plants had a totally submerged existence, their sperm cells could freely move through the water to fertilise their ovum cells. This method of fertilisation put some obvious limitations on where they could survive, and out of water their reproductive strategy would have been impossible. Although bryophytes lived on land, they still relied on a partially wet environment to transfer their sperm cells and spores to their female gametes. We know from bryophytes living today, such as mosses, that when their surroundings are saturated some species store up several times their own weight in water as a reserve, and they are also able to stop their metabolism if their habitat dries out for long periods. These water-dependent land plants were therefore best suited to colonise the damp tidal shores of lowland streams of the Devonian forests, and mosses and ferns can still be found to thrive in these environments today. The need of these amphibious plants to be linked to a moist external environment for their reproduction would have been very limiting in all but the dampest of habitats, and so any plant that was able to break this reliance on water would have had an immense