Lectures 8 & 9

Xylem 

I.  General Information

A. Xylem is the major water conducting tissue, but also important in supporting the plant. It is a complex tissue that includes more than one type.

B. Together with phloem it makes up the vascular tissue.  This tissue first evolved in the lycophytes.

C. Primary xylem develops from the procambium (apical meristem of Syringa) and is later produced by the cambium.
• transverse section of Medicago stem. Closer view, labeled.
• transverse section of Rubus stem
• transverse section of Helianthus stem
• Secondary xylem develops from the vascular cambium (transverse section of Pinus stem
We will see more about primary xylem below.

D. No secondary growth in annuals and most monocots.

II.  Secondary Xylem Structure

A.  Axial and radial systems.  Axial system = tracheary elements and fibers and parenchyma. Ray system = parenchyma cells of the rays (and some tracheids in conifers)
1.  Review of four planes of section. Diagram.  Actual wood of Acer saccharinum.  What planes of section are the three panels?
a.  Transverse (= cross section, XS)
b.  Tangential (TS)
c.  Radial (RS)
d.  Paradermal (PS)
2.  Block diagram and secondary vascular tissues (Fig. 8.1)
a.  Transverse plane.  Cells in XS.  Growth rings formed in woody plants with seasonal growth.  Note early wood (secondary xylem), late wood, rays, vessels. Example: Quercus velutina
b.  Radial plane.  One sees the axial system, where most cells are seen in their longest dimensions. One also sees rays whose cells look like a brick wall. Example: Quercus velutina.
c.  Tangential plane.  Also shows axial system with cells in their longest dimensions.  Rays seen "end on", appearing fusiform. Example: Quercus velutina.

B. Growth Layers. Fraxinus 3 year old stem XS unlabeled. Labeled.
1. Growth layer appears as rings when stem is cut in XS. 
2. The ring is really just the contrast between cells of different XS diameters. The widest cells are formed in the spring and these contrast with the narrow diameter cells from previous season (late wood)
3.  Some tropical trees do not form distinct growth rings

C.  Sapwood and heartwood (Photo of Taxus from Wikipedia)
1.  Sapwood contains the functional xylem where water moves
2.  Heartwood is non-functional.  Basically, the "dumping grounds" where materials are moved, by the rays, from other parts of the plant.

III.  Cell types in secondary xylem

A. Tracheary Elements (sometimes called conducting sclerenchyma). Esau, Figs. 8.2, 8.3.
1.  Composed of tracheids and vessel members.  When strung together, vessel members form a long tube called a vessel.
2.  Cells are non-living at maturity, lignified, with pitted walls.
3.  Main function is to move water
4.  Comparison of tracheids and vessel members.  Magnolia wood maceration showing vessel members, tracheids and fibers. 
Tracheids Vessel Members
longer shorter
narrower in diameter wider in diameter
pointed overlapping end walls transverse ends walls with less overlap
long chain of cells chain of vessel elements called the vessel
imperforated end walls (no holes) Perforated end walls
conduct water and support   conduct water only
found in all vascular plants found in most angiosperms, absent in most gymnosperms

4.  Perforation plates.  Openings in the walls of vessels (tracheids are imperforate). Simple (as in this Euphorbia or Zea) or multiperforate which includes scalariform (as in this Magnolia) and reticulate.

5.  Pits.  Recall structures from Chapter 4 (Fig. 4.7, 4.9).  Pit pairs exist as three types based upon what type of cell wall occurs in the two adjacent cells:
a.  Simple, Bordered, Half bordered
b. Figure 8.4 (lower part) shows the appearance of the pits with different secondary wall configurations for the participating cells.
c.  Bordered pits in conifer tracheids.  Fig. 8.5. Review terms: aperture, border, margo, torus, pit membrane, pit cavity, crassula.  Bordered pit in transverse section of Pinus early and late wood. Another view of the same showing the torus stuck to one side of the pit (aspirated state).
d.  Pits can form between vessel elements, tracheids, fibers and parenchyma cells in all possible combinations except one - tracheids and fibers.  Those pits involving a parenchyma cell will usually be a half-bordered pit because the parenchyma cell only has a primary wall.  But, sometimes parenchyma cells CAN have a secondary wall, and then the pits can be bordered.

B.  Fibers
1.  Primary function is support.
2.  Long cells, lignified secondary walls which vary in thickness. Either dead or alive at maturity.
3.  Two types: fiber tracheids and libriform fibers.  Quercus suber maceration showing vessel member (V), fiber-tracheids (Ft) libriform fibers (F), and parenchyma (arrow). [from ResearchGate, Helena Pereira]
a. Fiber tracheids are generally shorter and with a thinner wall than libriform fibers. Fiber tracheids have bordered pits with cavities smaller than tracheids or vessel elements from same wood. The pit pairs have circular outer apertures and slit-like inner apertures (Fig. 8.4, N, O).
b.  Libriform fibers have no pit cavity and the pit pair is simple (no border).  In reality, no sharp boundary between fiber tracheids and libriform fibers.
c.  Fibers can be septate and retain a protoplast. These store materials (like xylem parenchyma in function and difficult to distinguish between them). Gelatinous fibers form in reaction wood.

C.  Evolutionary specialization of tracheary and fiber cells
1.  Tracheids gave rise to vessels and fibers.
2.  Vessels originated and specialized first in secondary xylem, then late primary xylem (metaxylem), and finally in early primary xylem (protoxylem).
3.  Plants lacking vessels.
a.  Lacking in nearly all gymnosperms. The exception is Gnetales, an order containing three families, each with only one genus: Ephedra, Gnetum, and Welwitschia.
b.  ANITA clade and Magnoliales. In Drimys (Winteraceae) wood (tangential section) shows abundant tracheids but no vessels, as well as lots of axial parenchyma.
c.  Some parasites, mycoheterotrophs, and succulents
4.  Tracheary element evolutionary trends
a.  Fig. 8.4 (upper part) shows trend from scalariform to opposite to alternate pit types. Scalariform is shown here in Pteridium, bracken fern and Lycopodium, a lycophyte. Opposite can be seen in Zea or even an irregular pitting type.  The alternate type is shown in Carpinus.
b.  Tracheid evolutionary trend generally followed that of vessels. Tracheids are shorter, pitting in parallel with vessels, no increase in width
c.  Fibers - increased wall thickness, decrease in width, pits elongated --> circular, borders reduced, then lost. Trend is generally Tracheids -> Fiber Tracheids -> Libriform Fibers
d.  Fibers are shorter in more advanced plants (because of length of fusiform cambium initials). But fibers have intrusive growth (grow in length after they are formed), so they are longer than vessels.

D.  Parenchyma cells
1.  Two kinds of parenchyma: axial and ray
2.  Function is to store starch, oils, ergastic substances such as tannins, crystals.
3.  Walls may develop secondary thickenings and become lignified. Then, these cells can form bordered, half-bordered, or simple pits.  They can develop into sclereids.
4.  Axial parenchyma is derived from fusiform cambial cells. The resulting parenchyma cell may stay fusiform in shape or divide forming a parenchyma strand (Fig. 8.2 K).  But, they don't have intrusive growth like fibers.
5.  Radial parenchyma.  Rays can be uniseriate or multiseriate.  Viewed in radial section, the rays can be composed of upright and procumbent cells (heterocellular) or just procumbent (homocellular).  The upright cells generally occur on the upper and/or lower margin of the ray.
IV.  Primary Xylem

A.  Same cell types as seen in secondary xylem (tracheids, vessel members, fibers, parenchyma) - BUT - derived from the procambium at the apical meristem, not a vascular cambium.

