Wood & Charcoal Anatomy Basics

According to The Wood Handbook:

The fibrous nature of wood strongly influences how it is used. Specifically, wood is composed mostly of hollow, elongate, spindle-shaped cells that are arranged parallel to each other along the trunk of a tree. When lumber and other products are cut from the tree, the characteristics of these fibrous cells and their arrangement affect such properties as strength and shrinkage, as well as grain pattern of the wood (USDA: 2007:2-1).

Hardwoods and Softwoods

Trees are divided into two broad classes, usually referred to as "hardwoods" and "softwoods." Some softwoods, however, are actually harder than some of the hardwood and some hardwoods are softer than softwoods. For example, such softwoods as longleaf pine and Douglas-fir produce wood that is typically harder than the hardwoods basswood and aspen. Botanically, the softwoods are Gymnosperms or conifers which are plants with naked seeds, that is, they are not enclosed in the ovary of the flower. Usually softwoods are cone-bearing plants with needlelike or scale like evergreen leaves. Some conifers such as the larches and bald cypress, however, lose their needles during autumn and winter. Examples of softwoods are pines, spruces, redwoods, and junipers. Click here to view Softwood Key. Botanically, the hardwoods are Angiosperms whose seeds are enclosed in the ovary of the flower. Typically they are plants with broad leaves that, with few exceptions in the temperate region, lose their leaves in fall or during the winter. Most imported tropical woods are hardwoods. Click here to view Hardwood Key.

The phylogeny of 5 woods: Eastern White Pine (Pinus strobus), Bald Cypress (Taxodium distichum), Tulip Poplar (Liriodendron tulipifera), Pawpaw (Asimina triloba), Cherry (Prunus serotina) and Persimmon (Diospyros virginiana).

Gross Structural Features

Planes of Section

Wood is a three-dimensional material, with the cells of tissues oriented in three different planes of view (charcoal) or planes of section (wood). The plane that cuts across the trunk or branch perpendicular to the grain is called the transverse or cross section (XS). The plane that runs longitudinally from the center of the trunk or branch to the bark is called the radial section (R). The plane that is perpendicular to the radial plane and is tangential to the growth rings is called the tangential section (T).

Diagram showing planes of section, bark, growth rings, sapwood and heartwood.

Growth Increments (Rings)

For all ring porous and semi-ring porous hardwoods and some softwoods in Calvert County, there is enough of a difference between the earlywood of one year and the previous latewood to produce annual growth rings visible to the naked eye. For the most part, the age of a tree can be calculated by counting the growth rings either from a stump or from other cross sections of the trunk. The inner part of the growth ring formed first in the growing season is called earlywood and the outer part formed later in the growing season, latewood. Earlywood is characterized by cells having relatively large cavities and thin walls, while latewood cells have smaller cavities and thicker walls. The transition from earlywood to latewood may be noticeable or gradual, depending on the taxon and the growing conditions when it was being formed.

		End grain of Bald Cypress (Taxodium distichum) charcoal showing several growth rings.		End grain of Pawpaw (Asimina triloba) charcoal showing several growth rings.
Schematic diagram of a tree stem cut away to the center, exposing 6 growth sheaths or growth rings (bottom). The top of each sheath shows the height of the tree at the end of each growing season (right). From: Foulger, A.N. 1969. Classroom Demonstrations of Wood Properties. USDA/FPL. (Public Domain)

Earlywood & Latewood

During the course of a growing season, the tree will produce cells early in the season (earlywood) and cells towards the end of the season (latewood). During stress (drought, defoliation, fire) some trees produce so little earlywood that the growth ring disappears into the previous year’s latewood.

Click here to view larger image.		Click here to view larger image.
Cross-section of Yellow Pine Group      (Pinus sp.), showing 2 partial growth       rings.		Cross-section of Trumpet Vine      (Campsis radicans),showing the      growth ring boundary,the      earlywood (top) and the latewood      (bottom) of the previous growth ring.

Sapwood and Heartwood

Sapwood is located between the bark and the heartwood. The sapwood contains both living and dead cells and functions primarily in the storage of food and the mechanical transport of water or sap. The sapwood layer may vary in thickness and in the number of growth rings contained in it. Sapwood commonly ranges from 1-1/2 to 2 inches in radial thickness. As a rule, the more vigorously growing trees have wider sapwood layers. Many second growth trees of merchantable size consist mostly of sapwood.

