<P> During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant . By the Middle Devonian, the tracheid diameter of some plant lineages such as the Zosterophyllophytes had plateaued . Wider tracheids allow water to be transported faster, but the overall transport rate depends also on the overall cross-sectional area of the xylem bundle itself . </P> <P> While wider tracheids with robust walls make it possible to achieve higher water transport flow rates, this increases the problem of cavitation that occurs when the cohesive tension of the water column is broken, resulting in the formation of a bubble . Pits in tracheid walls have very small diameters, preventing air bubbles from passing through to adjacent tracheids., but at the cost of restricted flow rates . By the Carboniferous, Gymnosperms had developed bordered pits, valve - like structures that seal the pits when one side of a tracheid is depressurized . </P> <P> Growing to height also employed another trait of tracheids--the support offered by their lignified walls . Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem . However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems . Even when tracheids do take a structural role, they are supported by sclerenchymatic tissue . </P> <P> Tracheids end with walls, which impose a great deal of resistance on flow; vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel . The function of end walls, which were the default state in the Devonian, was probably to avoid embolisms . An embolism is where an air bubble is created in a tracheid . This may happen as a result of freezing, or by gases dissolving out of solution . Once an embolism is formed, it usually cannot be removed (but see later); the affected cell cannot pull water up, and is rendered useless . </P>

When did the tree of might take place