Dr. T logo BIO 410/510 Plant Anatomy
Vascular tissue: Xylem


Functions: water and mineral transport, support

Locations in the primary plant body

•  Stem

Helianthus stem, x.s.
Cross section of Helianthus (sunflower) stem with major tissues labeled
Micrograph by John Tiftickjian

•  In stems of seed plants, xylem and phloem make up vascular bundles.

•  Different arrangements occur in seedless vascular plants.

•  Root

Ranunculus root, x.s.
Cross section of Ranunculus root
Micrograph by Biodisc

•  In most seed plants, the root vascular system comprises a single strand near the center, with xylem in the center of this strand.

•  Leaf

Ligustrum leaf, x.s.
Cross section of a typical dicot leaf. Note upper and lower epidermis, palisade and spongy mesophyll, veins, stomata.
Micrograph by John Tiftickjian

•  In the leaf, the vascular bundles are called veins.

•  Each vein contains xylem and phloem.

•  In seed plants xylem is usually on the upper (adaxial) side of the vein; phloem on the lower (abaxial) side.

Mechanism of transport: cohesion/tension

•  Tracheary elements

•  Transport of water in the xylem occurs in elongate cylindrical cells known as tracheary elements.

•  These cells form long tubes specialized to carry water efficiently

•  The tube-like form can be easily seen in longitudinal sections of vascular bundles.

Helianthus stem, l.s.
Helianthus stem, l.s. showing xylem, phloem, and fibers
Micrograph by John Tiftickjian

•  Tracheary elements make up most of the mass of secondary xylem (wood).

Quercus wood
Transverse, radial, and tangential sections of wood oak wood
Micrograph by Biodisc

•  Transport depends on water potential gradient

•  Water potential = solute potential + pressure potential

•  Solute potential is due to the presence of solutes in the solution within a cell.

•  Solute potential is zero for pure water, becoming lower (negative) as solute concentration increases

•  Pressure potential (turgor pressure) is the pressure that the cell wall exerts on the fluid inside the cell.

•  At atmospheric pressure, the pressure potential is zero

•  For parenchyma cells, pressure potential is a positive number (unless the cell is plasmolyzed).

•  Pressure potential can be negative (usually only in xylem)

•  Water always moves down the water potential gradient, that is from higher water potential to lower water potential.

Water potential gradient
Water flows down a water potential gradient
Introduction to Plant Phyiology, Copyright John Wiley and Sons

•  Development of water potential gradient in the xylem

•  (1) Water evaporates from leaf (transpiration)

•  (2) solutes in leaf cells become more concentrated and turgor decreases. Solute potential and pressure potential of leaf cells have both decreased.

•  (3) tracheary elements now have higher water potential than leaf cells so water moves by osmosis from xylem to leaf cells.

•  (4) pressure potential in xylem decreases and becomes negative and tension is transmitted trough xylem of stem to root

•  (5) tension in root xylem reduces water potential below that of root parenchyma cells and water enters xylem from root cells by osmosis

•  (6) water enters root cells from soil by osmosis.

•  Summary of cohesion-tension mechanism

Xylem transport occurs because a series of events occurs that result in a water potential gradient being formed from the leaves, through the xylem, to the roots. Move the cursor over the numbered steps to reveal what is happening in each location in order for the water potential gradient to develop.

•  So flow of water depends on:

•  Tension developed by osmosis.

•  Cohesion and adhesion of water molecules

•  These forces are possible because of water's ability to form numerous hydrogen bonds.

•  Cohesion - attraction of water molecules to each other

•  Adhesion - attraction of water molecules to cell walls

•  Cohesive and adhesive forces must be strong enough to keep water columns intact. If water columns break (cavitation), then water flow stops.

•  Structure of the xylem cells must be such that they:

•  Resist collapse of xylem cells under tension

•  Provide an open channel for water to move through

•  Present as little resistance to water movement as possible

•  Prevent the spread of air bubbles if cavitation occurs

•  Structure of tracheary elements is a compromise related to these physiological requirements.

