In
vascular plantVascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the ferns, clubmosses, flowering plants, conifers and other gymnosperms...
s,
xylem is one of the two types of transport tissue,
phloemIn vascular plants, phloem is the living tissue that carries organic nutrients , particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
being the other. The word "xylem" is derived from classical
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
ξυλον (
xylon), "wood", and indeed the best-known xylem tissue is
woodWood is an organic material; in the strict sense wood is produced as secondary xylem in the stems of trees . In a living tree it transfers water and nutrients to the leaves and other growing tissues, and has a support function, enabling woody plants to reach large sizes or to stand up for themselves...
, though it is found throughout the plant. Its basic function is to transport water.
Physiology of xylem
The xylem is responsible for the transport of water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. Xylem
sapSap is a fluid transported in xylem cells or phloem sieve tube elements of a plant. Fluid found in the vacuoles of other cells is sometimes referred to as "cell 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 by maturity and no longer have living contents. Two phenomena cause xylem sap to flow:
- Transpirational pull
Transpirational pull is the main phenomenon driving the flow of water in the xylem tissues of large plants.-Mechanisms:Transpirational pull results ultimately from the evaporation of water from the surfaces of cells in the interior of the leaves. This evaporation causes the surface of the water to...
: the most important cause of xylem sap flow is the evaporationEvaporation is the vaporization of a liquid and the reverse, of condensation. A type of phase transition, it is the process by which molecules in a liquid state spontaneously become gaseous . Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed...
of waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
from the surfaces of mesophyll cells to the atmosphere. This transpirationTranspiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf surfaces are dotted with openings called stoma that are bordered by guard cells. Collectively the structures are called stomata...
causes millions of minute menisci to form in the mesophyll cell wall. The resulting surface tensionSurface tension is a property of the surface of a liquid. It is what causes the surface portion of liquid to be attracted to another surface, such as that of another portion of liquid .Applying Newtonian physics to the forces that arise due to surface tension accurately predicts many liquid behaviors...
causes a negative pressure or tension in the xylem that pulls the water from the roots and soil.
- Root pressure
Root pressure: osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day...
: If the water potentialWater potential is the potential energy of water relative to pure free water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects including surface tension. Water potential is measured in...
of the root cells is more negative than the soilSoil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. It is composed of particles of broken rock that have been altered by chemical and...
, usually due to high concentrations of solute, water can move by osmosisOsmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential to an area of low water potential...
into the root from the soil. This causes a positive pressure that forces sap up the xylem towards the leaves. In some circumstances, the sap will be forced from the leaf through a hydathodeA hydathode is a type of secretory tissue in leaves, usually of Angiosperms, that secretes water through pores in the epidermis or margin of leaves, typically at the tip of a marginal tooth or serration. They probably evolved from modified stomata...
in a phenomenon known as guttationGuttation is the appearance of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses. Guttation is not to be confused with dew, which condenses from the atmosphere onto the plant surface.- Process :...
. Root pressure is highest in the morning before the stomata open and allow transpiration to begin. Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in Vitis ripariaVitis riparia Michx, also commonly known as River Bank Grape or Frost Grape, is a native American climbing or trailing vine, widely distributed from Quebec to Texas, and Montana to New England. It is long-lived and capable of reaching into the upper canopy of the tallest trees...
but around zero in Celastrus orbiculatus.
Anatomy of xylem
Xylem can be found:
- in vascular bundle
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...
s, present in non-woody plants and non-woody parts of plants with wood
- in secondary xylem, laid down by a meristem
A meristem is the tissue in all plants consisting of undifferentiated cells and found in zones of the plant where growth can take place....
called the vascular cambiumThe vascular cambium is a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem and the secondary phloem , and is located between these tissues in the stem and root...
in woody plants
- as part of a stelar arrangement
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 French botanists P. E. L. van Tieghem and H. Doultion as a model for understanding the relationship...
not divided into bundles, as in many fernA fern is any one of a group of about 20,000 species of plants classified in the phylum or division Pteridophyta, also known as Filicophyta. The group is also referred to as Polypodiophyta, or Polypodiopsida when treated as a subdivision of tracheophyta...
s.
