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Wood Characteristics (Laboratory Assignment)

For the following exercises the wood samples can be viewed with the unaided eye, a 10x or 20x hand lens, and/or stereo-zoom microscope. The slides for exercises #9 and #13 can be viewed though the monocular microscopes.

Samples identified by (H-#) refer to wood samples from Hough, Romeyn B. The American Woods, Exhibited by Actual Specimens. Lowville, NY: Published and Sections Prepared by the Author, 1893, 2 volumes. These irreplaceable specimens are extremely fragile. Please handle them by the cardboard mounts, using great care.

Please do not touch the wood sections.

Make a note of what magnification (10x, 20x, 50x, etc.) you employ in distinguishing each pertinent feature.

References

The American Woods: Exhibited by actual specimens and with copious explanatory text , Volumes I-XIV, by Romeyn B. Hough, Loweville, New York.

Radial, tangential, and cross-sections of 350 North American woods from the fourteen-volume rare book The American Woods: exhibited by actual specimens and with copious explanatory text, published 1888-1910 by the author, Romeyn Beck Hough. The images can be accessed by volume number or by the scientific or common name of each tree. Available online from the Special Collections Department of the NCSU Libraries.

Plant Anatomy Laboratory: Micrographs of plant cells and tissues, with explanatory text. James D. Mauseth, Section of Integrative Biology, School of Biological Sciences at The University of Texas.

This web site is being developed as supplemental material for people studying plant anatomy. Its objective is to provide light micrographs of the types of cells and tissues that students typically examine in a plant anatomy course. All micrographs are accompanied by figure legends to help the viewer interpret and understand the structures presented. Wherever possible, the microscope slides that were photographed were obtained from companies such as Triarch or Carolina Biological so that they will be similar to slides that students are examining in their college courses. This web site is designed to complement a plant anatomy course, whether that is offered through a college or through individual study at home. The descriptions here emphasize objects and concepts that might arise as a person examines samples of plant tissues, and theoretical topics are given less attention.

1 Structure of Wood

Because of the arrangement of the layers of growth in the tree, as well as the vertical or horizontal orientation of the individual cells, it is appropriate to consider the structure of wood in three-dimensional terms. Wood is typically milled, purchased, worked, and analyzed with reference to these specific surfaces. However, scientific and commercial terms for these vary. The lumber industry and woodworkers may employ commercial terms with special reference to the grain. The scientific terms are listed below with some of their equivalent commercial terms:

Scientific (section) Commercial (grain) Illustration (Hough) Anatomy
transverse (cross-section) end
radial (through the pith) side
radial, edge, vertical
quarter, comb, rift
tangential (parallel to the pith) side
flat, plain, slash, side

These sections cuts are planar. An exception to this is veneer, which may involve slicing, peeling or sawing wood into tangential sheets. Radial and tangential cuts will exhibit figure, a distinctive appearance on a longitudinal wood surface resulting from anatomical structure, irregular coloration, or defects.

  • Examine transverse, radial and tangential sections of various species in the Hough collection to understand the geometry and physiology of the characteristic features of each specie and the surface appearance of these sections.
Typical "softwood" (coniferous), red pine, Pinus resinosa, Ait.
Top to bottom: transverse; radial, tangential. Hough pl.19.
Typical "hardwood" (deciduous), red oak, Quercus rubra, L.
Top to bottom: transverse; radial, tangential. Hough pl.15.


2 Growth Increment and Grain

The term grain may be used sometimes to indicate the degrees of contrast between earlywood and latewood of a growth ring (or growth increment), the layer of xylem or phloem added to the stem in a given growth period. In temperate zones, one layer is added per yearly growth period and is often termed annual ring.

Earlywood (popularly termed "springwood") is the first-formed portion of the growth ring, often characterized by larger cells and lower density.

Latewood (popularly termed "summerwood") is the portion of the growth ring formed after the latewood, often characterized by smaller cells or higher density. These features are sometimes termed springwood and summerwood, but these terms should not be employed because ring growth is not always defined by these seasons.

The grain may vary from being uneven grain (great contrast) to even-grained (little contrast). Intermediates can be indicated by compound modifiers such as fairly even or moderately uneven.

