Atlas of plant and animal histology

Vascular plants, unlike non-vascular plants, have specialized tissues for transporting water and both dissolved inorganic and organic compounds. These tissues are known as vascular tissues, which include the xylem and phloem. The xylem spezializes in transporting water with dissolved inorganic and organic substances from roots to other plant organs, while phloem mostly transports organic substances synthesized in organs, such as leaves and storage tissues, toward the rest of the plant.

1. Introduction

Physiologically, 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.

The formation of primary xylem and phloem occurs during the primary growth of the plant. The first functional vascular tissues of the plant that arise from the procambium meristem are the protophloem and protoxylem. It happens during the embryonic development and at the tips of stems and roots during later development. Later on, metaxylem and metaphloem arise from the procambium meristem too. As the organs develop, they replace protoxylem and protophloem as functional vascular tissues. If secondary growth takes place in the plant, the secondary xylem and secondary phloem are formed from the vascular cambium meristem, while the metaphloem and metaxylem become nonfunctional. Both primary and secondary xylem and phloem are always physically close to each other in all organs of the plant because they derive from the same meristem cells, either in the procambium or the vascular cambium. Xylem and phloem tissues are composed of various types of cells, some of which may provide phylogenetic clues. The organization of vascular tissue in stems and roots is different.

The set of vascular tissues of a shoot or root is known as the stele. There are various types of steles according to the organization of vascular tissues. For instance, when the vascular tissues form a solid cylinder, the stele is known as the protostele;however, if they form a hollow cylinder containing parenchyma, it is called a siphonostele (Figure 1).

2. Xylem

Xylem carries and distributes water along with dissolved minerals, mainly coming from the root, to all parts of the plant. It also carries some organic and signaling molecules. Furthermore, it serves as the primary tissue for providing mechanical support to plant organs, especially during secondary growth. The wood of trees and certain plants is mostly xylem.

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

Tracheary elements (a and b) are cell types containing lignin in their thick and hard secondary cell walls. These cells lose their cytoplasmic contents during differentiation. The secondary cell wall of the tracheary cells show a variety of thickenings that can be visualized with light microscopes, including annular, helical, reticulated, or dotted patterns. The kind of thickening depends on the developmental stage of the organ.

Vessel elements (a) are cells with a larger diameter and flat ends when compared to tracheids (Figures 2, 3, and 4). They are connected end-to-end to form long tubes, referred to as vessels. Water is conducted through the symplastic pathway, that is, within the cells, and it crosses from one cell to the next through the perforation plates, which are specialized perforated transverse cell walls found at both ends of each cell. Some vessels might lack these perforated plates. In addition, water can cross the lateral walls of the vessels through pits and pass to the lateral adjoining cells of the xylem. Vessel elements are the primary conducting cells of the xylem in angiosperms.

Tracheids (b) are the other conducting cell type found in vascular plants. It is the only tracheary component in pteridophytes and gymnosperms. Angiosperms have both tracheids and vessel elements. Tracheids are elongated, spindle-shape, and narrow cells. Water is conducted intracellularly and moves from one cell to the next by symplastic transport through areolate pits found at the walls of both ends of each cell, which overlap with those of the previous and following cells, respectively (Figure 2). Water is additonally laterally transported through pits located at the side cell walls. Because tracheids lack perforation plates, they have a lower capacity for water conduction than vessel elements. Furthermore, tracheids have thicker cell walls and a reduced inner volume compare to vessel elements. Conifers show tracheids with very large and rounded areolate pits along with an internal structure known as the torus, which is an oval thickening of the cell wall. The torus can regulate the flow of water through the pit.