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
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
found in xylem are the tracheary elements:
tracheidTracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...
s and
vessel elementA vessel element is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are typically found in the angiosperms but absent from most gymnosperms such as the conifers....
s. 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
parenchymaParenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, from Greek parenkhuma, visceral flesh, from parenkhein, to pour in beside : para-, beside + en-, in + khein, to pour.- In animals :The parenchyma are the...
, in addition to those that serve to transport water.
The branching pattern exhibited by xylem has been shown to follow
Murray's lawMurray's law, or Murray's principle is a formula for relating the radii of daughter branches to the radii of the parent branch of a lumen-based system...
.
Primary and secondary xylem
Primary xylem is the xylem that is formed during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. It is distinguished by wider vessels and tracheids. As it develops, the xylem can become
endarchEndarch is a botanical term that refers to primary xylem maturation in which the protoxylem is internal in the stem to the metaxylem . The protoxylem is therefore found on the inner side of the bundle . This means that the xylem tissue develops from the outside towards the centre of the plant stem...
or exarch.
Secondary xylem is the xylem that is formed during secondary growth from
vascular cambiumThe vascular cambium is a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem and the secondary phloem , and is located between these tissues in the stem and root...
. Although secondary xylem is also found in members of the "
gymnospermGymnosperm is a group of spermatophyte seed-bearing plants with ovules on scales, which are usually arranged in cone-like structures....
" groups
GnetophytaThe plant division Gnetophyta or gnetophytes comprise three related families of woody plants grouped in the gymnosperms. The gnetophytes differ from other gymnosperms in having vessel elements as in the flowering plants....
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
The term softwood is used to describe wood from conifers. It may also be used to describe these trees, which tend to be evergreen, notable exceptions being bald cypress and the larches....
.
- 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
The term 'hardwood' is used to describe wood from angiosperm trees . It may also be used for those trees themselves: these are usually broad-leaved; in temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen.Hardwood contrasts with softwood which...
.
Evolution of xylem
Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the
SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
(more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian period and is followed by the Silurian period...
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, it seems, 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 elementA vessel element is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are typically found in the angiosperms but absent from most gymnosperms such as the conifers....
. Vessel elements are joined by perforations into vessels. In vessels, water travels by
bulk flow, as 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.,
AmborellaceaeAmborellaceae is a family of flowering plants endemic to New Caledonia. The family consists of only a single species, Amborella trichopoda. It is currently accepted by plant systematists as the most basal lineage in the angiosperms clade.-Description:...
,
TetracentraceaeTetracentraceae is the botanical name for a family of flowering plants. Such a family has been recognized by many taxonomists.The APG II system, of 2003 , does not formally use this family; it assumes the plants involved to belong to family Trochodendraceae...
,
TrochodendraceaeTrochodendraceae is the botanical name for a family of flowering plants.The APG II system, of 2003 , also recognizes this family: it is unplaced as to order and left among the basal lineages of the eudicots...
, and
WinteraceaeThe Winteraceae are a family of flowering plants. The family includes 120 species of trees and shrubs in 9 genera.The Winteraceae are a mostly southern-hemisphere family associated with the Antarctic flora, found in tropical to temperate climate regions of Malesia, Oceania, eastern Australia, New...
), and their secondary xylem is described by
Arthur CronquistArthur John Cronquist was a North American botanist and a specialist on Compositae. He is considered one of the most influential botanists of the 20th century, largely due to his formulation of the Cronquist system. Two plant genera in the aster family have been named in his honor...
as "primitively vesselless". Cronquist considered the vessels of
GnetumGnetum is a genus of about 30-35 species of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms they possess vessel elements in the xylem....
to be convergent with those of angiosperms. Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis states that vessel elements originated in a precursor to the angiosperms and were subsequently lost.
See also
- Cohesion-tension theory
- Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
- Secondary growth
In many vascular plants, secondary growth is the result of the activity of the vascular cambium. The latter is a meristem that divides to produce secondary xylem cells on the inside of the meristem and secondary phloem cells on the outside...
- Transpirational pull
Transpirational pull is the main phenomenon driving the flow of water in the xylem tissues of large plants.-Mechanisms:Transpirational pull results ultimately from the evaporation of water from the surfaces of cells in the interior of the leaves. This evaporation causes the surface of the water to...
- Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
- Vascular bundle
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...
General references
recent update of the classic book on xylem transport by the late Martin Zimmermann