Abstract
This paper is chiefly a review of work published by the author and others on the structure of the outer secondary wall (Sl) of softwood tracheids and hardwood fibres. There is now considerable evidence that the fibrils of S1 form more than one helix. Two counter-rotating and symmetrical helices have been demonstrated and these have a helix angle of ±55° to ±75° in the tracheids of the softwoods that have been examined so far. In hardwood fibres, the helix angle is probably significantly smaller. The nature of longitudinal features associated with SI is discussed, together with the probable thickness of this wall. Micrographs are shown of S1 in situ in a wood tissue.
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