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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 root 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. From a phylogenetic point of view, some cells of the vascular tissues have been used as traits for evolutionary studies. The organization of vascular tissues depends on the type of organ, age and species we are considering.
Vascular tissues arise in different parts of the plant and during different developmental stages. During primary plant growth, xylem and phloem originate from procambium meristem. During this process, there are several stages: protoxylem and protophloem originate in the embryonic and postembryonic stages, but they are progressively replaced by metaxylem and metaphloem later on. If secondary growth takes place, secondary xylem and secondary phloem are originated from the vascular cambium meristem, while the metaphloem and metaxylem become non functional.
Vascular tissues are complex and they contain several types of cells, most of them produced by the same meristematic cells. That is why xylem and phloem are close to each other in each of the plant organ. However, how they are organized in roots and stems, as well as in different species, is different.
Different patterns of organization of xylem and phloem in vascular bundles (Modified of Futura et al., 2014).
XYLEM, which is the wood of some plants and trees, consists of four cell types: tracheids and vessel elements are the sieve conducting cells or tracheary elements, and parenchyma cells and sclerenchyma fibers work as storage and supporting cells, respectively.
Metaxylem and metaphloem. Vascular tissues of a dicotyledon plant (A, B y C) and of a monocotyledon plant (D). Both with primary growth.
Tracheary elements are cells with lignin in their thick hard secondary cell wall, that lose their cytoplasmic content after differentiation. The name tracheid comes from the similarity of the morphology of these cells and the shape of the insect tracheal tubes. The morphology of tracheids and vessel elements are due to the thickenings of their secondary cell walls, that form annular, helical, reticulated and dotted structures..
Tracheids are elongated cells, tight and fusiforms. Water is conducted intracellularly and cross from one cell to the other via symplast, through bordered pits located in the lateral cell walls. During evolution, it is thought tracheids arose from sclerenchyma fibers and that they are phylogenetically older than vessel elements. Tracheids are the only conducting cell type in pteridophytes and gymnosperms, but, although not abundant, they are also present in angiosperms.
Vessel elements are cells with larger diameter and more flattened than tracheids. They are joined longitudinally to each other to form tubes called vessels or tracheae. Water is conducted via symplast, but in this case, besides crossing the bordered pits of the side walls, it mostly crosses through the perforations of the transverse cell walls.
Main cell types of angiosperm plant primary xylem.
PHLOEM, also known as sieve tissue or bast, consists of more cell types than xylem. The conductive elements are the sieve cells and sieve tubes, and the non-conductive elements are sclerenchyma fibers and parenchyma cells. Parenchyma cells show different function and morphology compared to other parenchymatic tissues. Companion cells are found tightly joined to the conductive elements .
Both sieve cells and tube cells are living cells, although without nucleus. The primary cell wall is thickened by callose deposits. The 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 gymnosperm phloem.
Sieve tubes are formed by flattened cells arranged in longitudinal rows and communicate with each other through sieve plates. They possess sieve areas in the side cell walls to contact with adjacent sieve tubes and with companion cells. They are the main conductive element in angiosperms
Main cell types of angiosperm plant phloem.
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.
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Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo