Lectures 10 & 11

Xylem – Wood Structure

I.  General Information

A. Hardwood (dicotyledons). Dicot woods are strong, heavy, dense, with many fibers. Example: Ilex opaca.

B. Softwoods (gymnosperms). Gymnosperm wood is generally homogeneous with long, straight elements, many tracheids, few fibers. Example: Thuja occidentalis.

II. Conifer Wood

A. Tracheids
1.  conifers lack vessels; have imperforate tracheary elements (= tracheids). Fig. 9.1 showing a diagram of the wood of Thuja occidentalis. Transverse, radial and tangential sections of pine (Pinus strobus) wood in Fig. 9.2.
2.  libriform fibers absent
3.  tracheids 2-5 mm long, sometimes branched indicating intrusive growth.

B. Pits
1.  Early wood with circular bordered pits (Fig. 8.5) with circular apertures. Example: Thuja occidentalis; Late wood with oval apertures, thicker walls. Examples: Sequoia sempervirons (photo), Pinus strobus (photo).
2.  Pitting location on tracheids: on radial walls in growth layers, some on tangential walls in late wood.

C. Axial parenchyma.  May or may not be present (it is seen in Podocarpaceae, Cupressaceae).  May be associated with resin ducts.

D. Resin ducts – in both axial and radial systems. Fig. 9.3 of Pseudotsuga menziesii with two resin ducts and thick walled epithelial cells (example: Pinus strobus). Photo1 and Photo2 of Pseudotsuga menziesii wood showing resin ducts.

E. Rays.
1. Generally uniseriate or biseriate (Pseudotsuga). If resin duct is in ray, makes it appear multiseriate (Pinus strobus).
2.  Rays may be composed of parenchyma that are alive at maturity (Abies balsamea) plus tracheids that are not alive (photo, Taxodium).  These tracheids resemble parenchyma but have no protoplast at maturity, have secondary walls with bordered pits, often occur near margins of rays
3. Pits bordered: tracheids / axial tracheid and tracheid / ray tracheid.  Pits half bordered: tracheids / ray parenchyma. Examples: Pseudotsuga menziesii (photo), Thuja occidentalis (photo), Sequoia sempervirens (photo).
4. Ray crossfield pitting – pits in conducting cells (and fibers) where they border ray cells.  Fig. 9.3 of Larix laricina. Seen as a rectangle in radial section.  Ray cells have primary pit fields adjacent to the pits of the axial cells.  Rarely do ray cells have secondary walls but it occurs in gymnosperms, esp. Pinus. In pine, the pits on the ray cells are huge.  They look like windows and hence are called fenestriform pits.  Two ray cells in radial section, each with one fenestriform pit. Also in pine and other gymnosperms there are tracheids that are part of the rays.  They are called ray tracheids and are oriented at right angles to the tracheids in the axial system.  They occur on the borders of the rays.


III.  Dicot Wood

A.  More complex than gymnosperm wood (except primitive angiosperms like Drimys). Contains tracheids as well as vessels, fibers. Fig. 9.4 is a diagram of Liriodendron tulipifera wood. Photo of X.S. of Liriodendron showing early wood and late wood.

B.  Storied and non-storied wood
1. Seen in tangential section, rays and axial system may show seriation. If the rays line up in horizontal rows, wood is called storied.  Examples: Aesculus, Cryptocarya, Diospyros (Fig. 9.7B), Ficus, Tilia, Fabaceae.
2. If they overlap but don’t appear in horizontal rows, non-storied. Examples: Castanea, Fraxinus, Juglans, Quercus, Salix.  Examples: Salix nigra (Fig. 9.5), Quercus alba (Fig. 9.6), Carya illinoinensis (Fig. 9.7A), Ulmus americana (photo), Prunus serotina (photo),
3.  Storied woods derived from vascular cambium with short fusiform initials.
4.  Intermediate forms between storied and non-storied wood can occur

