The structure and degradation of bordered pits in Gymnosperm tracheids as revealed by the scanning reflection electron microscope

A study has been made of the bordered pit structure in the tracheids of Picea species, Pinus sylvestris and Pinus radiata with the aid of the scanning reflection electron microscope. Cellulase treatment was applied to gain more detailed information on the pit morphology. It was found that the pit membrane on both sides was covered with a thin membranous layer. The margo fibrils were digested in the cellulase preparation in nearly all samples. The central part of the Pinus torus was often found weakened and very flexible: in many pits it had entirely disappeared, as was found by Stemsrud (1956). After enzyme treatment the amount of this type of torus increased. In the branch wood of Pinus sylvestris many pits with aberrant membrane structures were found. As a whole the overall structure of the pit dome as described by Jutte & Spit (1963) and Murmanis & Sachs (1969) was confirmed. The pit porus in Picea species often seemed to be strengthened by the presence of a thick fibrillar strand, possibly representing the S3 layer. In the cellulase preparation the lignified cell walls looked smoother, proving that some changes had occurred in their chemical components. After cellulase treatment the inside of the pit domes in both Pinus species were readily lifted or peeled off in a flat helix, but not in Picea sp. The tori in the aspirated pits were very tightly attached to the porus whose edge seemed to be pushed into the torus. The torus was not removed during cellulase treatment.