Xylem

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In vascular plants, xylem is one of the two types of transport tissue in plants; phloem is the other vascular tissue. Xylem is the primary water-conducting tissue and phloem ciruculates a nutrient-rich sap throughout the plant. The term “xylem” is derived from classical Greek xúlon, "wood," and indeed the best known xylem tissue is wood. Xylem conducts sap from the root up the plant. The sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well.

Two forces cause xylem sap to flow:

• The soil solution (see soil) is more dilute than the cytosol of the root cells. Thus, water moves osmotically into the cells, creating root pressure. Root pressure is highly variable between different plants. For example, in Vitis riparia pressure is 145 kPa, but it is near zero in Celastrus orbiculatus (Tibbetts and Ewers 2000).
• The main phenomenon driving the flow of xylem sap is transpirational pull. The reverse of root pressure, this is caused by the transpiration of water. In larger plants such as trees, the root pressure and transpirational pull work together as a pump that pulls sap from the soil up to where it is transpired.

Xylem can be found:

• in vascular bundles, present in non-woody plants and non-woody plant parts
• in secondary xylem, laid down by a meristem called the vascular cambium
• as part of a stelar arrangement not divided into bundles, as in many ferns.

Note that, in transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only. The most distinctive cells found in xylem are the tracheary elements: tracheids and vessel elements. However, the xylem is a complex tissue of plants, which means that it includes more than one type of cell. In fact, xylem contains other kinds of cells in addition to those that serve to transport water.

Evolution of xylem

Photos showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips.

Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the Silurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks. The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first-formed xylem) of all living groups of plants. Several groups of plants later developed pitted tracheid cells, apparently through convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the metaxylem (following the protoxylem).

In most plants, pitted tracheids function as the primary transport cells. The other type of tracheary element, besides the tracheid, is the vessel element. Vessel elements are joined by perforations into vessels. In vessels, water travels by bulk flow, like in a pipe, rather than by diffusion through cell membranes. The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the angiosperms. However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae), and their secondary xylem is described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms. Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis being that vessel elements originated in a precursor to the angiosperms and were subsquently lost.

Vessel element

A vessel element is a cell type found in xylem, the water conducting tissue of plant. Vessel elements are typically found in the angiosperms; their absence from the conifers is noteworthy.

Vessel elements are the building blocks of vessels, which constitute the major part of the water transporting system in the plants where they occur. Vessels form an efficient system for transporting water (including necessary minerals) from the root to the leaves and other parts of the plant.

In secondary xylem, a vessel element originates from a fusiform initial in the cambium, at maturity the protoplast dies and disappears, but the lignified cell walls persist. It may be seen as a dead cell, which still has a function, and is still being protected by surrounding living cells.

The cell wall is strongly lignified. At both ends there are openings that connect the individual vessel elements. These are called perforations or perforation plates. These perforations may have a variety of shapes: the most common are the simple perforation (a simple opening) and the scalariform perforation (several elongated openings on top of each other in a ladder-like design). Other types include the foraminate perforation plate (several round openings) and reticulate perforation plate (net-like pattern, with many openings). The side walls will have pits, and may have spiral thickenings.

The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the flowering plants. It was once thought that vessel elements were an evolutionary innovation of flowering plants, but their absence from some basal angiosperms and presence in some members of the Gnetales suggest that this hypothesis must be re-examined; vessel elements in Gnetales may not be homologous with those of angiosperms, or vessel elements may have originated in a precursor to the angiosperms and were subsquently lost in some archaic or "basal" lineages (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae), described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms.

Vessel-like cells have also been found in the xylem of Equisetum (horsetails), Selaginella (spike-mosses), Pteridium aquilinum (bracken fern), and the enigmatic fossil group Gigantopteridales. In these cases, it is generally agreed that the vessels evolved independently. It is therefore not a stretch to believe that vessels may have appeared more than once among the angiosperms as well.

Vascular bundle

Cross section of celery stalk, showing vascular bundles, which include both phloem and xylem

A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in vascular tissue, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will include supporting and protective tissues.

The xylem typically lies adaxial with phloem positioned abaxial. In a stem or root this means that the xylem is closer to the centre of the stem or root while the phloem is closer to the exterior. In a leaf, the adaxial surface of the leaf will usually be the upper side, with the abaxial surface the lower side. This is why aphids are typically found on the underside of a leaf rather than on the top, since the sugars manufactured by the plant are transported by the phloem, which is closer to the lower surface.

The position of vascular bundles relative to each other may vary considerably: see stele.