Parenchyma cells (c) are arranged in the conducting tissues in two ways: radially or axially. Radial parenchyma cells form rows, or rays, that are perpendicular to the surface of the organ, while axial parenchyma cells form groups, or rows, that are arranged longitudinally in the xylem, occuring more frequently in the secondary xylem (see below). Radial parenchyma cells are elongated, parallel to the axis of the ray, and connected to one another by many plasmodesmata that allow the transport of substances. In conifers, the rays are either uniseriate or biseriate, meaning they have one or two rows of cells, while in angiosperms, they are usually multiseriate with many rows of cells, sometimes containing various types of cells. The rays of the xylem are continuous with the rays of the phloem. This occurs because one single cell of the vascular cambium can differentiate into radial parenchyma cells of either the xylem or phloem.

Parenchyma cells perform multiple functions, such us 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 initial form of xylem that arises during the formation of an organ in the plant. First, it is protoxylem, followed by the metaxylem. The protoxylem is developed from the procambium meristem during the growth of organs. The protoxylem fully matures and disappears later due to the compressive mechanical forces produced by the growth of the organs. 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. Together with secondary phloem, it is the mature conducting tissue in plants with secondary growth.

3. Phloem

The phloem, also known as sieve tissue or bast, consists of living cells. Its primary function is to carry and distribute organic molecules synthesized by photosynthesis or mobilized from storing tissues, along with signaling molecules like phytohormones.

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

Sieve tubes (a) (Figure 8) are the typical conducting cells of angiosperms. They lined up into longitudinal rows that communicate with each other through sieve plates located at both ends (transverse) of each cell. Sieve plates have large-size pores that allow direct communication between adjoining cytoplasms. In addition, there are sieve areas in the side walls that are depressions of the primary cell wall with pores for communicating with adjacent sieve tubes and with parenchyma companion cells. Sieve tubes are the main conductive element in angiosperms.

Sieve cells (b) (Figure 9) are typical of gymnosperms and pteridophytes. They are long cells with pointed ends that communicate laterally to one another via primary pore fields forming the sieve areas. They don't have sieve plates. Functionally and morphologically, sieve cells are associated with a type of specialized parenchyma cell called Strassburger's cells (albuminous cells). Sieve cells are the only conductive component the phloem found in in gymnosperms and pteridophytes.

Parenchyma cells (c) are components of the phloem. Companion cells are parenchyma cells closely associated with the sieve tubes. They maintain the metabolism of sieve tubes since sieve tubes lack nuclei and show reduced cytoplasm. Companion cells contain a large nucleus and cytoplasm with abundant organelles, which indicates a high metabolic rate. Nonetheless, they do not store starch. Only angiosperms show companion cells. The cells associated with the conducting cells (sieve cells) in gymnosperms are known as Strasburger's cells (albuminous cells), with similar functions to the companion cells.

Other types of parenchyma cells work as storage for substances transported by the phloem itself. In some species, there are cells in the phloem that are specialized for secretion. The relationship between parenchyma cells and conducting cells is strong, and when conducting cells die, parenchyma cells die too. In the primary phloem, parenchyma cells are elongated and vertically arranged, while in the secondary phloem there is an axial parenchyma, with elongated and vertically oriented cells, and a radial parenchyma containing isodiametric cells.

Sclerenchyma fibers and sclereids (d) are also found in the phloem, with supporting and protection roles.

The primary phloem is the initial phloem that becomes active in growing organs. The first form of primary phloem that develops 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, typically when the organ stops growing. The metaphloem contains sieve tubes and sieve cells that are thicker and longer than those in the protophloem. In angiosperms, it always contains companion cells. The sieve tubes have sieve plates. Metaphloem is the active conducting tissue in plants showing primary growth.

The secondary phloem arises from the vascular cambium meristem in plants showing 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 produce secondary cell walls, and therefore they are living cells. However, the cytoplasm of sieve elements may lack nuclei, microtubules, and ribosomes, and the border between the vacuole and the rest of the cytoplasm is not easily observed.

• Bibliography ↷

  • Lalonde S, Francesch VR, Frommer WB. 2001. Commpanion cells. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net. DOI:10.1038/npg.els.0002087.

    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 .