<P> An endodermis probably evolved during the Silu - Devonian, but the first fossil evidence for such a structure is Carboniferous . This structure in the roots covers the water transport tissue and regulates ion exchange (and prevents unwanted pathogens etc. from entering the water transport system). The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver . </P> <P> Once plants had evolved this level of controlled water transport, they were truly homoiohydric, able to extract water from their environment through root - like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size . As a result of their independence from their surroundings, they lost their ability to survive desiccation--a costly trait to retain . </P> <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 (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 . The increase in vascular bundle thickness further seems to correlate with the width of plant axes, and plant height; it is also closely related to the appearance of leaves and increased stomatal density, both of which would increase the demand for water . </P> <P> While wider tracheids with robust walls make it possible to achieve higher water transport pressures, this increases the problem of cavitation . Cavitation occurs when a bubble of air forms within a vessel, breaking the bonds between chains of water molecules and preventing them from pulling more water up with their cohesive tension . A tracheid, once cavitated, cannot have its embolism removed and return to service (except in a few advanced angiosperms which have developed a mechanism of doing so). Therefore, it is well worth plants' while to avoid cavitation occurring . For this reason, pits in tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate . Freeze - thaw cycles are a major cause of cavitation . Damage to a tracheid's wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel . </P>

Name and give the function of each cell of xylem