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 vascular plants. Several groups of plants later developed
pitted tracheid cells independently through
convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the
metaxylem (following the protoxylem).) is 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. To photosynthesize, plants must absorb from the atmosphere. However, this comes at a price: while stomata are open to allow to enter, water can evaporate. Water is lost much faster than is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis. The early Devonian pretracheophytes
Aglaophyton and
Horneophyton have structures very similar to the
hydroids of modern mosses. Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport. Bands on the walls of tubes, in fact apparent from the early Silurian onwards, are an early improvisation to aid the easy flow of water. and, when they form single celled conduits, are considered to be
tracheids. These, the "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure. uniting all tracheophytes (but they may have evolved more than once). By adjusting the amount 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, being present in the non-vascular hornworts. By the middle Devonian, the tracheid diameter of some plant lineages (
Zosterophyllophytes) had plateaued. 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. 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.
Patterns of protoxylem and metaxylem There are four primary patterns to the arrangement of protoxylem and metaxylem in stems and roots. •
Centrarch refers to the case in which the primary xylem forms a single cylinder in the center of the stem and develops from the center outwards. The protoxylem is thus found in the central core, and the metaxylem is in a cylinder around it. This pattern was common in early land plants, such as "
rhyniophytes", but is not present in any living plants. The other three terms are used where there is more than one strand of primary xylem. •
Exarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the outside inwards towards the center, i.e., centripetally. The metaxylem is thus closest to the center of the stem or root, and the protoxylem is closest to the periphery. The roots of
vascular plants are generally considered to have exarch development. •
Endarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the inside outwards towards the periphery, i.e., centrifugally. The protoxylem is thus closest to the center of the stem or root, and the metaxylem is closest to the periphery. The stems of
seed plants typically have endarch development. •
Mesarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the middle of a strand in both directions. The metaxylem is thus on both the peripheral and central sides of the strand, with the protoxylem between the metaxylem (possibly surrounded by it). The leaves and stems of many
ferns have mesarch development. == History ==