•  As tracheary element diameter increases:
•  Resistance to water flow decreases - GOOD
•  Tension that can be maintained decreases (cavitation more likely) - BAD
•  As cell wall thickness increases:
•  Resistance to collapse increases - GOOD
•  More energy expended to produce wall materials - BAD
•  Vessel elements completely remove their end walls (discussed below)
•  Resistance to water flow decreases - GOOD
•  If cavitation happens, all cells in the whole vessel stop working - BAD

Cell types of xylem

•  Tracheary elements

•  These are the cells that actually transport water

•  Always dead when mature. The lumen provides a tube that water can move through.

•  Tracheids

•  Present in all vascular plants

•  Elongate cells with tapering end walls

Pinus wood
Transverse, radial, and tangential sections of pine wood
Micrograph by Biodisc

•  Water moves from from cell to cell through:

•  Areas of cell lacking secondary wall (primary xylem)
•  Or through pits (primary and secondary xylem)
•  Pits connecting two tracheids are usually bordered
•  Pits are concentrated at end walls.
•  Tracheids have continuous cell walls (no perforations)

•  Vessel members

•  Found in nearly all angiosperms and rarely in a few other taxa.

•  Evolved from tracheids

•  Generally shorter and wider than tracheids

•  End walls are less sloped (nearer to horizontal)

•  Vessel members stack up in vertical series called vessels.

Vessel
Drawing from lecture comparing tracheids and vessels

•  Vessels in cross section and longitudinal views

Quercus wood
Transverse, radial, and tangential sections of wood oak wood
Micrograph by Biodisc

•  Perforation plates can be clearly seen in a whole vessel member from macerated wood.

Quercus vessel member
Quercus (oak) wood maceration showing a vessel member.
Micrograph by John Tiftickjian
•  Perforations are complete openings (not pits)
•  Before cell dies, end walls are digested to produce perforations.
•  Perforation plates may be simple or compound.

•  Parenchyma

•  Cells are living

•  May contain secondary cell wallls

•  This is unusual for parenchyma cells in other locations, but common in xylem, especially secondary xylem

•  Fibers

•  Provide support

•  Narrower than tracheids

•  Have very thick secondary walls

•  Have few pits or pits are lacking

•  Fiber-tracheids

•  Intermediate between tracheids and fibers

•  Have some pits (may transport some water)

•  Libriform fibers (wood fibers)

•  Have very few pits

•  Provides support but do not transport water

Cavitation and embolisms

•  When gas bubbles form in a xylem element, they may coalesce into larger bubbles. This bubble formation is called cavitation.

•  The large bubbles caused by cavitation are called embolisms.

•  Embolisms cause a loss of tension and stop water transport in any tracheids or vessels in which they occur.

•  Embolisms happen during times of water stress when xylem tensions are high.

•  Mechanisms of reducing the effects of embolisms.

•  Restricting spread of embolisms - pits (discussed below)

•  Production of new xylem to replace older non-functioning xylem

Pits and cavitation control

•  Remember that pits are openings in the secondary wall between two adjacent cells.

•  Simple pits

Simple pit
A simple pit seen in cross-sectional view.

•  The pit appears as a simple hole in face view.

•  Pits help prevent spread of embolisms.

•  Because of their small diameter, it is difficult for air to pass through from one cell to the next.

•  The high surface tension resist the air bubble deforming enough to squeeze through the opening.

•  Bordered pits are most effective in preventing embolism spread.

•  Bordered pits

•  Secondary wall arches over forming a pit chamber. In face view, this area appears as a border around the inner aperture.

Bordered pit
A bordered pit seen in cross-sectional view.
Drawing by John Tiftickjian

•  Outer pit aperture is wide and gives more surface area for water to move through

•  Inner pit aperture is narrow making it difficult for air to pass through.

