Figure 1. Drawing of the secondary thickened wall of a mature tracheary element showing the orientation of cellulose microfibrils in the different layers of the wall. Note the designation of the secondary wall layers and the average microfibril angle of each layer: S₁ is the outermost layer, S₂ is the middle layer and S₃ the innermost layer. Most of the wall thickness is determined by the thickness of the S₂ layer (the relative thicknesses are: primary wall, 1%; S₁,10 to 20%; S₂, 40 to 90% and S₃, 2 to 8%). Modified after Côté WA (1967) Wood Ultrastructure: An Atlas of Electron Micrographs. Seattle: University of Washington Press.

Figure 2. Drawing showing the relative sizes and shapes of some xylem cell types: (a) conifer tracheid with circular bordered pits, (b) fibre tracheid with bordered pits, (c) libriform fibre with simple pitting, (d) vessel element with scalariform perforations and (e) vessel element with a simple perforation. Note that conifer tracheids (3 to 5 mm) are usually much longer in relationship to fibres (0.8 to 2.3 mm) and vessel elements (0.2 to 1.3 mm).

Figure 3. Structure of a bordered pit in the secondary wall of a conifer tracheid showing the modification of the pit membrane to a torus and margo. Note the loose network of cellulose fibrils that forms the margo and the secondary thickening of the central region to form the torus. In angiosperms, the pit membranes of bordered pits are usually not modified.

Figure 4. Drawing of cross sections of young woody stems showing the cambial zone and secondary xylem development. (a) Dicot wood. (b) Conifer wood. Note the abrupt change in the size of tracheids from earlywood to latewood. M, differentiating xylem and phloem mother cells; C, cambial initial.