Vascular plants, unlike non vascular plants, have specialized tissues for transporting water and inorganic and organic substances. These tissues are known as vascular tissues: xylem and phloem, which appeared about 450 million years ago, when plants colonized the land. Xylem transports large amount of water and inorganic and organic compounds from roots to leaves, while phloem transports organic substances synthesized in plant body organs, such as leaves and storage tissues, toward the rest of the plant.
Physiologically, plants need vascular tissues to increase their size by distributing water and organic substances, but have also a role in supporting the aerial part of the plant, including stem, branches, and organs, such as leaves, flowers and fruits, and also give strenght to roots. Different parts of the plant body can communicate between each other by sending meaningful molecules, like some fitohormones, via the vascular system.
Vascular tissues arise in different parts of the plant at different developmental stages. During primary plant grow, xylem and phloem originate from the procambium meristem. Protoxylem and protophloem originate first at both the embryonary and postembryonary stages, but they are progressively replaced by metaxylem and metaphloem, which are the mature vascular system in organs that only show primary grow. If secondary grow takes place, secondary xylem and secondary phloem are originated from the vascular cambium meristem, while metaphloem and metaxylem become nonfunctional.
The organization of vascular tissues is diverse depending on the organ, the age and the species we are studying. For example, they are organized differently in the roots compare to the shoots, and also there are differences in the same organ (for instance, leaves) of different species. Vascular tissues are complex and they are made up of several types of cells, some of them are studied as relevant evolutionary traits. Most of the cellular types of the vascular tissues are produced by the same meristematic cells. That is why xylem and phloem are physically close to each other.
The vascular bundles in the shoots and roots are known as stele, and according to the organization there are different types of steles. When the vascular bundle are arranged as a solid cylinder is known as protostele, but if they form a hollow cylinder, with parenchyma inside, the stele is known as siphonostele. The eustele, a type of syphonostele, is like a cylinder with discontinuous walls and isolated vascular bundles (see figure).
XYLEM is responsible for transporting and distribution of water and mineral salts that mainly come from the roots, although it also transports some organic and signaling molecules. Furthermore, it is the main tissue for mechanical support of plants with secondary grow. It is the wood of trees and of some plants.
Xylem consists of four cell types: a) vessel elements and b) tracheids are the sieve conducting cells or tracheary elements, c) parenchyma cells work as storing and intercommunication cells, and d) sclerenchyma and sclereids as supporting cells.
Tracheary elements (a and b) are cells containing lignin in their thick and hard secondary cell wall. These cells lose their cytoplasmic content after differentiation. The secondary cell wall thickenings form different structures depending on the cell type, so that the tracheids and vessel elements are identified at light microscopy by the morphology of these thickenings, which form annular, helical, reticulated or dotted structures. M. Malpighi named the conducting cells as tracheae because the similarity between the morphology of these cells and the shape of the insect tracheal tubes.
Vessel elements (a) are cells with larger diameter and showing more flattened ends than tracheids. They are joined longitudinally to one other to form tubes or vessels. Water is conducted through symplastic pathway, by the interior of the cell. Besides crossing the pits of the side walls, the water mostly crosses from one cell to another through the perforations of the transverse cell walls located at both ends of the cell. Vessel elements are the main conducting cells of the xylem of angiosperms.
Tracheids (b) constitute the second conducting cell type of the vascular plants. It is the only tracheary element in pteridophytes and gymmnosperms. Angiosperms have both tracheids and vessel elements. Tracheids are elongated, fusiform densely arranged cells. Water is conducted intracellularly and crosses from one cell to the other by symplastic transport through bordered pits located in the lateral cell walls and in the overlapped walls at the ends of the cells. Because tracheids do not have perforation plates, they do not conduct so much water as vessel elements. Furthermore, tracheids have thicker cell walls and smaller inner volume than vessel elements. Conifers show tracheids with very large and round pits or areolas, with an internal structure known as torus, which is an oval thickening of the cell wall. The torus can regulate the flux of water crossing through the areola.
During evolution, it is thought that tracheids arose from sclerenchyma fibers and that they are phylogenetically older than the vessel elements. In some conifers, tracheids have a role as storing cells.
Parenchyma cells (c) are organized in the conducting tissues in two ways: radially and axially. Radial parenchyma cells form rows or rays perpendicular to the surface of the organ, whereas axial parenchyma cell groups are arranged longitudinally in the xylem, being more abundant in the secondary xylem (see below). Radial parenchyma cells are elongated, parallel to the axis of the ray, and are connected between each other by numerous plasmodesmata that permit the transport of substances. In conifers, the rays are uniseriate or biseriate, i.e. constituted by one or two rows of cells, whereas in angiosperms are usually multiseriate with many rows of cells, sometimes with different types of cells. Generally, there are less proportion of parenchyma cells in the xylem of conifers compared to the xylem of angiosperms. The rays of the xylem are continuous with the rays of the phloem. This is so because one single cells of the vascular cambium can differentiate in radial parenchyma cells of either xylem or phloem. Radial parenchyma cells of phloem are more difficult to distinguished than those of the xylem. Axial parenchyma cells are usually in contact with conducting cells and show different distribution patterns according to the species. Axial parenchyma cells are more abundant in phloem than in xylem.
