TRANSPORT IN PLANTS OCTOBER, 31 2011 CAPE BIOLOGY

MAIN VASCULAR TISSUE: XYLEM AND PHLOEM

The xylem and phloem are the two main types of conducting tissues in the plant. Together they make up the vascular tissue. Xylem conducts mainly water and mineral salts from the roots to other parts of the plant. As another function, secondary (older) xylem may form wood. The phloem conducts mainly organic food from the leaves both up and down the plant.

XYLEM consists of parenchyma cells and fibres together with two specialized cells: vessels and tracheids. both tissues are dead and function in transporting water and supporting various parts of the plant.

TRACHEIDS Are single cells that are elongated and lignified; Have tapering end walls that overlap, hence they have mechanical strength and give support to the plant. Are dead with empty lumen when mature. Water can pass unobstructed through the empty lumen. There are pits between adjacent tracheids through which water can pass via the “pit membranes” or through unlignified portions of the cell walls.

VESSELS Long, thin tubular structures formed by the fusion of several cells end to end in a row. Formed when the neighbouring vessel of a given row fuse as a result of their end walls breaking down.

The first vessels form the protoxylem located just behind the apical meristem and mature protoxylem can stretch as the surrounding cells elongate because lignin is not deposited over the entire cellulose wall but only in rings or in spirals. As growth of the plant continues, more xylem vessels develop and become more lignified thus forming mature, dead, non-stretching metaxylem. Metaxylem vessels show three basic patterns of lignification: scalariform, reticulate, and pitted.

PHLOEM Comprises parenchyma, sclereids and fibres together with specialized cells for translocation, called sieve tube elements, and companion cells.

SIEVE TUBES AND COMPANION CELLS Sieve tubes are long and are formed by the fusion of sieve tube elements and the partial breakdown of the cross walls between them to form sieve plates. The cell walls are thickened with cellulose and pectin, the cell lacks a nucleus and the cytoplasm is confined to the edges of the cell. This cytoplasm is still living but is dependent on the nucleus and active cytoplasm of an adjacent companion cell.

PHLOEM PARENCHYMA AND FIBRES Found in dicotyledons but not in monocotyledons. Cells are generally elongated. In secondary phloem, parenchyma occurs in medullary rays and vertical strands as for xylem parenchyma.

PHLOEM FIBRES AND SCLEREIDS Found in dicotyledons but not in monocotyledons. Form vertically running bands of cells in secondary phloem and help to resist pressure as growth continues in this region. Sclereids occur frequently in phloem, especially in older phloem.