Fine Structure of Secondary Wall Thickening and a Role of Microtubules in Primary Xylem Cells of Poplar

Abstract

Secondary wall thickening and organization of the cytoplasm during development of primary xylem cells of poplar were investigated by sectioning and freeze etching technique. Microtubules usually aggregate only above the helical thickenings or bands throughout the the development of primary xylem cells but not above the areas where no secondary wall deposition occurs, and run parallel to the micronbrils within the bands. In view of their general distribution throughout cell organelles, plasmalemma particles of primary xylem cells visible by freeze etching technique are questionable to be considered as the enzyme complex involved in the synthesis and orientation of cellulose micronbrils of secondary wall. Thus, microtubules may be considered as the only organella to determine the orientation of micronbrils. A possible association of microtubules and Golgi-derived vesicles during wall thickening is assumed by the fact that the position and alignment of microtubules are closely related to those of Golgi-derived vesicles. It seems that the microtubules play a part for the incorporation of Golgi-derived vesicles .into the destined site of cell wall.