The various types of plants that developed during Middle Devonian time had to overcome, one by one, the problems imposed by life out of water.(92 kb)
Drying out and insufficient gas exchange were the main risks, followed by the difficulty in maintaining structural support. Whereas aquatic algae were surrounded by a liquid medium and species could be soft and virtually support-free, land-dwelling plants had to struggle with the reality of gravity.
The thick epidermis of the small, tubular stems of the first plants was rigid enough to allow them to grow about a dozen centimetres high. Faced with competition from many other plant groups colonizing the land, taller species could disperse their spores farther, and were therefore the first to colonize distant virgin territory. Not to mention that a new biological innovation the leaf was creating shade, and taller plants had a distinct advantage in the race toward sunlight.
A stem that simply grows tall will eventually break under its own weight or will be blown over by strong winds. To grow higher, a plants stem must also become thicker and thus stronger. This ability first appeared among a few plant groups in Upper Devonian time. In addition to primary (upward) growth, secondary growth (in circumference) allowed lignified material to be added and accumulated in a continuous manner. This was the beginning of wood, a substance that permitted early trees to attain substantial height and form the first forests.
The Middle Devonian flora of Gilboa, in the State of New York, existed just before the first forests formed and includes a few plants that grew several metres tall. But these were restricted to thin bands that bordered the unstable channels of an alluvial plain, and it was not until the arrival of Archaeopteris
in Upper Devonian time that the first true forests were established. These great trees possessed an extensive root system that greatly accelerated soil formation, producing a new terrestrial habitat that encouraged the diversification of invertebrate animals venturing onto the land. Archaeopteris
forests, which lined the shores of the ancient Miguasha estuary, spread around the world during the Upper Devonian.
At long last, the success of terrestrial plants was assured. Their abundance notably increased photosynthetic activity across the planet, and over millions of years, more than 80% of atmospheric carbon dioxide (CO2) was gradually consumed. Scientists attribute this decrease in carbon dioxide a notorious greenhouse gas to the cooling period that occurred on Earth at the end of the Devonian Period.