<P> The evolution of plants has resulted in widely varying levels of complexity, from the earliest algal mats, through bryophytes, lycopods, and ferns, to the complex gymnosperms and angiosperms of today . While many of the groups which appeared earlier continue to thrive, as exemplified by algal dominance in marine environments, more recently derived groups have also displaced previously ecologically dominant ones, e.g. the ascendance of flowering plants over gymnosperms in terrestrial environments . </P> <P> Evidence for the appearance of the first land plants occurs in the Ordovician, around 450 million years ago, in the form of fossil spores . Land plants began to diversify in the Late Silurian, from around 430 million years ago, and the results of their diversification are displayed in remarkable detail in an early Devonian fossil assemblage from the Rhynie chert . This chert, formed in volcanic hot springs, preserved several species of early plants in cellular detail by petrification . </P> <P> By the middle of the Devonian, many of the features recognised in plants today were present, including roots and leaves . Late Devonian free - sporing plants such as Archaeopteris had secondary vascular tissue that produced wood and had formed forests of tall trees . Also by late Devonian, Elkinsia, an early seed fern, had evolved seeds . Evolutionary innovation continued into the Carboniferous and is still ongoing today . Most plant groups were relatively unscathed by the Permo - Triassic extinction event, although the structures of communities changed . This may have set the scene for the appearance of the flowering plants in the Triassic (~ 200 million years ago), and their later diversification in the Cretaceous and Paleogene . 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 CO and warm, dry conditions of the tropics over the last 10 million years . </P> <P> Land plants evolved from a group of green algae, perhaps as early as 510 million years ago; some molecular estimates place their origin even earlier, as much as 630 million years ago . Their closest living relatives are the charophytes, specifically Charales; assuming that the Charales' habit has changed little since the divergence of lineages, this means that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools . The alga would have had a haplontic life cycle: it would only very briefly have had paired chromosomes (the diploid condition) when the egg and sperm first fused to form a zygote; this would have immediately divided by meiosis to produce cells with half the number of unpaired chromosomes (the haploid condition). Co-operative interactions with fungi may have helped early plants adapt to the stresses of the terrestrial realm . </P>

When did the first grass appear on earth