Using these terms compare and describe the grain characteristics of the following species with reference to the “Cards of Wood” samples:

__________________ Ohio buckeye, Aesculus glabra
__________________ beech, Fagus grandifolia
__________________ aspen, Populus sp.
__________________ black walnut, Juglans nigra
__________________ Douglas fir, Pseudotsuga menziesii

__________________ Which plane or surface is exhibited by most of these samples?

 

Growth ("annual") rings

Transverse section of wood of Juniperus ashii (juniper, a conifer). Being a conifer or softwood, this juniper has wood whose axial system contains only tracheids. Parts of five annual rings are present in this micrograph; the center annual ring is wide with many cells in each row because the vascular cambium was active for a long time due to good growing conditions. The annual rings produced just before and just after the center ring are much narrower due to poor growing conditions in those years and the vascular cambium produced fewer cells.

Fig. 15.1-4, Plant Anatomy Laboratory, James D. Mauset, Integrative Biology, University of Texas

3 Density

Density (weight/unit volume) is the single most important indicator of strength in wood and may therefore predict such characteristics as hardness, ease of machining, and nailing resistance. Dense woods generally shrink and swell more, and usually present greater problems in drying.

Specific gravity (below: SG) is often termed density index and is stated as the ratio of the weight of a given volume of wood to the same volume of water.
Compare the relative size of the growth rings of the following species to determine whether a visual analysis of woods can indicate the relative density of woods.

Note the average width of a growth ring by measuring those in a 5cm or 10cm section and obtain an average. Again, be very careful with the samples. Do not touch samples with the ruler. Do not employ any sharp object, pens, or pencil to aid in counting. Place ruler on surrounding cardboard, not on the wood.

_____________ tamarack, Larix americana, Michx., SG:0.2636, (H-23)
_____________ cottonwood, Populus monolifera, Ait., SG:0.3889 (H-48)
                           [Populus deltoides, Marsh.]
_____________ chestnut, Castanea vesca, var. Americana, Michx.,
                           SG:0.4504, (H-40) [Castea Dentata, Borkh.]
_____________ red cedar, Juniperus Virginia, L., SG:0.4926, (H-25)
_____________ black locust, Gleditschia triacanthos, L., SG:0.6740,(H-28)
_____________ beech, Fagus ferruginae, Ait., SG:0.6883 (H-16)

  • Does the relative size of the growth rings of various species determine their relative density?
  • Which section did you use to measure the growth rings?

4 "Hardwood" and "Softwood"

Learn About Woods, Highland Hardwoods, Inc.

Hardwood is a popular term for wood produced by broad-leaved trees in the botanical group referred to as angiosperms (covered seeds). Softwood is a popular term for wood produced by coniferous trees in the botanical group referred to as gymnosperms (naked seeds).

The distinction between hardwood and softwood actually has to do with plant reproduction. All trees reproduce by producing seeds, but the seed structure varies.

Hardwood trees are angiosperms, plants that produce seeds with some sort of covering. This might be a fruit, such as an apple, or a hard shell, such as an acorn.

Softwoods, on the other hand, are gymnosperms. These plants let seeds fall to the ground as is, with no covering. Pine trees, which grow seeds in hard cones, fall into this category. In conifers ("cone-bearing") like pines, these seeds are released into the wind once they mature. This spreads the plant's seed over a wider area.

For the most part, angiosperm trees lose their leaves during cold weather while gymnosperm trees keep their leaves all year round. So, it is also accurate to say evergreens are softwoods and deciduous trees are hardwoods.

Compare the specific gravity of the following species (from exercise #4) and comment on the appropriateness of the terms “hardwood” and “softwood” which respectively describe these species.

_______ cottonwood, Populus monolifera, Ait., (H-48)
               [Populus deltoides, Marsh.]
_______ red cedar, Juniperus Virginiana, L., (H-25)

Comments:

 

 

5 Sapwood and Heartwood

Sapwood (duramen) is the physiologically active wood comprising one to many outermost growth rings. Some nonliving prosenchyma cells are active in conduction and the living parenchyma cells also store food. The heartwood (alburnum) is the central core of wood in mature stems where the prosenchyma cells cease to conduct sap and the parenchyma cells die. At one time heartwood was sapwood but it no longer conducts sap or has living cells (parenchyma cells).