Parenchyma cells perform multiple functions like storing carbohydrates (starch), water, or nitrogen, and facilitating the communication between xylem and phloem.
Sclerenchyma fibers and sclereids (d) provide support and protection.
Primary xylem is the first type of xylem during the development of a plant organ. It can be protoxylem or metaxylem. First, protoxylem is developed from the procambium meristem. During the organ grow, protoxylem matures completely and it desappears later by compression mechanical forces that are produced by the growing of the organ. The secondary wall of conducting cells of the protoxylem, vessel elements and tracheids, initially shows annular thickenings that later become helicoidal. Since protoxylem is formed close to apical meristems, it is the main water supplier to these meristems. Metaxylem appears after protoxylem, when the organ enlarges and matures once the growing slows, and it develops from the fascicular cambium. 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. Metaxylem is the mature xylem in the organs that don't go through secondary grow.
Secondary xylem is produced from vascular cambium in those organs with secondary grow. Together with secondary phloem, it is the mature conducting tissue in plants with secondary grow.
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 and sieve tubes, which are living cells, but without nucleus. The primary cell wall is thickened by callose deposits. Non conducting cells are parenchyma cells that include the abundant companion cells. There are also supporting cells associated to phloem, such as sclerenchyma fibers and sclereids.
Sieve cells are long and have pointed ends, communicating with each other laterally by primary pore fields that form the sieve areas. Functionally and morphologically, sieve cells are related with type of specialized parenchymal cell called Strassburger's (albumin) cells, which are present in gymnosperms. Sieve cells are the only conductive element in gymnosperms and ferns phloem.
Sieve tubes are typical conducting cells in angiosperms. They are flattened cells arranged in longitudinal rows that communicate with each other through sieve plates located in the end (transverse) walls. Sieve plates have large size pores that allow the direct connection between adjoining cytoplasms. Sieve tubes also possess sieve areas in the side walls that are discontinuities of the primary cell wall that allow the communication with adjacent sieve tubes and with parenchyma companion cells. Sieve tubes are the main conductive element in angiosperms.
Companion cells are parenchyma cells tightly associated to conducting cells of the phloem. They are needed to maintain the metabolism of the sieve tubes because sieve tubes lack nucleus and have a reduced cytoplasm. Companion cells show a large nucleus and the cytoplasm contains numerous and abundant organelles, which is indicative of the high metabolic rate. However, they do not store starch. Companion cells and conducting cells are differentiated from the same meristematic cells. Only angiosperms show companion cells. The cells associated to the conducting cells in gymnosperms are known as Strassburger cells (albumin) with similar functions to the companion cells.
Parenchyma cells are associated with phloem. They works as stores of substances transported by the phloem itself. In some species, there are other cells specialized in secretion. The interaction between parenchyma cells and conducting cells is strong and when conducting cells die parenchyma cell die too. In primary phloem, parenchyma cells are elongated and vertically arranged, whereas in the secondary phloem there are an axial parenchyma, with elongated and vertically organized cells, and a radial parenchyma with isodiametric cells.
Sclerenchyma fibers and sclereids are also found in the phloem with suppporting and protection roles.
Primary phloem is the first phloem to be functional in the developing organs. Primary phloem can be protophloem and metaphloem. The first to appear is protophloem and metaphloem is originated late later. Protophloem is formed from the procambium meristem. Protophloem contains sieve elements non well-developed in angiosperms, whereas in gymnosperms and ferns it contains sieve cells, also poorly developed. Companion cells are rare or absent. Metaphloem replaces protophloem during development, usually when the organ stop growing. Metaphloem is originated from the fascicular cambium meristem and contains sieve tubes and sieve cells that are thicker and longer than in the protophloem, and always contain companion cells. The sieve tubes have sieve plates. Metaphloem is the functional conducting tissue in plants with primary grow.
Secondary phloem arises from vascular cambium meristem in plants with secondary grow. The conducting cells are well-developed, as well as companion cells, and both axial and radial parenchyma are present. Unlike in xylem, secondary phloem cells do not synthesize secondary cell wall, and therefore they are living cells. However, the cytoplasm of sieve elements may lack nucleus, microtubules and ribosomes, and the border between vacuole and the rest of the cytoplasm is no easily observed. In actively growing trees, there is little secondary phloem involved in conduction. Sieve elements are short-living, commonly a year, and are squashed by the new cells that are differentiating from the vascular cambium.
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.