"Cooksonia" is an extinct grouping of primitive land plant dating from  433 to 393 million years ago. Cooksonia fossils are distributed globally, but most type specimens come from Britain, where they were first discovered in 1937. Cooksonia includes the oldest known plant to have a stem with vascular tissue and is thus a transitional form between the primitive non-vascular bryophytes and the vascular plants.  The first Cooksonia species were described by William Henry Lang in 1937 and named in honor of Isabel Cookson, with whom he had collaborated and who collected specimens of Cooksonia pertoni in Perton Quarry, Wales, in 1934.

12.1 Of course when life first left the watery oceans for dry land, the first resource it needed was, water. Not the salt water, however, of the oceans back home, but the fresh water of ponds and lakes, places in fact not dissimilar to Lake Annecy. Another lake, Lake Winnipeg is more than a large Canadian lake; it is also an area rich with fossil remains of marine plants and animals. The first fossils from this area were found in 1819 in limestone along the lake's shore, and new species continue to be found there. The first fossil algae were described and published in 1895 by J. F. Whiteaves in the Canadian Record of Science. The remarkable preservation of these Late Ordovician algae was not fully appreciated at the time. It is rare to find such a diversity of fossil algae in a single location, and even rarer for so many of these algae to be soft-bodied.

12.2 Land plants evolved from green algae perhaps as early as 510 Ma, from a division of freshwater green algae called Charophyta, and more specifically Charales - a branched, filamentous alga dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools. The first of these were non-vascular plants called bryophytes, including mosses, hornworts, and liverworts, the first evidence of which comes from spores of mid-Ordovician age (early Llanvirn, around 470 Ma). The earliest macrofossils to bear water-transport tubes are Silurian plants placed in the genus Cooksonia. These first explorers had limited success because they did not work out how to deal with the two great challenges posed by life on land – gravity and dessication – so they did little more than cover the surfaces of wet rocks as best they could. Consequently the early Devonian landscape was devoid of vegetation taller than waist height. Without the evolution of a robust vascular system, taller heights could not be attained. There was, however, a constant evolutionary pressure to attain greater height. The most obvious advantage is the harvesting of more sunlight for photosynthesis – by overshadowing competitors – but a further advantage is present in spore distribution, as spores (and, later, seeds) can be blown greater distances if they start higher

12.3 So before land plants could diversify much further they had to deal with similar challenges to those which were to face the tetrapods emerging from the sea on to land: how to support their structure out of water against the Earth’s gravity, and how to get access to water? The plants solved this in a series of seriously ingenious ways. First was the problem of how to suck up, against the force of gravity, the necessary fresh water and nutrients from the earth into the body of the plant. No problem, just exploit four of out the multiplicity of wonderful physical properties of water. First make use of the fact that water acts as a solvent for most things and so can dissolve and transport all the nutrients you need. Then use osmosis to allow water and dissolved nutrients into the roots. Next develop a vascular system within the plant, little tubes up which water could rise through its property of surface tension. Finally design a leaf from which water can evaporate into the air, thus drawing after it a column of water through the plant. Job done. The system so designed is called the Xylem. The plant has a continuing flow of water and vital nutrients up through it with no effort on its part. Elegant design!

12.4 However, this was only half of the design work put in by the Plants’ equivalent of Apple Labs back in the day. The other part was how to fuel growth.   Again the plants let others do the work. They co-opted the photosynthesising invention of Cyanobacteria into their Chloroplasts which they put in their leaves and let the leaves soak up the energy from the sun. Then all the Plants had to do was design a secondary vascular system to transport the stored chemical energy in the sugars produced by the chloroplasts throughout the plant to enable it to grow. This they did by designing the Phloem.
By the middle of the Devonian, many of the features recognised in plants today were present, including roots and leaves.

12.5 Finally in order to grow to the great heights they aspired to they invented lignin, a small molecule that serves to toughen the cell walls of plants which are made of cellulose. The invention of wood and its amazing properties is a wonder that Colin Tudge eulogies in a chapter dedicated to the subject, in similar fashion to Dr Servettaz’s encomnium on water. [Tudge,81] The first plants to develop this secondary growth, and a woody habit, were the ferns, and as early as the Middle Devonian one species, Wattieza, had already reached heights of 8 m and a tree-like habit.[Tudge,70] So the invention of wood enabled the next generations of vascular plants to grow to great heights and produce two further great lineages of trees, the lycophytes and the more sophisticated euphyllophytes. The lycophytes could grow to great heights and one tree the Lepidodendron grew as high as 40 metres from and from around 360 Ma to 270 Ma (late Devonian to early Permian) they produced great forests. However, their one drawback was they had no proper leaves.

