WOOD ANALYSIS

Introduction

Wood is encountered, in one form or another, in connection with a variety of crimes. Its use as evidence is limited primarily to three areas: (1) physical matches between broken pieces of wood; (2) as a substrate carrying toolmarks; and (3) as small pieces or particles of trace evidence. Other questions may be raised in special circumstances. In the first two categories the wood is only incidental. Toolmark examiners routinely perform physical matches and toolmark comparisons on a variety of materials. Although wood may present some special challenges, because of its fibrous nature, a knowledge of wood structure and anatomy is usually unnecessary to perform a successful comparison in these cases. Experience in interpreting physical matches and toolmarks and in recording the results are the primary knowledge needed for these types of examination. However, a background in wood technology is useful, as illustrated by the investigations by Arthur Koehler into the ladder used in the kidnapping of the Lindbergh baby. Koehler was able to trace pieces of the handmade wodden ladder back, not only to the mills that sawed the wood, but to the lumberyards which sold some of the boards, based on his intimate knowledge of wood and the lumber industry in all their aspects. After Richard Hauptman was identified as a suspect, Koehler was able to locate a board in his attic which compared in all respects to one of the rails from the ladder. Typical forensic wood examinations are not this elaborate. Most involve the identification of small pieces of wood or sawdust, and normally the only comparison that can be made is to complete accurate identifications of both the questioned and known wood samples. Exceptions occur when the wood is painted, impregnated or coated with something which permits further comparisons to be conducted. In a recent homicide investigation, detectives discovered that the murderers had broken down the door of the apartment in which the victim lived, showering down wood particles and plaster as the door gave way. Vacuum sweepings from the shoulders of the suspects’ clothing revealed the presence of the same genera of wood comprising the door frame and molding, as well as small particles of calcium sulfate consistent with plaster. One of the suspects was employed in mill-work, but none of them had an explanation for both the wood and plaster.


Wood Identification

Sample preparation

The identification of wood requires a knowledge of wood anatomy as well as skill in sectioning and mounting small particles for microscopical study. Particles of wood may be matchstick sized or smaller; all the way down to sawdust and sometimes even single fibers. The microscopical identification of wood from small particles requires that the specimen be sectioned in three specific directions to reveal the characteristic morphological features necessary for identification. A piece of wood, no matter how small, retains its orientation with respect to the tree in which it originally grew. Thus a transverse or cross-section lies in a plane perpendicular to the length of the tree. It is the plane one observes on the stump of a tree which has been sawn down. On a transverse section one can observe lines of cells radiating from the center of the tree, called rays, which are oriented like the spokes of a bicycle wheel. A section cut in a plane parallel to the length of the tree and aligned with one of these rays is called the radial section. The third direction lies in a plane parallel to the length of the tree, which cuts through the rays at a right angle and is referred to as the tangential section. These sections are therefore tangential to the circumference of the tree from which the wood originated. Thus, transverse sections are oriented perpendicular to the elongated fibrous cells comprising the wood, and radial and tangential sections are oriented parallel to the length of these fibers. These directions are not difficult to locate on a tree or a large piece of wood, but become a challenge as the size of the specimen decreases.
The first step in preparing a small sample of wood for identification is orientation. Since most pieces of wood, with diameters of the order of a toothpick, are elongated parallel to the fibrous elements of which they are composed, the easiest way to determine orientation is to cut a cross-section straight through the sliver using a sharp razor. The freshly cut surface is then held in the fingers (or if it is too small in a pair of forceps) and examined under a stereomicroscope. The directions of the rays will immediately reveal the true orientation of the fragment. The specimen is now oriented so that the required sections can be cut from it. If the piece of wood is large enough, it can either be mounted for sectioning in a microtome, or sections can be cut freehand with a microtome knife blade using a technique similar to that employed in peeling an apple. In most cases, however, the particles are much smaller and must be sectioned while being observed through a stereomicroscope. If a microtome with a freezing stage is available, the oriented particle may be placed on the stage of a sledge microtome, frozen in a drop of water in which a little gum is dissolved and sections sliced off. Sections may also be cut on a conventional rotary microtome after being impregnated and embedded in wax. This method requires considerable time in sample preparation. For most purposes, sections satisfactory for microscopical identification can be prepared under a stereo-microscope using a sharp single-edged razor. The oriented fragment is held down with an index finger or in a pair of forceps and a small slice is made along the proper direction. The remainder of the section is then peeled away with a sharp pair of pointed forceps. This has the effect of producing sections which are very thin on at least one edge, meaning that almost every section will be a good one as long as it was cut in the right direction. Once the sections have been cut they are placed on a microscope slide and mounted beneath a coverslip in a drop of a mixture of equal parts (v/v) glycerin and alcohol. They are then boiled to remove the air. Fig. 1 shows a freehand section prepared from a casework-sized sample. Particles of sawdust, which are too small to section, are sprinkled on a slide so that they do not lie on top of one another, boiled in glycerin-alcohol and observed directly under the microscope. Many of the sawdust particles will be cut in a generally radial or tangential section and, therefore, will often show enough characteristics to permit an identification to be made.
When comparing particles of wood in casework, it is essential that good known samples be obtained. A broken door and frame may produce particles from several different kinds of wood. If adequate known samples are not taken at the crime scene, the micros-copist may erroneously eliminate questioned wood particles found on a sledge hammer suspected in a break-in, for instance, from being associated with the incident. This problem can become quite taxing in certain situations, such as when sawdust from a workshop or particle board are involved. The number of different woods which may be involved can mean that many samples may have to be prepared and examined over a relatively long period of time before a conclusion can be reached.
 Freehand radial sections from wood fragment after boiling in glycerin alcohol. Original magnification x 40.
Figure 1 Freehand radial sections from wood fragment after boiling in glycerin alcohol. Original magnification x 40.

