In vascular plants, xylem is one of the two types of transport tissue in plants, phloem being the other one. The word "xylem" is derived from classical Greek ξυλον (xylon), "wood", and indeed the best known xylem tissue is wood, though it is found throughout the plant.
Physiology of xylem
The xylem transports sap from the root up the plant. The xylem is mainly responsible for the transportation of water and mineral nutrients throughout the plant. Xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well. This transport is not powered by energy spent by the tracheary elements themselves, which are dead at maturity and no longer have living contents. Two phenomena cause xylem sap to flow:
- Transpirational pull: the most important cause of xylem sap flow, is caused by the evaporation of water from the surface mesophyll cells to the atmosphere. This transpiration causes millions of minute menisci to form in the cell wall of the mesophyll. The resulting surface tension causes a negative pressure in the xylem that pulls the water from the roots and soil.
- Root pressure: If the water potential of the root cells is more negative than the soil, usually due to high concentrations of solute, water can move by osmosis into the root. This may cause a positive pressure that will force sap up the xylem towards the leaves. In extreme circumstances the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is most common in the morning before the stomata open and cause transpiration to begin. Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus .
Anatomy of xylem
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, such as parenchyma, in addition to those that serve to transport water.
Primary and Secondary xylem
Primary xylem, also known as Protoxylem, is the xylem that is formed during primary growth from procambium.
Secondary xylem, also known as Metaxylem, is the xylem that is formed during secondary growth from vascular cambium. Secondary xylem is also found in members of the "gymnosperm" groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta. The two main groups in which secondary xylem can be found are:
- 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.
- 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 seco
ndary xylem within the angiosperms. Many non-monocot angiosperms become trees, and the secondary xylem of these is marketed as hardwood.
Evolution of xylem
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 subsequently lost.
- Campbell, Neil A. and Jane B. Reece. 2002. Biology, 6th ed., published by 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 (link to abstract here).
- Cronquist, A. 1988. The Evolution and Classification of Flowering Plants. New York, New York: The New York Botanical Garden.
- Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
- Muhammad, A.F. and R. Sattler. 1982. Vessel Structure of Gnetum and the Origin of Angiosperms. American Journal of Botany 69: 1004-1021 (available online here).
- Tyree, Melvin T. Tyree and Martin H. Zimmermann. Xylem Structure and the Ascent of Sap, 2nd ed. (ISBN 3-540-43354-6) recent update of the classic book on xylem transport by the late Martin Zimmermann