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. Once cavitation has occurred, plants have a range of mechanisms to contain the damage. Small pits link adjacent conduits to allow fluid to flow between them, but not air – although these pits, which prevent the spread of embolism, are also a major cause of them. These pitted surfaces further reduce the flow of water through the xylem by as much as 30%. The diversification of xylem strand shapes with tracheid network topologies increasingly resistant to the spread of embolism likely facilitated increases in plant size and the colonization of drier habitats during the Devonian radiation. Conifers, by the Jurassic, developed bordered pits had valve-like structures to isolate cavitated elements. These torus-margo structures have an impermeable disc (torus) suspended by a permeable membrane (margo) between two adjacent pores. When a tracheid on one side depressurizes, the disc is sucked into the pore on that side, and blocks further flow. Other plants simply tolerate cavitation. For instance, oaks grow a ring of wide vessels at the start of each spring, none of which survive the winter frosts. Maples use root pressure each spring to force sap upwards from the roots, squeezing out any air bubbles. 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. 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