C. Distribution of vessels
1.  Vessels = pores in common language
2.  Types
a. Ring porous - early wood pores are abundant, large diameter; late wood pores are narrower and fewer. Examples: Castanea, Catalpa, Celtis, Fraxinus, Gleditsia, Morus, Quercus (Example: Fig. 9.6, photo Q. velutina), Robinia, Ulmus americana (photo),
b. Diffuse porous - vessel diameter and abundance constant through growth ring. Examples: Acer, Betula, Carpinus, Fagus, Juglans, Liriodendron, Platanus, Populus, Pyrus, and Salix.  Example: Salix nigra (Fig. 9.5).
c. Intermediate or semi-ring porous - vessel diameter progressively decreases during growing season
d.  Ring-porous wood conducts water ca. 10 times faster than diffuse-porous wood

D. Vessel (pore) groupings.  Seen in cross-section, the arrangement of vessels in relation to each other. Such patterns are used in identifying wood.
1. Solitary: vessels do not touch each other, are surrounded by other types of cells. Example: Albizia julibrissin.
2. Multiple: two or more pores together
a. Radial chains of vessels - uniserate chain of vessels in x-section. Example: Aesculus glabra.
b. Pore clusters - aggregate of vessels. Example: Liriodendron, Magnolia tripetala.
c. Oblique chains of vessels
d. Tangential chains of vessels
e. Tangential bands. Example: Ulmus americana.
f.  Dendritic patterns - branched groupings that are inter-connected. 

E. Imperforate elements
1. tracheids, fibers (fiber tracheids and libriform fibers), these make up the matrix of the wood and vary from wood to wood in their composition
2. elongate greatly and push into other cells, they are intrusive in their growth, this determines the hardness of the wood
3. gymnosperms lack fibers in wood
4. most dicots have fibers; some are primitively fiberless; others have lost them secondarily

F. Axial parenchyma (= AP). Tangential section of Maclura showing axial parenchyma cells. Can be seen on all sections to a limited extent, but based on X.S. one can classify in two categories: apotracheal and paratracheal.
1. Apotracheal – AP not associated with vessels. Figure 11.17 (modified from Evert). From Esau: Fig. 9.8, Fig. 9.9
a.  Diffuse singly. Example: Alnus [the parenchyma shows up as dark dots in the wood]
b.  Diffuse in aggregation. Example: Agonandra brasiliensis.
c.  In bands – may or may not be at margin of growth ring. Marginal may be in early wood (initial parenchyma) or late wood (terminal parenchyma). Example: Carya [the parenchyma shows up as green bands in the wood]
2. Paratracheal - AP associated with vessels
a.  Scanty – scattered, usually solitary, next to vessels. Example: Dillenia pulcherima, Acanthocereus (a cactus), Tilia.
b.  Vasicentric – surround the vessel. Example: Piptadeniastrum africanum, Ailanthus.
c.  Aliform (with wings) – AP surrounds the vessel and extends to either side in wings. Example: Microberlinia brazzavillensis.
d.  Confluent – wings connect adjacent vessels. Example: Peltogyne confertiflora.
e.  Boundary banded – in growth ring, forms sheath around vessels and narrow bands that connect with other vessels. Example:  Fraxinus

G. Rays
1. Ray classification in tangential section
a.  Uniseriate ray. Example: Salix nigra (Fig. 9.5), Ginkgo biloba (photo),
b.  Biseriate ray. Example: Sassafras variifolium (photo).
c.  Multiseriate ray (may be up to 50 cells wide, may have uniseriate wings). Both uniseriate and multiseriate rays can occur in one wood, such as Quercus alba (Fig. 9.6), Fagus grandifolia (photo), Platanus (photo),
e. Aggregate rays (grouped together) – may have fibers between rays
2. Ray classification in radial section. Cell types based on shape
a. Upright cells. Square to rectangular in shape, vertically elongated. Examples: Liriodendron, Drimys has mostly upright cells,
b. Procumbent cells.  Horizontally elongated. Example: Ilex.
c.  Homocellular (homogeneous) = just procumbent cells 
d.  Heterocellular = procumbent plus upright cells. Example: Liriodendron
e.  Heterogeneous = homocellular and heterocellular rays in one plant, 
3. Derived from ray initials; may be ephemeral; may form from conversion of fusiform initials to ray initials; may also get bigger in this way.
4. As with conifers, have cross-field pitting. These pits are highly modified, where the axial elements meet the rays.
5. Function of rays is to move (often stored) materials horizontally in the wood.  Radial transport of assimilates, esp. carbohydrates, to the activated cambium in the spring. Ray parenchyma cells can become sclerified as shown in this Zygogynum (Winteraceae) wood.