Secondary xylem

Secondary xylem is formed by a vascular cambium. The two main groups in which secondary xylem can be found are:

1. conifers (Coniferae): there are some six hundred species of conifers. All species have secondary xylem, which is relatively uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is marketed as softwood.
2. angiosperms (Angiospermae): there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem has not been found in the monocots. In the remainder of the angiosperms this secondary xylem may or may not be present, this may vary even within a species, depending on growing circumstances. In view of the size of this group it will be no surprise that no absolutes apply to the structure of secondary xylem within the angiosperms. Many non-monocot angiosperms become trees, and the secondary xylem of these is marketed as hardwood.

Secondary xylem is also found in members of the "gymnosperm" groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta.

Stele

In a vascular plant, the stele is the central part of the root or stem containing the vascular tissue and occasionally a pith. The concept of the stele was developed in the late nineteenth century by P. E. L. van Tieghem as a model for understanding the relationship between the shoot and root, and for discussing the evolution of vascular plant morphology. Now, at the beginning of the twenty-first century, plant molecular biologists are coming to understand the genetics and developmental pathways that govern tissue patterns in the stele.

Protosteles

The earliest vascular plants had both root and shoot with a central core of vascular tissue. This consisted of xylem in the center, surrounded by a region of phloem tissue. Around these tissues there might be an endodermis that regulated the flow of water into and out of the vascular core. Such an arrangement is termed a protostele.

There are three basic types of protostele:

• haplostele - the most basic of protosteles, with a cylindrical core of vascular tissue. This type of stele is the most common in roots.
• actinostele - a variation of the protostele in which the core is lobed. This type of stele is rare among living plants, but is found in stems of the whisk fern, Psilotum.
• plectostele - a protostele in which interconnected plate-like regions of xylem are surrounded and immersed in phloem tissue. Many modern club mosses (Lycopodiopsida) have this type of stele within their stems.

Siphonostele

Plants that produce complex leaves also produce more complex stelar arrangements. The hormones produced by the young leaf and its associated axillary bud affect the development of tissues within the stele. These plants have a pith in the center of their stems, surrounded by a cylinder containing the vascular tissue. This stelar arrangement is termed a siphonostele.

There are three basic types of siphonostele:

• solenostele - the most basic of siphonosteles, with a central core of pith enclosed in a cylinder of vascular tissue. This type of stele is found only in fern stems today.
• dictyostele - a variation of the solenostele caused by dense leaf production. The closely arranged leaves create multiple gaps in the stelar core. Among living plants, this type of stele is found only in the stems of ferns.
• eustele - the most common stelar arrangement in stems of living plants. Here, the vascular tissue in arranged in vascular bundles, usually in one or two rings around the central pith. In addition to being found in stems, the eustele appears in the roots of monocot flowering plants.

Siphonosteles may be ectophloic, with the phloem tissue positioned on one side of the xylem and closer to the epidermis. They may also be amphiphloic, with the phloem tissue on both sides of the xylem. Among living plants, many ferns and some Asterid flowering plants have an amphiphloic stele.

There is also a variant on the eustele found in monocots like maize and rye. The variation has numerous scattered bundles in the stem and is called an atactostele. However, it is really just a variant of the eustele.

References

ISBN links support NWE through referral fees

• Campbell, Neil A. and Jane B. Reece. 2002. Biology, (6th ed.) TK : Benjamin Cummings.
• Carlquist, S. and E. L. Schneider. 2002. The tracheid–vessel element transition in angiosperms involves multiple independent features: cladistic consequences. American Journal of Botany 89:185-195
• Cronquist, A. 1988. "The Evolution and Classification of Flowering Plants". New York, New York : The New York Botanical Garden
• Gifford, Ernest M. and Adriance S. Foster. 1988. Morphology and Evolution of Vascular Plants. (3rd ed.). New York : W. H. Freeman and Company ISBN 0-7167-1946-0
• Kenrick, Paul and Peter R. Crane. 1997. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C. : Smithsonian Institution Press ISBN 1-56098-730-8
• Niklas, Karl J. 1997. The Evolutionary Biology of Plants. Chicago and London: The University of Chicago Press ISBN 0-226-58082-2.
• Schweingruber, F.H. (1990) Anatomie europäischer Hölzer - Anatomy of European woods. Eidgenössische Forschungsanstalt für Wald, Schnee und Landscaft, Birmensdorf (Hrsg,). Haupt, Bern und Stuttgart.
• Tibbetts, T. J. and F. W. Ewers. 2000. Root pressure and specific conductivity in temperate lianas: exotic Celastrus orbiculatus (Celastraceae) vs. native Vitis riparia (Vitaceae). "American Journal of Botany" 87:1272-78
• Timonen, Tuuli. 2002. Introduction to Microscopic Wood Identification. Finnish Museum of Natural History, University of Helsinki.
• Wilson, K. and D. J. B. White. 1986. The Anatomy of Wood: its Diversity and variability. London : Stobart & Son Ltd
• Muhammad, A. F. and R. Sattler. 1982. Vessel Structure of Gnetum and the Origin of Angiosperms. American Journal of Botany 69:1004-1021

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