Longitudinal tracheid

Trees are classified into two major groups termed softwoods (gymnosperms) and hardwoods (angiosperms). The botanical basis for classification is whether or not the tree seed is naked as in softwoods or covered as in hardwoods. A more familiar classification, which with some exceptions is valid, is based on the retention of leaves by softwoods or the shedding of leaves by hardwoods. Thus the softwoods are often referred to as evergreen trees and hardwood as deciduous trees. The major difference with regard to wood anatomy is the presence of vessels in hardwoods. Vessels are structures composed of cells created exclusively for the conduction of water. Softwoods lack vessels but have cells termed longitudinal tracheids which perform a dual role of conduction and support. 

Since longitudinal tracheids constitute about 90 of the volume and are therefore largely responsible for the resulting physical and chemical properties of softwoods, a detailed... 

Since the longitudinal tracheid is a long, thin, cylindrical, tube-like cell, its appearance depends upon how it is viewed. 

It should be obvious that the structure of the longitudinal tracheid is well suited to perform the dual roles of conduction and support. Since water is translocated up the tree via the tracheids, the orientation of the long axis of the tracheid parallel to the vertical stem permits a longer passageway prior to interruption by a cell wall. The rigid cell walls, of varying thickness, provide adequate support. 

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)...

Figure 9. Innermost wall layer as seen from the inside of a springwood longitudinal tracheid showing two bordered pits. Note the circular pit apertures and the stringlike microfibrils which are oriented at approximately 90° to the long axis of the cell. Arrow indicates longitudinal axis of the cell. 3,000X...

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. 

Thus, most hardwood species contain four types of cells, vessel segments, fibers, transverse and axial parenchyma, whereas most softwood species possess two types longitudinal tracheids and transverse parenchyma. 

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...

Based on the wood anatomical descriptions presented, it is obvious that hardwoods and softwoods differ considerably from each other. For example, vessels are present in hardwoods and absent in softwoods. In hardwoods more cell types, shorter cells, more parenchyma and a more variable arrangement of cell types occur. The relative uniformity of softwood anatomy is the result of the preponderance of a single type cell, the longitudinal tracheid. 

Figure 19. Cell walls in cross-sectional view from contiguous springwood longitudinal tracheids depicting wall layering. C compound middle lamella. 1 Sj layer 2 S2 layer and 3 S3 layer. Note the S2 layers are the largest. 16,000X...

Figure 21. Cellulose microfibrils in the primary wall and Sj portion of the secondary wall from a longitudinal tracheid. Note the loosely packed ana randomly arranged microfibrils in the primary wall (P). The S layer (S) consists of tightly packed, parallel microfibrils. 12,000X...

The predominant cell type in softwoods is the vertically oriented (along the stem axis) longitudinal tracheid. More commonly known as fibers, these tracheids are hollow, square to rectangular in cross section, have closed and tapering ends, and are arranged so that their... 

Figure 1.1. The transverse and tangential-longitudinal faces of the softwood European larch, Larix decidua. The wood comprises longitudinal tracheids forming the axial system of cells, and radial parenchyma mostly in uniseriate rays. Axial and ray canals are also present. Magnification X 125.

The differences in the penetration behaviors have been addressed several times in the literature. Brady and Kamke [22] reported that fractures were abundant along the surface of earlywood flakes, opening additional flow paths. Rays were found to be a more important flow path into fir tissue. Most of the radial penetration of adhesive is probably through the ray cells and between the longitudinal tracheids... 

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