Helical thickenings in the tracheids of Taxus and Pseudotsuga as revealed by the scanning reflection electron microscope

Helical thickenings in the tracheids of Pseudotsuga and Taxus were studied by the scanning reflection electron microscope. From this it was found that both were part of the S3 layer. In Pseudotsuga they were branched in the radial wall, there having a much smaller angle of inclination than the spirals in the tangential wall, which were not branched. Their direction is the same as from the “streaming figure” in the bordered pit. The thickenings in Taxus showed up as sharply demarcated ridges of about the same width at regular intervals in the radial plane. The tangential plane was not studied. The helices here ran in a direction opposite to the “streaming figure” around the pit apertures. They seemed to be loosely attached to the S3 layer but no proof of that could be found.