Wood anatomy

The microscopic features by which wood is identified have been studied and compiled over almost two centuries. Wood is the highly lignified, fibrous tissue (xylem) which is differentiated inside the cambium of a stem towards its center. As a wood-bearing tree grows from a stem, the xylem increases in proportion until it is the dominant tissue in the plant. Although it is possible to name a complete tree in great detail, giving not only its genus and species but perhaps a varietal name as well, this is normally not the case with the wood alone. In most cases, wood can be identified only to genus. On occasion, identification may be carried down to the species level by an experienced microscopist well trained in wood anatomy. If the wood is not exotic, a microscopist trained in wood identification will recognize the common woods almost at sight, based on a few key characteristics. An exotic wood which is rare in commerce will normally make the best evidence. Particles of wood recovered from the scalp of a murder victim were identified as a species of Kokka, a rather rare tropical wood not commonly found on the market. An examination of a pool which belonged to the prime suspect showed that it was made from the same wood and had sustained damage to its thick end. This was the most important piece of physical evidence in the trial which resulted in the conviction of the defendant.
The first step in a wood identification based on microscopic anatomy is to decide if the sample is a hardwood or softwood. The terms are perhaps unfortunate, as some hardwoods are softer than some softwoods (e.g. hard pines, which are harder than basswood). A more accurate terminology is based on the fact that the softwoods are all gymnosperms and the hardwoods all originate from angiosperms. Transverse sections of a softwood (Pinus resinosa) and hardwood (Acer saccbarum) are shown in Figs 2 and 3, respectively. Softwoods are most easily distinguished from hardwoods on the basis of bordered pits which occur on the radial side of the fibrous elements (called tracheids) that make up the gymnosperm xylem. Fig. 4 illustrates tracheids with their bordered pits in a radial section of P. resinosa. It is often possible to make an identification from the transverse section of a hardwood. Fig. 5 illustrates a transverse section of wood from Quercus alba (white oak). The arrangements of the large and small vessels and the single (uniseriate) rows of rays and the broad (multi-seriate) rays are quite characteristic of the oaks. Fig. 6 shows the tangential section of oak. The uniseriate and multiseriate rays can be seen here as they appear ‘head on’. On occasion, the arrangement of the vessels in transverse section is so characteristic that identification to the species level is possible For example,
Transverse section of softwood {Pinus resinosa). R, radial direction, and T, tangential direction as seen in this orientation; r, resin duct. Original magnification x 40.
Figure 2 Transverse section of softwood {Pinus resinosa). R, radial direction, and T, tangential direction as seen in this orientation; r, resin duct. Original magnification x 40.
Transverse section of a hardwood (Acer saccharum). R, radial direction; T, tangential direction; v, vessel element. Original magnification x 40.
Figure 3 Transverse section of a hardwood (Acer saccharum). R, radial direction; T, tangential direction; v, vessel element. Original magnification x 40.
Figs 7 and 8 show the endgrain (transverse section) of Ulmus fulva and U. americana, two species of elm. The relative sizes, distribution and patterns formed by the large and small vessels make it possible to tell one species from another on the basis of this feature alone. In practice, however, it is often difficult or impossible to make identifications from the transverse sections alone, as the total area of the cross-section is so small that only portions of a few rays and one or two vessels are present. In these cases, radial and tangential sections must be prepared in order to observe other morphological details. Fig. 9 shows a radial section through U. fulva and illustrates the appearance of the vessels and rays in this orientation.
