Woodworker's Journal 2008-32-5, страница 42
Softwoods and Hardwood Trees are popularly divided into softwoods (needle-leaved) and hardwoods (broad-leaved). It's believed that softwoods preceded hardwoods on our planet, because their structure is simpler and more primitive. Unlike hardwoods, inspecting softwoods with a lOx lens doesn't reveal much more than can be seen by gross inspection. Closer inspection, however, shows important similarities: both of them are composed mainly of cellulose, and both go through a sapwood to heartwood g.g growth phase. Elements in Softwood The cells in the cambium which divide and form wood tissue are initials, so-called because they initiate the formation The cambium is a microscopically thin layer of living cells that sheathes the tree from ground to crown. The cambium cells grow and divide. Half of the new cells make either wood or bark; the other half remain in the cambium. New cells on the inside of the cambium become one of the woody elements (fiber, parenchyma, rays. etc). The outside cells become bark. of the specialized progenitor cells after cell division. There are two types of initials: fusiform initials and ray initials. In a typical softwood, the fusiform initial makes only one type of element called a tracheid. Tracheids come in varying types and comprise the vast bulk'of softwood. In the main, they are 80 to 100 times longer than their diameter. They also vary greatly in wall thickness, depending upon when they are deposited during the growing season. In this way, they perform the tasks of transport and support. Ray initials in softwood are complex, because they can function to store and provide food for some time in the growing season after other elements have completed their growth. Most rays are only one cell wide and not visible by gross inspection. Elements in Hardwood In hardwood, the fusiform initial makes three different elements called vessels, fiber and parenchyma. The ray initial is responsible for only one element, rays. Each element plays a specific role in the tree, and each is sufficiently well-defined that it can be identified with the aid of a lOx hand lens when viewed on a transverse section. Vessels conduct water and nutrients from roots to leaves. They are long tube-like elements with thin walls and large cavities. Although each vessel is short, they are connected in vertical series and can extend for quite some distance up the tree. Fiber is the mechanical or support element of the wood. It's laid down in the later part of the growing season. You can describe this element as the reverse structure of a vessel. It's short and has a pointy end, a thick wall and a small cavity that isn't visible with a lOx lens. Parenchyma is essentially storage tissue. It's deposited vertically in different species in a variety of ways throughout the tree's growing season. Rays are the most complex of the four elements. In some cases, they continue to function as the growing season progresses to provide or store food. Their size, shape and number varies enormously according to species. In a few species, such as poplar and willow, the rays are only one cell wide and therefore not visible. In the majority of species, rays are easily visible. Rays in oak are relatively huge structures, hundreds of cells high and tens wide. Seen on the transverse section, rays radiate outward like spokes on a wheel. When you split a log, it generally splits along a series of rays. You can then see them on the split side of the log in their front elevation or as a plate. ^ It all ^ begins with the Cambium Hardwood Softwood Cambium Fiber Tracheid Fusiform Initial Fiber Parenchyma Vertical Ray Initial Cambium 42 October 2008 Woodworker's Journal |
