Softwoods cell wall structure, tracheid

Figure 2.3 A schematic representation of the structure of the primary (P) and secondary (SI, S2 and S3) cell walls of a softwood tracheid (ML = middle lamella).

Bailey and his co-workers worked out the basic structure of the softwood tracheid (Figure 2.12) in the 1930s, e.g. Bailey and Kerr (1935) the orientation of the microfibrils within the cell wall was determined by polarizing microscopy and by... 

The organization of a typical softwood tracheid or hardwood fiber is presented in Fig. 1. In the primary wall (P) the cellulose microfibrils form a disordered network, whereas the outer layer of the secondary wall (Si) has a crossed, fibrillar structure. The middle layer of the secondary wall (S2) represents the bulk of the cell. Here, the cellulose microfibrils run almost parallel to the fiber axis. The microfibrils in the inner layer of the secondary wall (S3), sometimes referred to as the tertiary wall, are oriented at a steep angle to the fiber axis. Toward the central lumen, the S3 layer is often terminated by a warty membrane. 

---