A = Xylem, B = Phloem.
The wood and the liber, respectively called xylem and phloem, are vascular tissues that carry out the transport of sap in plant organisms (tracheophytes): pteridophytes, gymnosperms and angiosperms, communicating the root system to the leaf structures, mediated by the stem.
The transport of raw sap, consisting of water and mineral salts, is carried out from the absorption capacity by the roots and distribution to the rest of the plant with essential destination to the leaves, through the xylem. Its basic composition brings together elements of vessels, tracheal elements, tracheids, fibers and parenchyma cells.
In the transport of elaborate sap, whose basic components are organic substances synthesized in photosynthesis, starting from the leaves towards the other organs, they use the phloem for transport. Formed by sieve tube elements, companion cells, sclerid fibers and parenchyma cells.
These conducting vessels, both in the roots and in the stems, have the following disposition: in the roots, the vessels occupy the region of the central cylinder, and in the stem, the vessels integrate the liberoligneous bundles, where in both the xylem with a more internal position and the phloem more externally. , are produced by differentiation of the vascular cambium.
In the stem, the xylem performs the growth in thickness (secondary growth), due to the inactivation of the crude sap transport mechanism in the central layers, forming the heartwood.
In general, in monocots the vascular vessels are diffusely spread throughout the stem parenchyma, while in dicots the vascular bundles have an organized arrangement surrounding the medulla.