Atlas of plant and animal histology

Vascular plants have specialized tissues for transporting water with both inorganic and organic substances. Plants need vascular tissues to increase their size by distributing water and organic substances to feed the cells. These tissues also have a mechanical role in supporting the aerial parts and giving consistency to the underground organs, acting as a skeleton. Another function of vascular tissues is to allow communication between distant organs of the plant body by transporting meaningful molecules, like some phytohormones.

1. Xylem

Xylem is responsible for transporting and distributing water and mineral salts, which mainly come from the roots, to the plant body. It also transports some organic and signaling molecules. Furthermore, it is the main tissue for the mechanical support of plant organs, particularly during secondary growth. The wood of trees and some plants is largely xylem.

Xylem consists of four cell types: a) vessel elements and b) tracheids are the conducting cells, or tracheary elements; c) parenchyma cells work as storing and communication cells; and d) sclerenchyma and sclereids are supporting cells.

Tracheary elements (a and b) are cells containing lignin in their thick and hard secondary cell walls. These cells lose their cytoplasmic content during differentiation. The secondary cell wall of the tracheary cells forms different thickenings, which can be annular, helical, reticulated, or dotted structures.

Vessel elements (a) are cells with a larger diameter and more flat ends when compared to tracheids. They are connected longitudinally to one another to form long tubes, also known as vessels. Vessel elements are the main conducting cells of the xylem of angiosperms (Figure 1).

Tracheids (b) are the second conducting cell type of the vascular plants. It is the only tracheary element in pteridophytes and gymnosperms. Tracheids are elongated, fusiform, thin cells. They have a lower capacity for water conduction than vessel elements because they have thicker cell walls and a smaller inner volume than vessel elements.

Parenchyma cells (c) are organized in the conducting tissues in two ways: radially or axially. Radial parenchyma cells form rows, or rays, perpendicular to the surface of the organ, while axial parenchyma cell form groups, or rows, arranged longitudinally in the xylem. Parenchyma cells perform multiple functions, like storing carbohydrates (starch), water, or nitrogen, and facilitating communication between xylem and phloem.

Sclerenchyma fibers and sclereids (d) provide support and protection.

The primary xylem is the first type of xylem developed during the formation of a plant organ. The protoxylem is developed from the procambium meristem during the organ growth, matures completely and disappears later due to the compression of mechanical forces that are produced by the growth of the organ. The secondary wall of conducting cells of the protoxylem (vessel elements and tracheids) initially shows annular thickenings that later become helical. The metaxylem appears after the protoxylem, when the organ enlarges and matures once the growth begins to slow. It also arises from the procambium. Their cells show larger diameters than those of the protoxylem, and the cell walls of the conducting cells have reticulated thickenings first and perforated thickenings later. The metaxylem is the mature xylem in those organs that don't go through secondary growth.

The secondary xylem is produced from the vascular cambium meristem in those organs with secondary growth. It is the mature conducting tissue in plants with secondary growth.

2. Phloem

The phloem, also known as sieve tissue or bast, consists of living cells. Its main role is transporting and distributing organic molecules synthesized by photosynthesis or mobilized from storing tissues, as well as signaling molecules like hormones.

Phloem is made up of more cell types than xylem. There are conducting and non-conducting cells. Conducting cells are the sieve cells (a) and sieve tubes (b) (Figures 5, 6 and 7), Both cell types are living cells, without nucleus, and their primary cell wall is thickened with callose deposits. Non conducting cells are parenchyma cells that include the abundant companion cells (c). There are also supporting cells (d) associated with the phloem, such as sclerenchyma fibers and sclereids.

Sieve tubes (a) are the typical conducting cells of angiosperms. They are formed of cells lined up into longitudinal rows that communicate with each other through sieve plates located at both ends (transverse) of each cell. In addition, there are sieve areas in the side walls that are discontinuities of the primary cell wall for communication with adjacent sieve tubes and with parenchyma companion cells (Figure 2).

Sieve cells (b) are typical of gymnosperms and pteridophytes. They are long cells with pointed ends. Parenchyma cells (c) maintain the metabolism of sieve tubes since sieve tubes lack nuclei and show a reduced cytoplasm. In addition, they are storage cells.Sclerenchyma fibers and sclereids (d) are also found in the phloem, with supporting and protection roles.

The primary phloem is the first phloem to be functional in developing organs. The first type of primary phloem to appear is the protophloem, which is later replaced by the metaphloem. Both the protophloem and metaphloem are formed from the procambium meristem. In angiosperms, protophloem contains non-well-developed sieve tubes and cells, while in gymnosperms and ferns, it shows poorly-developed sieve cells. Companion cells are rare or absent. Metaphloem replaces protophloem during development, usually when the organ stops growing. The metaphloem contains sieve tubes and sieve cells that are thicker and longer than in the protophloem. In angiosperms, it always contains companion cells. The sieve tubes have sieve plates. Metaphloem is the functional conducting tissue in plants with primary growth.

The secondary phloem arises from the vascular cambium meristem in plants with secondary growth. The conducting cells are well-developed, as are the companion cells, and both axial and radial parenchyma are present. Unlike in xylem, secondary phloem cells do not synthesize secondary cell walls, and therefore they are living cells.

• Bibliography ↷

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    Furuta KM, Hellmann E, Helariutta Y. 2014. Molecular control of cell specification and cell differentiation during procambial development. Annual review of plant biology. 65:607-638.

    Spicer R. 2014. Symplasmic network in secondary vascular tissues: parenchyma distribution and activity supporting long-distant transport. Journal of experimental botany. 65: 1829-1848 .