H. Tyloses (Fig. 9.10)
1.  Axial and ray parenchyma – forms bubble-like outgrowths into vessel members through the pit cavities.  This inactivates the vessel in terms of conducting water serves to strengthen and protect it. 
2. Ray cell produces a tylose-forming layer (also called the protective layer) – see Fig. 9.10 which is composed of polysaccharides and pectins.  Seals off the entire wall facing the vessel member. 
3.  Pit membrane degrades, tylose baloons out, into the vessel member lumen, nucleus and cytoplasm can migrate in.
4.  Tyloses can store ergastic substances, develop secondary walls, and differentiate into sclereids. This occurs when cells are being converted to heartwood.
5.  Tyloses are in hardwoods, rarely in softwoods. Examples: Quercus (white oaks), Robinia, Fraxinus, Catalpa, Juglans, Maclura.

IV.  Secondary Xylem Development

A.  Auxin and other influences on growth rings
1. The intermittent activity of the vascular cambium strongly affects the nature of the growth rings.
2. Auxin is high when spring wood is forming, low when late wood is formed. Differentiating vascular tissue is a source of auxin (not just meristems and leaves).
3. Work with Populus and Pinus shows that pressure is important in proper cambial growth and development and differentiation of xylem.  Disjunctive parenchyma and disjunctive tracheids - cells disrupted in shape by an expanding vessel which forces cells apart if there is no space.
4. Growth ring width is affected by water, mineral supply, and climate. Studies of these factors in the field of Dendrochronology (Wiki page here). Studies of Fraxinus stems showed growth rings most affected by water potential (more than IAA and GA).  

B.  Reaction wood. See treedictionary.com web page here.
1.  Formed on branches (and roots) under stress (e.g. gravity) and endogenous growth substances (plant hormones). Cells become smaller, thicker, lignified walls.
2.  Conifer wood.  Forms on lower sides of branches, called compression wood. Formation associated with high [auxin] - article on topic here.  Wood darker, denser, tracheids shorter, cell walls rounded in X.S., lignified, S3 (innermost) secondary wall layer missing. SEM photo.
3.  Dicot wood. Forms on top sides of branches, called tension wood. Formation associated with low [auxin].  Vessels reduced in width and number, fibers have thick, refractive inner layer = gelatinous layer (labeled G in this photo) consisting of cellulose. 2-4 layers in total. A nice SEM photo showing the G layer shrinking compared to other wall layers in a fiber.
4. See also: Donaldson, L. A. (Phytochemistry 2001) entitled "Lignification and lignin topochemistry — an ultrastructural view" HERE.

C.  Wood Identification
1.  Age of wood (twig vs. trunk) and environmental conditions (such as presence of reaction wood) affects the way wood appears in section.  Good to obtain samples from multiple individuals to account for variability.
2.  Species in a genus may have few wood characters that could be used to separate them.
3.  Characters
a.  Color
b.  Grain.  Straight, wavy, spiral, interlocked, irregular.
c.  Texture. Relative size and size variation in elements in growth ring (coarse, fine, even)
d.  Figure. Patterns seen in longitudinal section (decorative types)
e.  Doing a Google search on "wood grain texture" shows just how much variation there is in the above characters!
3.  Discussion of differences between woods in Esau (pp. 139-142) very detailed.  Best to code these in a matrix!
Last updated: 12-Oct-22 / dln