Radial section of wood from Pinus resinosa showing vertical tracheids with bordered pits arranged on their surfaces. Original magnification x 383.
Figure 4 Radial section of wood from Pinus resinosa showing vertical tracheids with bordered pits arranged on their surfaces. Original magnification x 383.
Transverse section of oak {Quercus alba). br, multiseriate ray; r, uniseriate ray; v, large and small vessels. Compare with Fig. 5. Original magnification x 40.
Figure 5 Transverse section of oak {Quercus alba). br, multiseriate ray; r, uniseriate ray; v, large and small vessels. Compare with Fig. 5. Original magnification x 40.
Most of the diagnostic structure of softwoods is best observed on the radial section, although transverse sections are still essential for orienting small pieces before sectioning. Resin ducts, which are an important diagnostic character, can best be observed in transverse and radial sections, although they can occasionally be observed in radial sections, as illustrated in Fig. 10, which also shows the bordered pits and ray parenchyma cells which make up the rays. Fig. 11 shows a crossfield region from P. resinosa at higher magnification. The four horizontal rows of ray parenchyma cells in the center of the figure contain large window-like pits which are typical of the commercial soft pines and red pine (P. resinosa).
Tangential section of oak. br, multiseriate ray; r, uniseriate ray; v, the hole left by a vessel which was torn away in sectioning. Vessels are tubes, composed of cells connected end to end {vessel elements), which are aligned roughly parallel to the other fibrous elements in the wood. Compare with Fig. 5. Original magnification x 40.
Figure 6 Tangential section of oak. br, multiseriate ray; r, uniseriate ray; v, the hole left by a vessel which was torn away in sectioning. Vessels are tubes, composed of cells connected end to end {vessel elements), which are aligned roughly parallel to the other fibrous elements in the wood. Compare with Fig. 5. Original magnification x 40.
Transverse section of elm [Ulmus fulva) showing pattern formed by the large and small vessels. Compare with Fig. 8. Original magnification x 40.
Figure 7 Transverse section of elm [Ulmus fulva) showing pattern formed by the large and small vessels. Compare with Fig. 8. Original magnification x 40.
This type of pitting is referred to as ‘fenstriform’. The small ray cells with bordered pits above and below these are called ‘ray parenchyma’. If these cells contain tooth-like projections, such as those shown in the figure, they are referred to as ‘dentate’. The crossfield pitting of spruce (Picea sitchensis)is shown for comparison in Fig. 12. Note that the single row of ray tracheids above and below the crossfield pits in this wood are not dentate. The small pits in the ray parenchyma cells between the ray tracheids are small and slit-like. These are called ‘piceoid pits’ and are found also in Larix spp. and Pseudotsuga spp. Other types of crossfield pits are found in different softwoods.
Transverse section of American elm [Ulmus americana) showing pattern formed by the large and small vessels. Compare with Fig. 7. Original magnification x 40.
Figure 8 Transverse section of American elm [Ulmus americana) showing pattern formed by the large and small vessels. Compare with Fig. 7. Original magnification x 40.
Radial section of hardwood Ulmus fulva showing 'brick wall-like' appearance of the rays {r) and pieces of vessel elements {v) which were not torn out of the xylem during sectioning. Original magnification x 40.
Figure 9 Radial section of hardwood Ulmus fulva showing ‘brick wall-like’ appearance of the rays {r) and pieces of vessel elements {v) which were not torn out of the xylem during sectioning. Original magnification x 40.