In most species, extractives impart a darker color to heartwood. Various inclusions are collectively termed extractives, or extraneous material, which are not part of the wood substance but are deposited in cell lumina (cavities) and cell walls. These are compounds of varying chemical compositions such a gums, fats, resins, sugars, oils, starches, alkaloids, and tannins. The term is based on their possible (at least partial) extraction with water or neutral organic solvents. The proportion of extractives varies from less than 1% (e.g., poplar) to more than 10% (e.g., redwood) of oven-dry weight of wood.

The extractives make the heartwood portion much more fungal resistant than the sapwood. The heartwood may be denser, more dimensionally stable, slower to dry, and more difficult to impregnate with chemical preservatives. The basic strength of the wood is not affected by sapwood cells changing into heartwood cells.

A significant aspect of heartwood extractives is color, for the sapwood of all species ranges from whitish or cream to perhaps yellowish or light tan. However, the heartwood of various species may exhibit very distinctive colors.

The color of the heartwood tends to darken with age due to oxidation and other factors.
Examine the following species to note (when possible) the difference between the sapwood and heartwood color. Record the characteristic color of the heartwood based on the radial section.

__________________ black walnut, Juglans nigra, L. (H-35)
__________________ red cedar, Juniperus virginiana, L. (H-25)
__________________ black spruce, Picea nigra, Poir., (H-20) [red spruce, Picea rubens, Sarg.]
__________________ white pine, Pinus strobus, L.H. (H-49)
__________________ poplar, Populus grandidentata, Michx., (H-18)

  • Do you think the common and scientific name of a wood is determined by the color of the wood or by the character of other features: leaves, bark, etc.?
  • Does the common - and, at times, botanical - name of each specie accurately describe the color of the heartwood?
  • What is the difference between the color of the heartwood in transverse and radial sections?

 

6 Growth Rings and their Relation to Sapwood and Heartwood

As the girth (the measure of anything cylindrical in form) of the tree increases with the addition of new sapwood, the diameter of the heartwood zone also expands proportionally. The width of the sapwood is characteristic in some species, or at least relatively consistent. Some retain only one or two growth rings of sapwood while mature specimens of another specie may retain as many as one hundred. While the sections in Hough do not include all rings in either the heartwood or sapwood of a particular tree, they do exhibit the relationship of these two features.

Examine the relative proportion of growth rings in the heartwood and sapwood in the following species:

  • chestnut, Castanea vesca, Var. americana, Michx., (H-40, with two sap rings visible); 3-4 sapwood growth rings
  • black walnut, Juglands nigra, Poir., (H-35, with ten sap rings visible); 10-20 sapwood growth rings
  • tupelo, Nyssa multiflora, Wang., (H-9, with over fifty sap rings visible); 80-100 sapwood growth rings
  • red cedar, Juniperus virginia, L., (H-25, with approximately twenty-five sap rings visible)

Growth-Ring Size and Nutrients

Growth-ring size in nature is typicaly determined by moisture available, site conditions, canopy cover and other competing trees.

In managed forests, growth — and the resulting increase in growth-ring size — is enhanced through fertilizers, here "biosolids".

Consider what affect this has on the quantity, and the quality, of wood — and its preservation.

 

7 Texture in Gymnosperms

Gymnosperms (naked seeds), which are sometimes termed “softwoods” are mostly conifers (conus/ferre: cone bearing). Most of the cells (90-95% by volume) found in coniferous wood are tracheids: fiberlike cells about 100 times longer than they are in diameter, with an average lengths range from 2mm to about 6mm. Diameters of tracheids range from 20 to 60 microns (1 micron equals 0.001 mm) and serve as the basis for classifying texture in wood.

Texture ranges from being:

    1. coarse-textured, through
    2. medium-textured, to
    3. fine-textured.

With a 10x lens, texture can be estimated on the basis of how clearly individual tracheids can be seen in cross section. Texture, a valuable aid in wood identification, is also related to surface smoothness and finishing qualities.

Tracheids in longitudinal section

Longitudinal section of pine wood (Pinus). All cells shown here are tracheids, and all the circular structures are circular bordered pits. The outer white area is known as the margo, the inner dark area is the torus. The margo is an area where the pit membrane is very thin, partially digested away in some species such that water passes relatively easily through it. The torus, in contrast, is a thickened area that can seal off the pit if one of the tracheids becomes filled with air.

