Conductive tissues ensure the transport of substance through the plant’s body. These tissues can be classified into two types: xylem and phloem.
Conductive tissues are plant tissues whose primary function is to conduct water and other substances throughout the plant’s body. The conduction of substances occurs through two types of tissue: the phloem and the xylem . Both xylem and phloem are continuous throughout plant organs, forming a true conduction system .
Xylem and phloem can be primary or secondary . The primaries are formed from the procambium , and the secondaries are formed from the lateral meristem called vascular cambium . The primary conducting tissues are organized in the axial system (parallel to the major axis of the organ), and the secondary conducting tissues are organized in the axial and radial system (arranged horizontally).
The xylem is the tissue responsible for ensuring the transport of water and mineral salts to all parts of the plant. In addition to ensuring transport, this tissue stores substances and also guarantees support for the plant’s body.
This tissue is considered complex, since it is formed by different cell types. In addition to conducting elements, the xylem is also formed by parenchyma cells and fibers, for example.
The xylem elements responsible for conduction can be of two types: tracheids and vessel elements . These two cell types do not have a live protoplast at maturity, a characteristic that favors the transport of substances. Tracheids are imperforate elements, and vessel elements are distinguished by the presence of perforation plates (openings in the walls).
Analyzing the plant groups, we realized that tracheids are found in gymnosperms and in more primitive angiosperm groups. Vessel elements, in turn, are found in angiosperms and also appear in some more evolved groups of gymnosperms.
Phloem conducts organic and inorganic material such as amino acids, hormones, sucrose and nucleic acids. The transport of these substances occurs from the producing organ to the consuming organ.
The phloem, like the xylem, is a complex tissue due to the presence of different cell types. In addition to the elements responsible for conduction, there are parenchyma cells, fibers and sclereids. Among the parenchyma cells, the companion cells stand out, which seem to be related to the distribution of substances in the sieve tube.
Phloem conducting cells are of two types: sieve cells and sieve tube elements. These cells have sieved areas (with pores), are alive and do not have a nucleus at maturity. Sieve cells have sieve areas on all walls, and sieve tube elements have sieve plates (with larger pores) on the end walls and sieve areas on the sides.