Xylem Maturation
Xylem maturation is the pattern formed when xylem cells mature, as observed in transectional area. These xylem cells are formed from the apical cells/meristem (=primary xylem). Maturation patterns can be ascertained by locating the position of protoxylem and metaxylem cells.
Xylem cells
Cells have different thickening patterns depending on when the xylem was created
Protoxylem (PX)
First formed primary xylem cells in a plant shoot/root
PX are the smallest xylem cells in diameter
PX cells stop maturing early in development of a shoot, while the shoot is still expanding longitudinally
PX cells are formed close to the apical cells/meristem; and transport water to the activitely dividing apical cells
PX cells have annular and helical thickenings, allowing them to stretch as the surrounding tissues expand longitudinally
Metaxylem (MX)
Later formed primary xylem cells in a plant shoot/root
MX cells are larger in diameter than PX cells
MX cells mature after PX cells during the final primary development of a shoot: small amounts of longitudinal expansion
MX cells serve to conduct water throughout the primary body of a plant
MX cells have sclariform thickenings and pitted walls, which are rigid and not able to stretch
Above: cross section of a root, showing tissues. See below for labels of vascular tissues: 1) late metaxylem cells; 2) early metaxylem cells; 3) protoxylem cells, 4) phloem cells
Above: Xylem maturation patterns in stems (e.g. centrarch, exarch, endarch)
Maturation Patterns
The development of the primary xylem cells can indicate to which group a fossil plant belongs. Below are the different patterns by which xylem cells mature in plant tissues:
Centrarch
Vascular cells mature from the center of the axis to the periphery
This condition has only been found in plants with haplosteles.
A single protoxylem strand is found in the center of the axis, and the metaxylem form around it.
This pattern was common in early land plants, such as rhyniophytes, cooksonioids, trimerophytes.
Exarch
Vascular cells develop from the outside, inwards towards the center, i.e. centripetally.
The protoxylem is found on the periphery, and metaxylem is closest to the center of the stem or root
The stems of lycopods (lycophytes), and the roots of seed plants (spermatophytes), have exarch development.
Endarch
Vascular cells develop 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 closest to the periphery.
The stems of seed plants (spermatophytes) typically have endarch development.
Mesarch
Vascular cells develop from the middle of a strand in both directions, radially.
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 (monilophytes) have mesarch development.
