Comparative development of intertracheary pit membranes in Abies firma and Metasequoia glyptostroboides

This study compares intertracheary pit membrane structure and ontogeny in Abies firma (Pinaceae) and Metasequoia glyptostroboides (Cupressaceae). Initial phases of pit membrane development are the same for both species. Branched plasmodesmata are present in the earliest stages of pit membrane development observed. Torus thickening of the pit membrane occurs early in pit development prior to pit border initiation. During pit border enlargement, plastids frequently occlude the apertures. Cell lysis is associated with complete wall matrix removal from pit membranes of Metasequoia. By contrast, cell lysis in Abies results in loss of matrix material from the margo, whereas the torus remains largely unaffected. Torus extensions in pit membranes of A.firma retain variable amounts of matrix material. Either a difference in chemical composition of the torus or a difference in autolytic enzymes is hypothesized to explain developmental differences between pit membranes of the two species.