In general, heartwood is comprised of inactive cells that neither conduct water or store food. Frequently these extractives darken the heartwood and give species such as black walnut and cherry their characteristic color. Heartwood extractives in some species such as black locust, western redcedar, and redwood make the wood resistant to fungi or insect attack. Sapwood of all species, however, is not resistant to decay. In some species, such as the ashes, hickories, and certain oaks, the pores (vessels become plugged to a greater or lesser degree with ingrowths known as tyloses. Heartwood having pores tightly plugged by tyloses, as in white oak, is suitable for tight cooperage, since this prevents the passage of liquid through the pores. Tyloses also make impregnation with liquid preservatives difficult.

End grain view of Red Mulberry (Morus rubra), showing bark.

Four different woods showing heartwood (dark areas) and sapwood (light areas).   Left to right: Persimmon (Diospyros virginiana), Cherry (Prunus serotina), Red   Mulberry (Morus rubra), and Eastern Red Cedar (Juniperus virginiana).

Anatomical Features

Wood Cells

Wood cells make up the structural elements of wood and are composed of various sizes and shapes. Dry wood cells may be empty or part filled with deposits, such as gums and resins. Vessels can be occluded with tyloses, neighboring parenchyma cells that balloon into the vessel, closing it up. The main wood cell types that function in transport and support (are elongated and pointed at the ends) are the fibers and tracheids. In addition to fibers, hardwoods have cells of relatively large diameter that function in the transport of fluids, the vessels (or pores). The transport function is also performed by the tracheids in softwoods and hardwoods. Both hardwoods and softwoods have cells that are grouped into tissues oriented horizontally in the radial direction from the pith towards the bark. These cells conduct fluids radially and are called rays. The rays are most easily seen on end-grain (XS) or quartersawn (R) surfaces. Wood also has other cells, known as longitudinal or axial parenchyma that function mainly for the storage of food, extractives and/or crystals. All of these cell types and their arrangement into tissues, three dimensionally, constitutes the basis for the microscopical description and identification of woods.

Cells Involved (Softwoods vs. Hardwoods

Softwoods

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Diagram showing types of softwood cells and the cambial initials that produced them. The cambial initials are shown from approximately the tangential aspect. The two types of pit arrangement are shown in the tracheids. Modified from Jane, F.W. 1970. The Structure of Wood.

Axial/Vertical Tracheids

Axial tracheids are the main cell type in softwoods, comprising over 90% of cells in the wood. Their radial diameter changes depending on growth rates, the order formed during the growing season (earlywood tracheids are larger than latewood tracheids) and environmental conditions. In some softwoods, like Bald Cypress (Taxodium distichum) and Sequoia (Sequoia sempervirens), the first formed tracheids are always wide enough to develop inter-tracheid pitting that has more than one row of pits and is of diagnostic value.

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Radial section of Bald Cypress (Taxodium distichum) showing biseriate (2-wide) bordered pits (arrows) in the axial tracheids.

Spirals

A few softwoods have spiral thickenings on the walls of their tracheids and these can be diagnostically significant. However, archaeological or degraded wood frequently has spiral checks in the cell walls, appearing like spiral thickenings.

Ray Tracheids

Ray tracheids are like small axial tracheids oriented radially. Like axial tracheids, they have circular bordered pits (CBP), just smaller. The rays tracheids can have smooth walls like White Pine Group or the walls have pointed projections (dentate) within them as in the Yellow and Red Pine Groups. The main difference between spruce and larch is the shape of the cross section CBP in the ray tracheids.

Radial section of White Pine Group (Pinus sp.) showing 1 row of ray tracheids and 5 rows of ray parenchyma cells below.		Radial section of Yellow Pine Group Pinus sp.) showing 3 rows of ray tracheids.

Axial/Vertical Parenchyma

Axial parenchyma in softwoods is usually present diffusely in the latewood part of the growth ring. They usually contain colored contents and are easy to see at low magnifications. In tangential view, the walls separating adjacent cells will either be smooth or nodular.

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Tangential section of Bald Cypress   (Taxodium sp.) showing axial   parenchyma with nodular endwalls.

Ray Parenchyma

Ray parenchyma in softwoods have either smooth end walls or end walls with distinct nodules on them (nodular end walls).

Radial section of Hemlock (Tsuga spp.) showing ray parenchyma cells with nodular end walls.		Radial section of Bald Cypress (Taxodium   distichum) showing ray parenchyma cells with   smooth end walls.