B.  Primary xylem is not organized as axial and radial systems (e.g., no rays)

C.  Present in stems, leaves, flowers. Xylem and phloem organized in vascular bundles (Fig. 8.7). Area between vasc. bundles is called medullary rays (part of ground tissue)

D.  Protoxylem and metaxylem (vascular bundle XS in Helianthus)
1.  Protoxylem differentiates in parts of the primary plant where growth has not ceased. Composed of tracheary elements and parenchyma.
2.  During maturation, protoxylem is subjected to stresses (stretching) that influences the cell anatomy.  In some cases, protoxylem maturation means the cells are destroyed. In roots protoxylem may persist longer.
3.  Protoxylem is formed first, then metaxylem, but there is intergradation so the two are not always easy to tell apart.  This XS of the stem of Ipomoea batatas shows protoxylem transitioning into metaxylem. The vessel has a simple perforation plate. At the bottom is a protoxylem vessel with a partial arc of its helical wall showing.
4.  Metaxylem also initiated in growing plant parts but it matures after elongation has mostly stopped. Contains tracheary elements and parenchyma as well as fibers.
5.  Parenchyma in metaxylem may be in rows or stands radially, thus resembling rays (which form from secondary growth).
6.  Metaxylem may remain functional in mature plant organs or it may be destroyed.

E.  Cell walls in primary tracheary elements (Fig. 8.8)
1.  Progressively more secondary wall material is laid down on the primary wall resulting in:
a. annular (rings, not connected) - in protoxylem. Example: Zea mays.
b. helical (spiral) - in protoxylem. Example: Pastinaca.
c. scalariform (ladder-like, with coils connected) - in metaxylem. Example: Pteridium. Narrow tracheids in Lycopodium.
d. reticulate (netted) - in metaxylem
e. pitted (with holes) - in metaxylem
2.  Some cells may have more than one type of secondary wall
3.  Openings in the reticulum of the secondary wall may resemble pits seen in secondary xylem.
4.  Annular and helical thickenings are first to be deposited on protoxylem. These types don't inhibit the stretching that occurs upon growth and elongation. Example of Zea with annular rings stretched. In metaxylem, less stretching so the wall coils of helices can be less steep.
5.  Differentiating primary and secondary xylem not always easy. The last formed primary xylem is generally longer than the first formed secondary xylem.
6. Example (on Wikipedia): Primary xylem in a stem of Sambucus (Adoxaceae). Overall view 100X magnification. Closer view 400X magnification. Identify the protoxylem to metaxylem. Do their secondary cell wall thickenings vary ?

F.  Differentiation of tracheary elements
1.  Diagram of the stages in the development of a vessel member from a parenchyma cell (tracheary element precursor) in the protoxylem (Fig. 8.9).
a.  Secondary wall deposits begin
b.  Protoplast begins disintegration, as does the primary walls at the future pore site.
c.  Protoplast hydrolyzed, pores fully open at both ends
2.  ER and Golgi are abundant near area where new secondary wall is forming (Fig. 8.10)
3.  Microtubules, at first evenly dispersed, later concentrate near sites for secondary wall thickenings (Fig. 8.11)
4.  Details of the hydrolytic process.
a. enzymes that destroy the primary wall appear to originate in vacuole.
b. Primary wall not protected by secondary wall is digested.  Side walls just a fine network of cellulose microfibrils (Fig. 8.12A).
c. Fig. 8.12B shows the end walls completely gone.  Wall at site of future perforation is different (Fig. 8.12C).
d. TEM views of a vessel member potential perforation (Fig. 8.13) and later as a mature perforation plate (Fig. 8.14).
e. Hydrolysis of the non-lignified primary walls is followed by the incorporation of a glycine-rich protein (GRP1.8) into the wall.  This is what makes the wall flexible and able to expand greatly upon organ elongation.
5.  Polar flow of auxin is involved in tracheary element morphogensis.
6.  Tracheary element differentiation has been studied in vivo using Zinnia elegans (Asteraceae) mesophyll cells. A number of cytological, biochemical, and molecular markers have been identified for this system. The commitment to cell death, at the transition from Stage II to III involves brassinosteroids. This page shows photos from a study by Mao et al. (2006) where the microtubules were lit up by anti-tubulin immunofluorescence.

Last updated: 10-Oct-22 / dln