•  Bordered pits in conifers are even more elaborate. The pit membrane is swollen to form the torus.

Bordered pit with torus
Drawing of bordered pit with torus, common in conifers. The middle of the pit membrane is modified to form the torus.
Drawing by John Tiftickjian

•  When an embolism forms in the tracheid on one side of the pit, the tension of the water on the uncavitated side pulls the torus against the border blocking the flow of air into the tracheid that is still functional.

Aspirated bordered pit
The valve-like action of the torus in a bordered pit prevents the spread of cavitation.
Drawing by John Tiftickjian

•  Micrographs of bordered pits (with tori) in tracheids

•  Face view

l
Bordered pits, face
Circular bordered pits of pine tracheids seen in face view
Micrograph by John Tiftickjian

•  Cross sectional view

Bordered pits, section
Bordered pits of pine tracheids seen in cross sectional view
Micrograph by John Tiftickjian

Primary xylem

•  Protoxylem

•  Primary xylem that differentiates before growth is complete

•  Tracheary elements have relatively little secondary wall (allows for stretching)

•  Cells have relatively small diameters

•  Usually begins on inside or outside of procambium strand.

•  Metaxylem

•  Primary xylem that differentiates after growth is complete

•  Tracheary elements have extensive secondary wall (stretching not required)

•  Cells have larger diameters than in protoxylem

•  Xylem differentiation in the horizontal plane

•  Exarch. Protoxylem outside metaxylem.

•  Exarch xylem in dicot root

Casparian strips
Ranunculus young root x.s. showing Casparian strips.
Micrograph by John Tiftickjian
•  Exarch xylem is common in roots of most plants and in stems of some seedless vascular plants.

•  Endarch. Protoxylem inside metaxylem.

•  Exarch xylem is common in stems of angiosperms. The protoxylem is on the side the vascular bundle that contacts the pith.

•  Mesarch. Protoxylem surrounded by metaxylem.

•  Mesarch xylem is not common in seed plants, but can be found in certain lower vascular plants.

•  Direction of xylem differentiation

Xylem differentiation
Drawing from lecture showing acropetal xylem and phloem differentiation. In typical stem, differentiation is endarch (seen in cross section)

•  Patterns of secondary wall thickening

•  Secondary wall is deposited in various patterns

•  From first-formed to last-formed the sequence is:

Xylem wall patterns
Drawing from lecture showing successive patterns of secondary wall deposition in primary xylem elements.br>

•  Protoxylem cells usually annular and helical

•  Metaxylem cells usually helical through pitted

•  Example of helical wall thickening in corn stem

Zea stem, l.s.
Longitudinal section of a Zea (corn) stem. Helical thickening of secondary wall on a xylem tracheary element is clearly visible. Sieve tubes visible to left of xylem.
Micrograph by John Tiftickjian

Evolution of xylem cells from tracheids

•  Vessel member and fibers have both evolved from tracheids.

•  Primitive vascular plants have only tracheids in their xylem. They serve two functions: water transport and support.

•  From this starting point, there are two major evolutionary trends.

•  One trend is toward modifications that increase efficiency of water transport - leading to vessel members.

•  The second trend is toward modifications that increase support - leading to fibers

•  Steps in the two evolutionary pathways:

Xylem evolution
Diagram showing presumed evolution of both fibers and vessel members from tracheids.

Secondary xylem

•  Secondary xylem develops from the vascular cambium - a lateral meristem.

•  The vascular cambium (or just "cambium") is the lateral meristem that produces both secondary vascular tissues - xylem and phloem.

•  Secondary xylem = wood

•  Secondary growth is a characteristic of woody plants.

•  Some herbaceous (non-woody) plants have a small amount of secondary growth.

•  Understanding secondary xylem requires an understanding of how lateral meristems work, so we will postpone detailed discussion of secondary xylem until after secondary growth in general has been covered.

Delta State University > DSU Sciences > Dr. Tiftickjian > Plant anatomy > Xylem