Epithelial Cells of Resin Canals

In many softwood and some hardwoods, cells are arranged axially or radially into canals or ducts which produce resins, tannins and other secondary compounds used in protection against pathogens like insects and fungi. In softwoods, the resin canals are easily seen in both Xs and T views. The can have thick cell walls (epithelia) as in Spruce (Picea sp.) and Larch (Larix sp.) or thin walls as in Pines (Pinus spp.).

Cross-section of Pine (Pinus sp.) showing a single resin duct with thin-walled epithelial cells.		Cross-section of Spruce (Picea sp.) showing 3 resin ducts with thick-walled epithelial cells.

Tangential section of Pine (Pinus sp.) showing a   single resin duct with thin-walled epithelial cells.   Lucky hand section that retained epithelia.		Tangential section of Pine (Pinus sp.) showing   a single resin duct with thin-walled epithelial   cells. Normal hand sections with torn out   epithelia.

Radial section of Hemlock (Tsuga spp.) showing   ray parenchyma cells with nodular end walls.		Radial section of Bald Cypress (Taxodium   distichum) showing ray parenchyma cells with   smooth end walls.

Tangential section of Spruce (Picea sp.) showing   a single resin duct with thick-walled epithelial   cells.		Tangential section of Spruce (Picea sp.)showing   a single resin duct with thick-walled epithelial   cells. (Polarized Light).

Cross-Field Pits (not cells)

When rays intersect with axial tracheids, the cells that are adjacent to each other have intercellular connections called cross-field pits. They are where the axial system and ray system cross and are important diagnostic characters.

Diagrammatic representations of pitting on radial walls of ray parenchyma cells in softwoods. [x - pits in section cut along the x axis; y - pits in section cut along the y axis; cross-field view - pits as seen in surface view in a cross-field] Modified from Jane, F.W. 1970.

The Structure of Wood

Hardwoods

Cell Types

Hardwoods are distinguished form softwoods by having (usually) vessels and fibers in addition to parenchyma and tracheids. A few, primitive hardwoods are vessel-less. The more primitive tracheid is a good conductor of fluids and is good as a support element. Fibers evolved to be a poor conductors, but an excellent supportive elements, while vessels evolved for excellent conduction but poor support. Parenchyma can contain colored compounds or crystals or be segmented and aligned horizontally in a tissue. Vessel elements are connected vertically (along the grain) into vessels, that have perforation plates of various types. All four types of cells have evolved into different sizes, shapes and ornamentations in separate taxa and are the basis of all accurate microscopic identifications.

Diagram showing types of hardwood   cells and the cambial initials that   produced them. The several types of   perforations are shown in the   vessels. Modified from Jane, F.W.   1970. The Structure of Wood.		The evolution of fibers and vessel   members from tracheids (division of   labor/function).

Tyloses

In some hardwoods, during heartwood formation, vasicentric parenchyma balloon into the interior of adjacent vessels elements. The tyloses efficiently block the unused vessels as passage ways for pathogens like insects and fungi. The presence of tyloses in white oak allows it to be used as “tight cooperage” that will not leak fluids (wine, whiskey, beer, water, etc.), as opposed to red oak that is used as “slack cooperage” for dry goods. Tyloses can appear small or large, with thick or thin walls and is a good diagnostic character when used properly.

Radial section of American Black    Walnut (Juglans nigra) showing    tyloses.		Radial section of Black Locust (Robinia     pseudoacacia) showing tyloses.

Perforation Plates

Vessels are distinguished from tracheids by having perforations at their ends to allow for efficient transport of fluids. These openings can be completely open (simple perforation) or can have bars (ladder rungs) across it (scalariform perforation). The bars can be few and thick or many and thin and are of diagnostic value.

Radial section of Wax Myrtle (Myrica cerifera)   showing 2 vessels, 1 with a simple perforation   and another with a scalariform perforation.

Inter-vessel (IV) Pits

The connections between adjacent vessel members are called inter-vessel (IV) pits. They are easily seen in tangential view, except when the vessel distribution is solitary. They vary in size from the limits of most optical microscopes (2µ) to linear pitting that nearly spans the vessel (to 50µ).

Tangential section of Birch (Betula sp.)    showing minute inter-vessel pits (2 – 4µ).		Tangential section of Maple (Acer sp.) showing    inter-vessel pits of medium size (5 – 10µ).

