As a long-time woodworker I have an interest in being able to tell one wood from another and in the process of learning how to do that, I've developed some knowledge about wood anatomy that I think will be of interest to Biology students as well as woodworkers. There is an enormous variety of wood anatomy characteristics and this article discusses just a few so as to spark interest more than to provide an extensive exposition on those characteristics. Since this article is introductory in nature and doesn't even express all of my own limited knowledge of the subject, I've included at the end some references for those interested in further information.
There are quite a large number of different characteristics of wood that would be of interest to a wood anatomist, but I have limited my own knowledge to the subset of those characteristics that are readily discernible to the average woodworker in his own shop/garage/basement with minimal effort and no elaborate equipment. It usually comes as QUITE a surprise to people, even long-term woodworkers, just how much information is very readily available from the wood with a pretty small effort. I want to emphasize that all of the images in this article were taken of wood that I processed in my garage with nothing but a random orbital sander. This is not an elaborate process. THE PROCESS
The most interesting set of easily available characteristics of wood are those in the end grain of temperate zone hardwoods and that's what this article discusses.
When you look at the end grain of hardwoods, there are two primary attributes that are of interest, the growth rings including the pore distribution within the rings, and the parenchyma. Pores are the little tubes that are used by the tree to move sap/nutrients up/down the tree and parenchyma is other wood tissue that can be arranged in quite a large number of different ways.
There are three steps involved in obtaining and using the anatomy wood information discussed here to identify wood.
EXPOSE You have to clean off the end grain sufficiently to be able to see the characteristics. Here's a discussion of how I use a random orbital sander to clean the end grain (the process is shown at the bottom of the page) clean up end grain
EXAMINE You have to look at the characteristics at a magnification that is easily supplied by a simple 10X jeweler's loupe or you can (as I do) take a closeup of it with a camera that allows that. For my own photographic closeups, I use a camera that easily gives me a 12X view.
USE You have to have a reference source or sources to help you understand what you are seeing. I have provided links to a number of those at the end of this article.
So ... now on to the actual wood anatomy.
Trees in the temperate zones have growth seasons that start in the spring and continue through the summer and into the fall. This growth cycle results in annual growth rings that contain areas that are called, appropriately enough, earlywood and latewood.
GROWTH RING COUNT: Fast growing trees have few growth rings per inch and slow growing trees have a lot more, so one characteristic of interest is how many growth rings does a particular piece of wood have. This can vary a great deal within many species so often is not a reliable species identification factor.
1" wide sections showing growth rings/inch
TYPES OF GROWTH RINGS: The pore distribution has several different characteristics, the most fundamental of which is the "type" of porosity (how the pores are arranged relative to the growth rings), as illustrated here:
1/4" x 1/4" sections showing growth ring porosity
semi ring porous
semi diffuse porous
Ring porous: large earlywood pores dropping off quickly to smaller latewood pores
Semi ring porous: large earlywood pores dropping off gradually to smaller and smaller pores into the latewood
Semi diffuse porous: almost uniform pore distribution but with some thinning out and/or smaller pores as the latewood progresses
Diffuse porous: uniform pores throughout
RAYS --- rays are batches of tissues that run from the center of the tree out to the bark. They are shown in these images running from top to bottom. There are two aspects of rays that are relevant to wood identification and they are the size of the rays and the density (how many per inch). Here, I show the various categories of thickness.
1/4" x 1/4" sections showing ray thicknesses
strong in macadamia
medium in oxhorn
weak in Chilean beech
faint to invisible in arborvitae
PORE DENSITY: Another characteristic of pores is how many of them there are unit of area. The general categories are shown here. "Uncountable" is a figurative term since with a good microscope you COULD count them, but for practical purposes there's no need.
1/4" x 1/4" sections showing pore density (amount per unit area)
PORE SIZE: Pores range in size from so small you can't even see individual ones with a 10X loupe up to really large ones on woods such as red oak which are easy to see with the naked eye. In many hardwoods the pore size will vary throughout the grown ring, as you can see in the sparse pore image directly above.
PORE Multiples: Often pores will grow adjacent to each other and share a cell wall. When that happens, it's referred to as a pore "multiple". Multiples can occur as groups all the way up to 8 or 9 but that's rare. Groups of 2 and 3 pores are quite common. The orientation of the pore groups is also significant. Sometimes they are radial and sometimes tangential and they can even be random. Here are some examples. The orientation of all of these images is that the center of the tree is at the bottom and the outside of the tree is at the top, so bottom-to-top is radial and left to right is tangential.
1/8" x 1/8" sections showing pore multiples
DENDRITIC GROUPS: "Dendritic" means "tree-like" and this particular arrangement of pores appears, rather loosely, to be in the shape of tree branches. There are not, as far as I am aware, any formal designations for the different types with the dendritic group but I use the rather obvious designation shown here:
1/4" x 1/4" sections showing dendritic groups
skinny groups in Eastern hophornbeam
fat groups in chittamwood
V-shaped groups in chinkapin
WAVY BANDS: Pores group together in a wide variety of ways. The dendritic groups directly above are one such and another is wavy bands, which is just what it sounds like:
1/4" x 1/4" sections showing wavy pore bands
AILIFORM PARENCHYMA: "Aliform" means "wing-shaped" or "winglike" and there are two groupings of parenchyma cells that fall in this category. Since aliform means wing-shaped, it's a bit weird but the first type is called wing shaped aliform parenchyma, which is bit like calling it wing shaped wing shaped, but this is because (1) this form LOOKS solidly wing shaped and (2) there is another form, lozenge shaped, that it needs to be distinguished from. I really find the wing shaped aliform parenchyma funny because if you get a really good example, like the one shown here, it's easy to see it as a squadron of WWII fighter jets bearing down on you. The lozenge shaped form looks at bit like a cough lozenge. The parenchyma cells surround the pores and move off to either side in one of the two forms shown here.
1/4" x 1/4" sections showing aliform parenchyma
wing shaped in padauk
lozenge shaped in kempas
A FEW OTHER PARENCHYMA TYPES The images below show a few of the other parenchyma types, shown here as horizontal lines in the images.
1/4" x 1/4" sections showing some other parenchyma types
banded in hickory
confluent in wenge
marginal in mahogany
What I've gone over above here is just a sample of the characteristics, just to show some of them.
And now for the fun part. Here's a small sample of how the characteristics described above look in various combination for a few woods. These are all 1/4" x 1/4" end grain cross sections. Once you have some familiarity with the various characteristics, using them to identify woods can be (but isn't always) fairly easy. Just look at the variety in this batch: