As mentioned earlier, protoxylem consists of those trache-ary elements (usually smaller in diameter) that are the first to mature. Xylem maturation patterns describe the location of the protoxylem in relation to the metaxylem; these terms are used only for primary xylem. When the protoxylem is situated toward the outside of the stem, with metaxylem toward the center, the maturation, or development, of the xylem is described as being centripetal (from the outside in) or exarch. The exarch condition is found in almost all roots and in many primitive plants, such as the lycopsids. In axes with a pith, when the protoxylem is next to the pith, that is, closest to the center of the axis, and the metaxylem develops outside of it, the maturation pattern is endarch (centrifugal development of xylem). This is the typical maturation pattern found in the primary xylem of most seed plants. In axes with a solid core of xylem, called a protostele (see section "Stele Types") , if the protoxylem occupies the center of the protostele, this type of maturation is called centrarch. Centrarch xylem maturation is relatively common in several groups of Devonian plants, for example, the Rhyniophyta and Trimerophytophyta (Chapter 8). In axes that have a pith, if the protoxylem occupies the center of the xylem and the metaxylem develops on both the outside and the inside of the protoxylem, this pattern is called mesarch (development is both centrifugal and centripetal). Mesarchy occurs in many ferns.
These primary xylem configurations can be seen in anatomically preserved fossils and have been used when attempting to determine relationships among groups. While it is not possible to observe the actual sequence of development in fossils, it is often possible to infer the maturation pattern based on the relative cell size and secondary wall patterns of protoxylem and metaxylem elements.
Was this article helpful?