Identification

By locating and recognizing the types of features described above, the wood anatomist can narrow the possibilities until only one or a few possible woods remain. For example, the identification of a single cell such as a tracheid bearing bordered pits or a vessel element will serve to distinguish a softwood from a hardwood. Although many common woods can be recognized almost at sight, after experience is gained, small fragments of wood may not show all of the features one would like to see in order to make an identification. Secondly, a single person can only remember so many woods from memory alone.
Radial section of softwood Prunus resinosa showing 'brick wall' of rays. rd, resin duct. Resin ducts ere essentially empty tunnels, surrounded by a layer of epithelial cells unlike vessels which consist of tubular elements. Original magnification x 40.
Figure 10 Radial section of softwood Prunus resinosa showing ‘brick wall’ of rays. rd, resin duct. Resin ducts ere essentially empty tunnels, surrounded by a layer of epithelial cells unlike vessels which consist of tubular elements. Original magnification x 40.
Detail of radial section of Prunus resinosa showing horizontal ray parenchyma cells with large window-like pits (f) where they cross over vertical tracheids. Above and below these are horizontal rows of ray tracheids with small bordered pits. The ray tracheids in this wood contain tooth-like projections (d). Cells of this type are called dentate ray tracheids. Original magnification x 383.
Figure 11 Detail of radial section of Prunus resinosa showing horizontal ray parenchyma cells with large window-like pits (f) where they cross over vertical tracheids. Above and below these are horizontal rows of ray tracheids with small bordered pits. The ray tracheids in this wood contain tooth-like projections (d). Cells of this type are called dentate ray tracheids. Original magnification x 383.
Since the absence of a particular diagnostic feature does not mean that it would not be present if a larger piece of wood were available, the use of a conventional botanical key can be frustrating for identifying woods under these circumstances.
The first solution to this problem was the use of punch cards, which contained coded information on the microscopical anatomical characteristics of hard-and softwoods. The cards were punched or notched around the edges, depending on whether or not a particular characteristic was present in that wood. Each card carried all of the characteristics exhibited by a particular species. A knitting needle was inserted into the stack and all the cards for woods which did not bear this characteristic would fall out. As the needle was pushed through the stack, the number of cards remaining would become fewer and fewer until only one or a few cards remained. Today these cards have been largely replaced by computer programs, which greatly enhance the microscopist’s ability to make identifications from very small pieces of unfamiliar or unusual woods. The International Association of Wood Anatomists has published a glossary of standard terminology for use with these wood identification programs. Once a tentative identification has been made, the specimen must be prepared with known slides or photomicrographs made from authentic wood.
Detail of radial section of spruce wood (Picea sitchensis) showing slit-like piceoid pits in the crossfield. The ray tracheids are nondentate. Original magnification x 383.
Figure 12 Detail of radial section of spruce wood (Picea sitchensis) showing slit-like piceoid pits in the crossfield. The ray tracheids are nondentate. Original magnification x 383.

Interpretation of Wood Evidence

Once the particles of wood have been identified, it becomes necessary to interpret the results. The first consideration, aside from the integrity of the evidence itself, must be the accuracy of the identifications. These must be based on a sound knowledge of wood anatomy and comparison to reference specimens of authentic woods, either as physical specimens, prepared microscope slides or photographic atlases. Whenever possible, identifications should be checked by a second analyst, also skilled in the techniques of wood identification. This is a general rule in forensic science and helps to insure the quality of the results obtained. It is also important to take photomicrographs or make drawings of important identification features so that the analyst’s notes will reflect the basis for his or her conclusion. Enlarged photographs may also make useful exhibits for trial. The next step must be to consider how common or rare the wood is. In this regard, it is important to know, or at least be able to look up, something about the uses of a particular wood. Even a relatively rare wood may be common in a certain application. Knowledge of the uses of different woods will help prevent mistakes of this kind. The final conclusion of a positive wood examination will be that the questioned and known woods are both the same type of wood. Any further interpretation must take into account the factors discussed above. Of course, two woods could not have come from the same piece if they are different genera or species. Only in the case of a physical match will it be possible to say that two pieces of wood could have come from the same piece.

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