Fig. 7-2-6, Plant Anatomy Laboratory, James D. Mauset, Integrative Biology, University of Texas

 

The evenness of grain is determined by the overall variation from the earlywood (large diameter and thin-walled tracheids, being well suited for conduction) and latewood (thicker-walled, smaller diameter tracheids, best serving as structural support). There may be a corresponding increase in density. Evenness of grain affects abrasion performance and color reversal when stained.

Examine in theHough samples with a stereo-zoom microscope. Due to the condition of these samples some of these features are difficult to detect. Describe the texture and evenness of grain found in the following species:

__________________ red cedar, Juniperus virginiana, L. (H-25)
__________________ white pine, Pinus strobus, L.H., (H-49)
__________________ hemlock, Abies [Tsuga] canadensis, Michx., (H-21)
__________________ pitch pine, Pinus rigida, Miller, (H-50)

8 Resin Canals in Coniferous Wood

 

Some coniferous species have resin canals, tubular passageways lined with living parenchyma (epithelial)cells, which exude resin or “pitch” into the canals.

Growth ring in softwood

Transverse section of wood of Thuja occidentalis (American arbor-vitae, a conifer or softwood). The double-headed arrow indicates a single thick annual ring. Rays are narrow and rather far apart, and the axial system of the wood consists of just tracheids, with no fibers (that is why conifers are called “softwoods”) and no vessels. The latewood tracheids make up a relatively narrow band of darker red cells–they are dark because their secondary walls are thick and therefore stain intensely. Earlywood tracheids make up almost all the annual ring, and they have such thin secondary walls they do not take up enough stain to be dark red.

Fig. 15.1-2, Plant Anatomy Laboratory, James D. Mauset, Integrative Biology, University of Texas

Resin canals in softwood

Transverse section of wood of pine (Pinus). Pine wood is slightly more complicated than that of Thuja because it has abundant resin canals. Consequently, it has parenchyma not only in its rays but also in the axial system, the part of the wood produced by the fusiform initials of the vascular cambium.

Fig. 15.1-3, Plant Anatomy Laboratory, James D. Mauset, Integrative Biology, University of Texas

Especially in sapwood, resin is quite fluid and will flow to the surface, bleeding though paint films on finished woodwork. Proper kiln drying will set the resin adequately to minimize bleed-out problems.
Resin canals are found in four genera of the conifers, all within the family Pinaceae: Pinus (pine), Picea (spruce), Larix (larch) and Pseudotsuga (Douglas fir). Examine the following species for the presence and characteristics of resin canals:

__________________ white pine, Pinus strobus, L.H., (H-49)
__________________ black spruce, Picea nigra, Poir., (H-20)
__________________ pitch pine, Pinus rigida, Miller, (H-50)

9 Drawing of a Coniferous Wood

Detailed microscopic analysis will reveal many physiological features at the cellular level.

Select a prepared slide of a softwood and draw both:

    1. a transverse and
    2. tangential or radial view.

Use the “Wood Identification: Visual Analysis” sheets provided. Identify and label as many features as you can. Employ the various reference books provided. Cite each reference you use, giving the author, date, and page or plate/illustration number.

Note: If you do not have time in class to finish this exercise you may complete it in the Historic Preservation office by appointment.

10 Angiosperms

Angiosperms (covered seeds), sometimes termed “hardwoods” are mostly deciduous. Angiosperms evolved later than gymnosperms and are characterized by having many more types of cells and more variation in their arrangement than those in softwoods.

Vessel elements are extremely large diameter, thin-walled cells which form in the hardwood trees in end-to-end vertical arrangement and, when their end walls are lost, form continuous “pipelines” ideal for sap conduction. Vessels vary in size among and within species and are associated with fibers: small diameter, thick walled cells, poorly suited for conduction but contributing to the strength of wood.

Transverse cuts (across the grain) reveal the open end of the vessels, sometimes referred to as a pore. As all hardwoods possess vessels they may be termed porous; softwoods are consequentially termed non-porous.

The size, number, and distribution of vessels and fibers are determining factors in the appearance and uniformity of hardwoods.

