The first indication that a tissue conducts liquids is the presence of vascular bundles. They consist of water conducting xylem and assimilate transporting phloem (4.1). Primitive vascular bundles in mosses are composed of elongated parenchyma cells or fibers (4.2, 4.3). The cell walls of water conducting hydroids are lignified and the cell walls of assimilate transporting leptoids are not lignified (4.2). In contrast, the xylem of vascular bundles in ferns (4.4) and seed plants (conifers 4.5, angiosperms 4.6, 4.7- 4.13) consist of vessels, fibers and parenchyma, and the phloem contains sieve tubes, accompanying cells and parenchyma. Generally, the bundles appear together with groups of sclereids (4.9). The primary vessels of the protoxylem demonstrate annular and those in the metaxylem screw-shaped thickenings (see 3.14, p. 44).