Paraxylem

Paraxylem is a type of xylem tissue found in vascular plants. Xylem is the primary water-conducting tissue, and it also provides structural support. Paraxylem is characterized by the arrangement of its protoxylem elements, which are the first formed xylem. In paraxylem, the protoxylem lacunae, or large air spaces, are typically located towards the center of the stem or root, indicating centrifugal development. This means that the protoxylem formed first was positioned internally relative to the metaxylem, which forms later.

The development of xylem proceeds from protoxylem to metaxylem. Protoxylem consists of narrower, more elongated tracheary elements, while metaxylem has wider, shorter elements. The presence and position of paraxylem are often significant in distinguishing between different plant organs and species. For example, in stems, paraxylem development is centrifugal, leading to the protoxylem being closer to the pith. Conversely, in roots, xylem development is often described as exarch, with protoxylem located towards the periphery. Therefore, paraxylem is a descriptor of xylem development and arrangement, specifically relating to the centrifugal positioning of its earliest formed elements.