Angiosperms

Hardwoods come from angiosperms, which means the plant seeds are encased in a “seed vessel” like a fruit or a nut. There are two types of angiosperms: monocots (such as palm and bamboo) and dicots (such as oak and rosewood). Almost all hardwoods come from dicots. Most dicots in temperate climates are deciduous, meaning they lose their leaves during winter dormacy. Dicots in warmer parts of the world don’t lose their leaves — at least, not all at once. So, contrary to the standard definition, not all hardwoods come from deciduous trees.

Gymnosperms

Softwoods come from gymnosperms, plants with naked seeds. As the seeds develop on the cones, they are not encased in any tissue. Within this botanical group, there is a subdivision of conifers, which are characterized by needle and scale-like foliage. All softwoods come from coniferous gymnosperms.

MICROSCOPIC ATTRIBUTES

Botanical categories, while interesting, make little difference to most craftsmen. To find the traits that affect your woodworking, you must look closer.

Hardwood Cell Structure

Study a piece of hardwood under a microscope and you’ll find five major types of cells. Four of these are longitudinal cells, running parallel to the length of the trunk or limb.

• Most hardwood tissue is composed of fibers 100 times longer than they are wide. These have thick cell walls, sometimes appearing almost solid.
• Interspersed among the fibers are vessel elements. These appear much larger in diameter and shorter than fibers, and they are always aligned to form long, longitudinal pipelines. When sliced open, they appear as pores in the wood surface.
• You may also find a small number of tracheids and parenchyma cells with much thinner cell walls than fibers. These are few and far between and are completely missing in some hardwood species.

The fifth cell type runs perpendicular to the others:

• Ray cells form radial pipelines running out from the center of the tree.

Hardwood Grain

The appearance of a hardwood’s grain depends on the size, shape, and number of each cell type. And because there are so many possible combinations, hardwood grain is enormously varied. For example, the rays can grow quite large, producing a pronounced ray fleck in some species. In quartersawn oak, for example, the rays are the first thing you see. In other woods, they are barely noticeable.

Study a piece of hardwood under a microscope and you’ll find five major types of cells. Four of these are longitudinal cells, running parallel to the length of the trunk or limb.

Grain differs with the distribution of cells in the growth rings. When the cells in the summerwood are roughly the same size and density as the springwood, the wood has an even grain texture. When the summerwood cells are noticeably smaller and denser than those in the springwood, the texture is uneven.

Vessel elements and the pores they create also have a conspicuous effect. Hardwoods with large pores are said to have an open grain. Those with extremely small pores (too small to be seen with the naked eye) have a closed grain.

Softwood Cell Structure

Softwoods have a much simpler structure. Between 90 and 95 percent of the wood is composed of tracheids, while the rest are ray cells. There may also be a small number of parenchyma cells. There are no fibers or vessel elements, although some softwoods have resin canals lined with epithelial cells.

Softwood Grain

Because there are no fibers in softwoods, they tend not to be as dense or as hard as most hardwoods. And because there are fewer types of cells, there is less variety in the grain and appearance. For example, the absence of vessel elements eliminates the pores on softwoods. The grain is neither open nor closed — these terms don’t apply to softwoods.

Chemical Attributes

Although most wood tissue is composed of strands of cellulose fibers stuck together with lignin, there are many other chemicals and minerals embedded in this matrix.

Wood Extractives

Extractives, as the embedded substances are known, differ from species to species. A particular mix is what gives wood its characteristic color. But color is just one of the effects of extractives.

• Extractives can effect the density and strength of the wood species. The more extractives that are embedded in the fibers, the denser and stronger the wood.
• Many extractives are minerals. These act as fine abrasives, dulling your cutting tools as you work.
• Some are resinous and waxy substances. These build up as pitch on the cutting surfaces of power tools and interfere with the cutting action. They can also make a wood difficult to glue or finish.
• Many are antibiotics, killing molds and bacteria that would cause disease and rot. Woods high in these chemicals are resistant to decay.
• A few of these antibiotic chemicals are toxic to humans as well as microbes and have been known to cause or aggravate allergic reactions, respiratory ailments, and other health problems.

These botanical, microscopic, and chemical attributes all combine in a wood species to give it distinct characteristics or properties. These can be divided into two general categories, mechanical properties and physical properties.