It is convenient to distinguish between primary and secondary fibre orientations (Neville, 1967c). The first two sections of this chapter (5.1 and 5.2) concern primary orientation, in which fibres are laid down in their final positions. In Section 5.3, secondary orientation is considered; here fibres are laid down in one type of orientation, but afterwards are changed by some physiological force to a new position. There are two major categories of primary fibre orientation mechanisms. In Section 5.1, fibres are positioned by self-assembly, powered by intermolecular forces. In Section 5.2, fibres are positioned by directed assembly, driven by some type of cellular mechanism.

Where does the assembly of extracellular architecture take place? In the case of plant cell walls, cellulose is synthesized on the plasma membrane, whereas hemicellulose is synthesized in the Golgi apparatus and then transported by membrane flow to the plasma membrane. These and other components mix in the periplasm, which is a narrow region constrained between the most recently deposited cell wall layers (on its outer side) and the cell plasma membrane (on its inner side). Such mixing of matrix and microfibrils is confirmed by direct observation's on the developing periplasm of quince seed epidermis (Section 4.3f). It is fortunate for future work that the quince periplasm is so large; a major experimental problem with most systems is that the zone where assembly takes place is so thin.

The existence of helicoidal structure in some plant cell walls is becoming gradually (if sometimes reluctantly) accepted. This requires that somewhere there is a rotary mechanism.