Tangential section of Walnut (Juglans sp.)   showing large inter-vessel pits (arrowheads)   of 10-16µ.

IV. Arrangement

The arrangement of IV pits can be opposite each other, alternating horizontally or composed of long openings, spanning the vessel member (linear or scalariform).

Tangential section of Tulip Poplar (Liriodendron tulipifera) showing opposite inter-vessel pits (arrows).		Tangential section of Maple (Acer sp.) showing alternate inter-vessel pits arrows).

Tangential section of Red Gum (Liquidambar styraciflua) showing linear inter-vessel pits arrowheads).

Ray-Vessel (RV) Pitting

The connections between vessel elements and adjacent ray parenchyma cells are called ray-vessel (RV) pits. It can be the same as the IV pitting or much larger (salicoid) to linear.

Radial section of Elm (Ulmus sp.) showing ray-vessel pits.

Spirals

Some hardwoods have spiral thickenings on the walls of their vessel members. A few taxa have spirals that are limited to vessel element tips. Some spirals can be thin and evenly arranged, not passing through inter-vessel (IV) pitting, like Maple (Acer sp.). Other spirals are thicker and coarsely laid down and do pass through IV pitting like Cherry (Prunus sp.), Basswood (Tilia sp.) and Holly (Ilex sp.). However, archaeological or degraded wood frequently has spiral checks in the cell walls, appearing like spiral thickenings.

Fine Spirals (Maple)		Coarse Spirals (Cherry & Basswood)
Tangential section of Maple (Acer sp.) showing fine spirals.		Tangential sections of Cherry    (Prunus serotina) showing  coarse    spirals.

Although softwoods are composed mostly of tracheids, hardwoods can have both vascular tracheids (like softwoods) and vasicentric tracheids that surround vessels in woods like Oak.

Tangential section of Oak (Quercus sp.) showing vasicentric tracheids (arrowhead) with circular bordered pits.

Fibers

The main type of supporting cell in hardwoods is the libriform fiber. These cells can have partitions within them (septate) or not (non-septate). There is also a group of cells (fiber tracheids) that are intermediate in structure between fibers and tracheids. These cells can also be categorized as septate or non-septate.

Cross section of Oak Group (Quercus sp.), showing moderately thick walled fibers.		Cross section of Black Locust (Robinia pseudoacacia) showing extremely thick walled fibers.

Axial Parenchyma

Parenchyma strands that are oriented vertically, or with the grain, within the wood are called axial parenchyma. They are grouped into two broad categories; apotracheal parenchyma and paratracheal parenchyma. Apotracheal means “without vessels” and is axial parenchyma that does not touch any vessels in cross section. Types of apotracheal parenchyma include diffuse, reticulate or broken banded and banded. Paratracheal means “around the vessels” and is axial parenchyma that either completely or partially surrounds vessels in cross section. Types of paratracheal parenchyma include vasicentric (“surrounding vessel”), aliform (winged) and confluent (coalesced wings).

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Diffuse                                       Reticulate                                  Banded

Diagram of different types of apotracheal parenchyma in hardwoods, as seen in cross-section.
Vessels are represented by circles, rays by vertical lines and parenchyma by red areas.           

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Vasicentric                                     Aliform                                       Confluent   

Diagram of different types of paratracheal parenchyma in hardwoods, as seen in cross-section.
Vessels are represented by circles, rays by vertical lines and parenchyma by red areas.           

Rays

Rays are composed of parenchyma cells in hardwoods and of parenchyma and ray tracheids in softwoods. In hardwoods, rays are categorized by cell width and heterogeneity. Rays can be one cell wide in tangential view (uniseriate) or many cells wide (multiseriate). They can be of just one type of cell (square or procumbent), called homocellular, or it can have both procumbent cells in the body of the ray and one to many rows of squares or upright cells along the periphery (heterocellular).

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Uniseriate Rays Multiseriate Rays               Homocellular Rays Heterocellular Rays

Different types of rays in hardwoods as seen in tangential, radial and cross sections.

Radial section of Maple (Acer sp.) showing a homocellular ray composed exclusively of procumbent cells.		Radial section of Tulip Poplar    Liriodenron tulipifera) showing a    heterocellular ray composed of    procumbent cells (top half of figure)    and uprights (arrowheads).

Radial section of Willow (Salix spp.)   showing a heterocellular ray   composed of procumbent cells   (bracket) and uprights (arrowheads).