Early * primary xylem. The protoxylem matures before the organ completes its longitudinal growth and is thus often distorted or destroyed as the surrounding tissues elongate. In higher plants, the protoxylem of the stem occurs at the innermost edge of the vascular bundles whereas in the root it occurs external to the metaxylem as protoxylem poles. The protoxylem is composed mostly of parenchyma cells with a relatively small number of tracheary cells. The tracheary elements of the protoxylem usually possess spiral, annular, or sometimes reticulate thickening and nonpitted secondary cell walls. Compare metaxylem , protophloem. |