  1. Ring-porous woods have the pores concentrated in the early pores, causing pronounced uneven grain (in the case of ash, permitting the wood to be separated for basketry and other woodwork).
  2. Diffuse-porous woods have pores distributed fairly evenly.
  3. Semi-ring-porous or semi-diffuse-porous woods exhibit no distinct zoning between early and late wood.

Due to the condition of these samples some of these features are difficult to detect. Examine the species below and describe the arrangement of the pores in each:

__________________ black walnut, Juglands nigra, Poir., (H-35)
__________________ white ash, Fraxinus americana, L., (H-10)
__________________ chestnut, Castanea vesca, Var. americana, Michx., (H-40)
__________________ sugar maple, Acer saccharinum, Wang., (H-7)
__________________ butternut, Juglans cinerea, L., (H-14)
__________________ paper birch, Betula papyacea, Ait., (H-43)

11 Tyloses in Deciduous Woods

In some hardwoods, as the transition from sapwood to heartwood takes place tyloses (outgrowths of ray parenchyma cells into the vessel element interior appearing as froth- or bubble-like structures) form in the lumens (cavities) of the vessel elements, usually in conjunction with heartwood formation.

Whether a wood forms tyloses when vessels become air-filled generally is related to the size of the interconnections (pits) between vessels and ray parenchyma. If the vessel-ray parenchyma pits are relatively "large" then tyloses occur, if the vessel-ray parenchyma pits are relatively "small" then tyloses will not occur. They may be absent, sparse, unevenly distributed, or numerous. Woods without tyloses retain their porous nature, while woods densely packed with tyloses prohibit the passage of liquid.

Examine these species and indicate where tyloses are present.

__________________ black locust, Gleditschia triacanthos, L., (H-28)
__________________ black cherry, Prunus serotina, Ehrh., (H-29)
__________________ white oak, Quercus alba, L., (H-38)
__________________ red oak, Quercus rubra, L., (H-15)

  • Of the two Quercus species which would be better for barrel staves. Why?

Tyloses
Transverse section of Robinia pseudoacacia (black locust or false acacia). Most of the vessels here are plugged with tyloses. They had cavitated at some point, and subsequently the adjacent paratracheal parenchyma cells sealed them with tyloses. This plugging is not confined to just this level of the vessels; if a longitudinal section were available, almost all the length of the vessel would be seen to be plugged.

Fig. 15.3-14, Plant Anatomy Laboratory, James D. Mauset,
Integrative Biology, University of Texas

 

12 Rays in Deciduous Woods

Hardwoods contain rays, flattened bands of tissue composed of parenchyma cells, extending horizontally in a radial plane through the tree stem. These may contribute to forming planes of structural weakness. In drying wood, stresses often create checks (separation of wood cells along the grain). The weakness of the rays may also provide a natural cleavage plane to assist in splitting, roughing out, or riving woodwork. When rays are characteristic on a radial surface it is termed ray fleck.

Examine and describe the rays and ray fleck (when possible) of the following species.

__________________ sugar maple, Acer saccharinum, Wang., (H-7, 7a, 7b)
__________________ beech, Fagus ferruginea, Ait., (H-16)
__________________ sycamore, Platanus occidentalis, L., (H-13)
__________________ cherry, Prunus serotina, Ehrh, (H-29)

Rays of living maple

Transverse section of wood of maple (Acer). This material was collected from the sapwood of a living tree, so the ray cells (arrows) were alive and filled with starch. If it had been collected from heartwood or from a dead piece of lumber, the cells would have been empty. In this particular sample, there are very few axial cells with any starch in them.

Fig. 15.3-2, Plant Anatomy Laboratory, James D. Mauset, Integrative Biology, University of Texas

13 Drawing of a Deciduous Wood

Beside vessels, the other types of longitudinal cells in hardwoods are of uniformly small diameter and difficult or impossible to examine with a hand lens. But because of their difference in cell-wall thickness, masses of them are distinguishable. Fiber masses usually appear darker; tracheids and parenchyma cells are lighter. However, detailed microscopic analysis will reveal many physiological features at the cellular level.

Select a prepared slide of a hardwood and draw both (1) the transverse and (2) the tangential or radial view.Use the “Wood Identification: Visual Analysis” sheets. Identify and label as many features as you can. Employ the various reference books provided. Cite each reference you use, giving the author, date, and page or plate/illustration number.