Ray tracheids

Fig. 1-6. Cells of coniferous woods. An earlywood (a) and a latewood (b) pine tracheid, an earlywood spruce tracheid (c), ray tracheid of spruce (d) and of pine (e), ray parenchyma cell of spruce (f) and pine (g) (llvessalo-Pfaffli, 1967).

Fig. 1-7. Radial section of a spruce ray (above) and radial and tangential section of a pine ray (below), (a) Longitudial tracheids. (b) Rows of ray tracheids (small bordered pits), (c) Rows of ray parenchyma, (d) Pits in the cross fields leading from ray parenchyma to longitudial tracheids. (e) A bordered pit pair between two tracheids. (f) A bordered pit pair between a longitudial and a ray tracheid (llvessalo-Pfaffli, 1967).

The width of a ray usually corresponds to one cell. Several parenchyma cell files are placed on top of one another. Ray tracheids are often located at the upper and lower edges of this tier (Fig. 1-7). Parenchyma cells are thin-wailed, living cells. In Norway spruce and Scots pine their length and width vary between 0.01-0.16 mm and 2-50 tm, respectively. The ray tracheids are of the same size and also provide liquid transport in the radial direction. Rays in Scots pine, for example, contain 25-31 ray tracheids per square millimeter in a tangential section. 

TABLE 5-1. Data on Resin, Parenchyma Cells, and Ray Tracheids of Scots Pine (Pinus sylvestris) and Norway Spruce (Picea abies)a... 

The rays in some species also contain cells known as ray tracheids, which are similar in size to parenchyma but are dead at maturity see Figure 11). Presence and/or type of ray tracheid is sometimes a very useful diagnostic feature in identification of a particular wood genus or species. With or without ray tracheids, rays are usually several cells high, but in softwoods they are generally only one cell wide (uniseriate) (Figure 13), except in special cases where they can be up to several cells wide (multiseriate). 

The parenchyma-cell content of hardwoods is, on the average, much greater than that of softwoods. This situation is a result of the wider rays (1-50 cells) and greater ray volume of hardwoods, and also the relatively high proportion of longitudinal parenchyma (2). Additionally, the rays are all parenchyma—no ray tracheids. 

Figure 22. SEM of ray cross-field pits in softwoods as seen on the wood rcmial surface. Key A, pits to ray parenchyma (RP) in western white fir and B, pits to ray parenchyma and ray tracheids (RT) in lodgepole pine.

Figure 1.16. Ray axial parenchya cells (upper) and ray tracheids (lower) in Pinus radiata. x550.

Ray tracheids are present in a number of softwoods including Pinus (Figure... 

Radial permeability is not as well understood as longitudinal or tangential permeability and indeed the tree has no path for - or interest in - radial flow except within ray tissue. However, after drying (and pit aspiration) the minimal radial permeability in some species can exceed tangential permeability. The greater radial air-dry permeability for Pinus sylvestris compared to either Picea sitchensis or Pseudotsuga menziesii is attributed, in part, to the pine having many more ray tracheids. 

AG type II is most abundant in the heartwood of the genus Larix and occurs as minor, water-soluble components in softwoods. Certain tree parts of western larch (I. occidentalis) were reported to contain up to 35% AG [378]. The polysaccharide is located in the lumen of the tracheids and ray cells. Consequently, it is not a cell-wall component and, by definition, not a true hemicellulose. However, it is commonly classified as such in the field of wood and pulping research. This motivated us to include the larch AG in the review. 

Figures 11 and 12 show that the coalified product derived from the fiber-tracheid walls, although lighter in color, is similar to that derived from such walls in the case of the other woods. Worthy of note, however, is the colorless material shown in both small and large strands in Figure 12. These appear to be remnants of portions of fiber-tracheid walls (the small strands) and ray parenchyma cell walls (the larger strand in the right half of the photograph). Material of this type studied thus far has proved to be isotropic.

Figure 11. Tangential section of coalified Gordonia wood showing amber-brown material produced from, or in the position of ray parenchyma walls. Note nearly colorless cell walls in the two rays on the left. Matrix of yellowish-brown material has been formed from alteration of fiber-tracheid walls. 465X...

Figure 12. Transverse section of coalified Gordonia wood. The yellow-brown matrix material is derived from fiber-tracheid walls, the reddish material from ray parenchyma inclusions. Note colorless material in right half of photograph. 607X...

The ray parenchyma cells behave in a manner similar to the tracheids, with a dissolution of fibrillar material from the Si as well as from the wall region near the lumen. 

Figure 8. View of internal cell walls of springwood longitudinal tracheids. The circular dome-like structures are bordered pits which permit liquid flow between contiguous longitudinal tracheids. The smaller egg-shaped pits in clusters lead to adjacent transversely oriented ray cells. 400X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)...

The other major cell wall structure found on longitudinal tracheids is termed a ray crossing and is illustrated in Figures 7 and 8. Ray crossings consist of pits which interconnect longitudinal tracheids to ray parenchyma. Due to the diverse structure of ray crossing pits they are extremely useful in the identification of wood and wood fibers. However, since identification is beyond the scope of this review, a description of the different types of pits found in ray crossings is not included. 

Figures 8 and 15 reveal ray crossing pits as seen from the inside of longitudinal tracheids. The considerably higher magnification in Figure 15 shows a solid pit membrane. Openings in the pit membrane would expose the cytoplasm to the hostile environment of the longitudinal tracheid lumen and result in the death of the parenchyma cell. Thus, the membranes are solid and do not provide a passageway for free liquid flow.

Obviously, softwood anatomy is relatively simple as only two types of cells, longitudinal tracheids and ray parenchyma, constitute the bulk of the wood. Hardwoods have a more complex anatomy as more kinds of cells are present. The roles of conduction and support are carried out by different cells and in addition to the transverse ray parenchyma, food-storage cells oriented in the longitudinal direction are present. Parenchyma oriented longitudinally are called longitudinal or axial parenchyma. Vessel segments perform the conduction role, and fibers the support role. 

Figure 15. View from the inside of a longitudinal tracheid showing pits connecting a longitudinal tracheid to a ray cell. Note the lack of openings within the pit membrane. 2,500X...

Fig. 1-2. Transverse section of xylem and phloem of red spruce (P/cea rubens). CZ, cambial zone DP, differentiating phloem MP, mature phloem with sieve cells (sc) and tannin cells (tc) DX, differentiating xylem with ray cells and tracheids (tr) MX, mature xylem, earlywood (EW) with resin canals (rc), lined with epithelial cells (ec) LW, latewood. Note that each ray continuous from the xylem, through the cambial zone, and into the phloem. Light micrograph by L. W. Rees. Courtesy of Dr. T. E. Timell.

Water conduction in a tree is made possible by pits, which are recesses in the secondary wall between adjacent cells. Two complementary pits normally occur in neighboring cells thus forming a pit pair (Fig. 1-5). Water transport between adjacent cell lumina occurs through a pit membrane which consists of a primary wall and the middle lamella. Bordered pit pairs are typical of softwood tracheids and hardwood fibers and vessels. In softwoods the pit membrane might be pressed against the pit border thus preventing water transport, since the torus is impermeable. The pits connecting tracheids, fibers, and vessels with the ray parenchyma cells are half-bordered. Simple pits without any border connect the parenchyma cells with one another. 

The wood substance in softwoods is composed of two different cells tracheids (90-95%) and ray cells (5-10%). 

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