12.6 The earliest tree-like organisms were tree ferns, horsetails and lycophytes, which grew in forests in the Carboniferous period – hence the name, by the way. The first tree may have been Wattieza, fossils of which have been found in New York State in 2007 dating back to the Middle Devonian (about 385 million years ago). Prior to this discovery, Archaeopteris was the earliest known tree. Both of these reproduced by spores rather than seeds and are considered to be links between ferns and the gymnosperms which evolved in the Triassic period.

12.7 The invention of true leaves was done by the euphyllophytes (good leaf tree) and it was this lineage of trees that went on to conquer the world.   The euphyllophytes in turn split into two further lineages, the monilophytes and the spermatophytes. The former, which includes ferns, continued to reproduce in the customary fashion - alternately sexually by producing eggs and sperms and asexually by means of spores.   The latter invented a whole new, more efficient, means of reproduction - they invented seeds. “Seeds were, and are, a marvellous innovation. Spores obviously do a good job. The plants that make use of them include many that were and are hugely successful.   But although it has become politically correct to argue that there is no progress in evolution, there very clearly is, of a technological kind: and seeds, beyond doubt, are a technological improvement. Spores are little more than groups of relatively undifferentiated cells wrapped in a protective coating, light enough to be carried away by wind or water. Unless they land somewhere very favourable indeed (and in particular very damp) they perish. Spores are like children setting out on a wild adventure with nothing but high spirits and a bag of toffees. Seeds, by contrast, contain embryos that have already developed significantly while still attached to the parent plant, and are equipped with a fgood store of carbohydrate, ptotein and fat. The embryo and its attendant hamprer is encased witho a coat ( a ‘testa’) that is custom built for the cirucumstances that are liable to be met, and commonly contains (chemical) instructions on when to germinate including devices to delay germination for several years, for not every season is favourable.” [Tudge,75]

12.8 The trees armed with this new invention were called spermatophytes and around 360 Ma, at the end of the Devonian, they gave rise to a great lineage of seed bearing trees called the gymnosperms, including cycads, ginko, conifers and gnetals. Ginkgophyta was once a widespread diverse group of which the only survivor is the maidenhair tree Ginkgo biloba. This is considered to be a living fossil because it is virtually unchanged from the fossilised specimens found in Triassic deposits. The conifers, by contrast, were so successful they have dominated the landscape ever since. [Tudge,76]

12.9 In the words of Mr Tudge. “So that’s it, by the time we have seed plants, all the transformations required to take us from inchoate clouds of noxious gases to plants that can manifest as oaks and redwoods have taken place. There were many refinements to come including the evolution of flowers. But the basics were in place at least 150 million years before the first dinosaurs. Such antiquity is hard to comprehend; yet, botanically speaking it was the beginning of modernity.” [Tudge, 76]

12.10 However there was one final, brilliant invention to be made by the plants. Flowers. This is the last and final evolution of the plants since their green algae ancestors’ first tentative steps onto land – and the angiosperms flourish to this day across the world in amazing variety. These two lineages, the conifers and flowering plants, now dominate terrestrial ecosystems and account for at least 99% of all trees. Flowering plants also brought with them an upgrade to the seed. The term "gymnosperm" comes from the Greek for "naked seeds", after the unenclosed condition of their seeds. Their naked condition stands in contrast to the seeds and ovules of flowering plants, called angiosperms, which are enclosed within an ovary.

12.11 One of the first flowering plants was the Amborella which represents a line of flowering plants that diverged very early on, about 130 Ma, during the early Cretaceous period. This makes it of great interest to plant systematists because comparing characteristics of this early angiosperm, other flowering plants and fossils may provide clues about how flowers first appeared—what Darwin called the "abominable mystery".

12.12 The latest major group of plants to evolve were the grasses, which became important in the mid-Paleogene, from around 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years.

12.13 During the Mesozoic (245 to 66 million years ago) the conifers flourished and became adapted to live in all the major terrestrial habitats. Subsequently the tree forms of flowering plants evolved during the Cretaceous period. These began to dominate the conifers during the Tertiary era (66 to 2 million years ago) when forests covered the globe. When the climate cooled 1.5 million years ago and the first of four ice ages occurred, the forests retreated as the ice advanced. In the interglacials, trees recolonised the land that had been covered by ice, only to be driven back again in the next ice age.

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