Helical Thickenings in Normal and Compression Wood of Some Softwoods

Compression wood in some softwoods having helical thickenings on the inner surface of normal wood tracheids were examined using a scanning electron microscope. Helical thickenings of Taxus, Torreya and Cephalotaxus have narrow bases, and are loosely attached to the innermost layer of the secondary wall, while those of Pseudotsuga, Picea and Larix have broad bases blended tightly with the microfibrils of the S3 layer in normal wood. The transition from normal to compression wood entails a preservation of the thickenings in Taxus, Torreya and Cephalotaxus, while they are replaced by helical ridges and cavities in Pseudotsuga, Picea and Larix. The direction of helical thickenings gradually changes from an S- to a Z-helix, or a Z- to an S-helix in the course of the transition from normal to compression wood, or vice versa in Taxus, Torreya and Cephalotaxus. Helical checks never occur in these species. In Pseudotsuga, however, helical thickenings can be deposited as an additional layer on the helical ridges. The results obtained in the present investigation revealed that the orientation of the thickenings did not always coincide with that of the innermost microfibrils of the secondary wall layers, indicating that helical thickenings may be considered as a layer independent of the secondary wall.