<P> Until recently, the differential pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a pressure bomb to counteract it . When the technology to perform direct measurements with a pressure probe was developed, there was initially some doubt about whether the classic theory was correct, because some workers were unable to demonstrate negative pressures . More recent measurements do tend to validate the classic theory, for the most part . Xylem transport is driven by a combination of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated . </P> <P> Xylem appeared early in the history of terrestrial plant life . Fossil plants with anatomically preserved xylem are known from the Silurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks . The earliest true and recognizable xylem consists of tracheids with a helical - annular reinforcing layer added to the cell wall . This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first - formed xylem) of all living groups of plants . Several groups of plants later developed pitted tracheid cells, it seems, through convergent evolution . In living plants, pitted tracheids do not appear in development until the maturation of the metaxylem (following the protoxylem). </P> <P> In most plants, pitted tracheids function as the primary transport cells . The other type of tracheary element, besides the tracheid, is the vessel element . Vessel elements are joined by perforations into vessels . In vessels, water travels by bulk flow, as in a pipe, rather than by diffusion through cell membranes . The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the angiosperms . However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae), and their secondary xylem is described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms . Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis states that vessel elements originated in a precursor to the angiosperms and were subsequently lost . </P> <P> To photosynthesize, plants must absorb CO from the atmosphere . However, this comes at a price: while stomata are open to allow CO to enter, water can evaporate . Water is lost much faster than CO is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis . Early plants sucked water between the walls of their cells, then evolved the ability to control water loss (and CO acquisition) through the use of stomata . Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels . </P>

What do a group of xylem vessels form