Pit membranes in hardwoods?Fine structure and development

Abstract

1. The structure and development of the membranes (intervessel, vessel-parenchyma, parenchymal-intercellular and interparenchymatous) in wood cells of several Leguminosae species were investigated. 2. In very young cell elements the pit membrane represents the continuation of the middle lamella and primary walls of the adjacent cells, displaying however more structural details than inside the cell wall. 3. During the course of development, the intervessel pit membrane swells and loses dense incrusting material, becoming very transparent to electrons. This also happens to the vessel-parenchyma pit membranes, but assymmetrically, the modification being more accentuated in the layers of the vessel side. Subsequently, dense substances reappear and in completely developed membranes they can be quite abundant. 4. In vessel-parenchyma pits an additional layer (protective layer) is deposited between the plasmalemma of the parenchyma cell and the pit membrane. It is formed after degeneration of the vessel cytoplasm and shows a spongy texture. A similar layer is found at the parenchymal-intercellular membranes, thus suggesting the function of protecting the living cytoplasm against an external different medium, only from which it is separated by a thin permeable membrane. 5. Significant evidence was found indicating that vessel membranes consist of various fibrillar lamellae alternating with granular layers (incrusting material). At each side from the median plane, there are at least two possibly monofibrillar layers. The microfibrils of the more internal layers tend to group in a parallel arrangement and are continuous with the primary wall of the vessel. The more external ones show a loose, crossed texture. 6. Typical for the interparenchymatous pit membranes is the perforation by numerous plamsodesmata, a common feature of pit membranes separating living protoplasts. No openings were seen in developed vessel pit membranes up to 100,000X.