7 Transverse sections of wood showing cell walls and lumina. (A) Softwood: All the rectangular cells are of the same type, some with thicker cell walls and narrower lumina, and others with thinner walls and wider lumina in Pseudotsuga mensiezii. (B) Hardwood: The large round cells have thick cell walls and very large lumina. Other cells have thinner walls and narrower lumina in Quercus rubra . Scale bars = 50 μ m. 

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... structural differences between the two. Many authors use the general term porosity to describe growth rings (recall that vessels and pores are synonymous.) Nonporous woods (woods without vessels) are softwoods. Softwoods can exhibit any of the three general patterns noted above. Some softwoods such as Western red cedar ( Thuja plicata ), northern white cedar ( Thuja occidentalis ), and species of spruce ( Picea ) and true fir ( Abies ) have growth increments that undergo a gradual transition from the thin-walled wide-lumined earlywood cells to the thicker-walled, narrower-lumined latewood cells (Figure 2.6B). Other woods undergo an abrupt transition from earlywood to latewood, including Southern yellow pine ( Pinus ), larch ( Larix ), Douglas fir ( Pseudotsuga menziesii ), bald cypress ( Taxodium disticum ), and redwood ( Sequoia sempervirens ) (Figure 2.6C). Since most softwoods are native to the north temperate regions, growth rings are clearly evident. Only in species such as araucaria ( Araucaria ) and some podocarps ( Podocarpus ) do you find no transition within the growth ring (Figure 2.6A). Many authors have reported this state as growth rings being absent or only barely evident (Phillips 1948, Kukachka 1960). Porous woods (woods with vessels) are hardwoods, which have two main types of growth rings, and one intermediate form. In diffuse porous woods, the vessels either do not significantly change in size and distribution from the earlywood to the latewood or the change in size and distribution is gradual and no clear distinction between earlywood and latewood can be found (Figure 2.6D). Maple ( Acer ), birch ( Betula ), aspen/cottonwood ( Populus ), and yellow poplar ( Liriodendron tulipifera ) are examples of diffuse porous species. This pattern is in contrast to ring porous woods in which the transition from earlywood to latewood is abrupt, i.e., the vessels reduce significantly (often by an order or magnitude or more) in diameter and often change their distribution as well. This creates a ring pattern of large, earlywood vessels around the inner portion of the growth increment, alternating with denser, more fibrous tissue in the latewood, as is found in hackberry ( Celtis occidentalis ), white ash ( Fraxinus americana ), shagbark hickory ( Carya ovata ), and northern red oak ( Quercus rubra ) (Figure 2.6F). Sometimes the vessel size and distribution pattern falls more or less between these two definitions, and this condition is referred to as semi-ring porous (Figure 2.6E). Black walnut ( Juglans nigra ) and black cherry ( Prunus serotina ) are temperate-zone semi-ring porous woods. Most tropical hardwoods are diffuse porous except for Spanish cedar ( Cedrela ) and teak ( Tectona grandis ), which are generally semi-ring porous. There are no distinctly ring porous species in the tropics and only a very few in the Southern Hemisphere. It is interesting that in genera that span temperate and tropical zones, it is common to have ring porous representatives in the temperate zone and diffuse porous species in the tropics. The oaks ( Quercus ), ashes ( Fraxinus ), and hackberries ( Celtis ) that are native to the tropics are diffuse porous, while their temperate relatives are ring porous. There are numerous detailed texts with more information on growth increments in wood, a few of which are of particular note (Panshin and deZeeuw 1980, Dickison 2000, Carlquist 2001). To understand a growth ring in greater detail, it is essential to begin with an understanding of the structure, function, and variability of the cells that compose the ring. A single plant cell consists of two primary domains: the protoplast and the cell wall. The protoplast is the sum of the living contents that are bounded by the cell membrane. The cell wall is a non-living, largely carbohydrate matrix extruded by the protoplast to the exterior of the cell membrane. The plant cell wall protects the protoplast from osmotic lysis and can provide significant mechanical support to the plant at large (Esau 1977, Raven et al. 1999, Dickison 2000). For cells in wood, the situation is somewhat more complicated than this highly generalized case. In many cases in wood, the ultimate function of the cell is borne solely by the cell wall. This means that many mature wood cells not only do not require their protoplasts, but indeed must completely remove their protoplasts prior to achieving functional maturity. For this reason, it is a common convention in wood literature to refer to a cell wall without a protoplast as a cell. Although this is technically incorrect from a cell biological standpoint, it is a convention common in the literature and will be observed throughout the remainder of the chapter. In the case