<P> The high CO concentrations of the Silurian and early Devonian, when plants were first colonising land, meant that the need for water was relatively low . As CO was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant water acquisition and transport mechanisms evolved . Plants then needed a robust internal structure that contained long narrow channels for transporting water from the soil to all the different parts of the above - soil plant, especially to the parts where photosynthesis occurred . By the end of the Carboniferous, when CO concentrations had been reduced to something approaching today's, around 17 times more water was lost per unit of CO uptake . However, even in these "easy" early days, water was at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation . Even today, water transport takes advantage of the cohesion - tension property of water . Water can be wicked along a fabric with small spaces, and in narrow columns of water, such as those within the plant cell walls or in tracheids, when molecules evaporate from one end, they pull the molecules behind them along the channels . Therefore, transpiration alone provides the driving force for water transport in plants . However, without dedicated transport vessels, the cohesion - tension mechanism can cause negative pressures sufficient to collapse the water conducting cells, limiting the transport water to no more than a few cm, and therefore limiting the size of the earliest plants . </P> <P> To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system . During the early Silurian, they developed specialized xylem cells, with walls that were strengthened by bands of lignification (or similar chemical compounds) This process was followed by cell death, allowing the cell contents to be emptied and water to be passed through them . These wider, dead, empty cells, the xylem tracheids were much more conductive than the inter-cell pathway, and more resistant to collapse under the tension caused by water stress, giving the potential for transport over longer distances . </P> <P> The early Devonian pretracheophytes Aglaophyton and Horneophyton have unreinforced water transport tubes with wall structures very similar to the hydroids of modern moss sporophytes, but they grew alongside several species of tracheophytes, such as Rhynia gwynne - vaughanii that had well - reinforced xylem tracheids . The earliest macrofossils known to have xylem tracheids are small, mid-Silurian plants of the genus Cooksonia . Plants continued to innovate ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport . Thickened bands on the walls of tubes are apparent from the early Silurian onwards are adaptations to increase the resistance to collapse under tension . and, when they form single celled conduits, are referred to as tracheids . These, the "next generation" of transport cell design, have a more rigid structure than hydroids, preventing their collapse at higher levels of water tension . Tracheids may have a single evolutionary origin, possibly within the hornworts, uniting all tracheophytes (but they may have evolved more than once). </P> <P> Water transport requires regulation, and dynamic control is provided by stomata . By adjusting the rate of gas exchange, they can restrict the amount of water lost through transpiration . This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, since they are present in the sporophytes of mosses and the non-vascular hornworts . </P>

During which period did the land plants grow rapidly and fill in much of the earth's landscape