of a mature cell in wood in which there is no protoplast, the open portion of the cell where the protoplast would have existed is known as the lumen. Thus, in most cells in wood there are two domains: the cell wall and the cell lumen (Figure 2.7). The lumen is a critical component of many cells, whether in the context of the amount of space available for water conduction or in the context of a ratio between the width of the lumen and the thickness of the cell wall. The lumen has no structure per se, as it is really the void space in the interior of the cell. The relevance of the lumen to the formation of wood composites is the subject of Chapter 15. The cell walls in wood are important structures. Unlike the lumen, which is a void space, the cell wall itself is a highly regular structure, from one cell type to another, between species, and even when comparing softwoods and hardwoods. The cell wall consists of three main regions: the middle lamella, the primary wall, and the secondary wall (Figure 2.8). In each region, the cell wall has three major components: cellulose microfibrils (with characteristic distributions and organization), hemicelluloses, and a matrix or encrusting material, typically pectin in primary walls and lignin in secondary walls (Panshin and deZeeuw 1980). To understand these wall layers and their interrelationships, it is necessary to remember that plant cells generally do not exist singly in nature; instead they are adjacent to many other cells, and this association of thousands of cells, taken together, forms an organ such as a leaf. Each of the individual cells must adhere to others in a coherent way to ensure that the cells can act as a unified whole. This means that they must be interconnected with one another to permit the movement of biochemicals (e.g., photosynthate, hormones, cell signaling agents, etc.) and water. This adhesion is provided by the middle lamella, the layer of cell wall material between two or more cells, a part of which is contributed by each of the individual cells (Figure 2.8). This layer is the outermost layer of the cell wall continuum, and in a non-woody organ is pectin rich. In the case of wood, the middle lamella is lignified. The next layer, formed by the protoplast just interior to the middle lamella, is the primary wall (Figure 2.8). The primary wall is characterized by a largely random orientation of cellulose microfibrils, like thin threads wound round and round a balloon in no particular order, where any microfibril angle from 0 to 90 degrees relative to the long axis of the cell may be present. In cells in wood, the primary wall is very thin, and is generally indistinguishable from the middle lamella. For this reason, the term compound middle lamella is used to denote the primary cell wall of a cell, the middle lamella, and the primary cell wall of the adjacent cell. Even with transmission electron microscopy, the compound middle lamella often cannot be separated unequivocally into its component layers. The compound middle lamella in wood is almost invariably lignified. The remaining cell wall domain, found in virtually all cells in wood (and in many cells in non- woody plants or plant parts) is the secondary cell wall. The secondary cell wall is composed of three layers (Figure 2.8). As the protoplast lays down the cell wall layers, it progressively reduces the lumen volume. The first-formed secondary cell wall layer is the S 1 layer (Figure 2.8), which is adjacent to compound middle lamella (or technically the primary wall). This layer is a thin layer and is characterized by a large microfibril angle. That is to say, the cellulose microfibrils are laid down in a helical fashion, and the angle between the mean microfibril direction and the long axis of the cell is large: 50 to 70 degrees. The next wall layer is arguably the most important in determining the properties of the cell and, thus, the wood properties at a macroscopic level (Panshin and deZeeuw 1980). This layer, formed interior to the S 1 layer, is the S 2 layer (Figure 2.8). This is the thickest secondary cell wall layer and it makes the greatest contribution to the overall properties of the cell wall. It is characterized by a lower lignin percentage and a low microfibril angle: 5 to 30 degrees. There is a strong but not fully understood relationship between the microfibril angle of the S 2 layer of the wall and the wood properties at a macroscopic level (Kretschmann et al. 1998), and this is an area of active research. Interior to the S 2 layer is the S 3 layer, a relatively thin wall layer (Figure 2.8). The microfibril angle of this layer is relatively high and similar to that of S 1 : 70 + degrees. This layer has the lowest percentage of lignin of any of the secondary wall layers. The explanation of this phenomenon is related directly to the physiology of the living tree. In brief, for water to move up the plant (transpiration), there must be a significant adhesion between the water molecules and the cell walls of the water conduits. Lignin is a hydrophobic macromolecule, so it must be in low concentration in the S 3 layer to permit adhesion of water to the cell wall and thus facilitate transpiration. For more ...