Changes in the Distribution of Plasmodesmata in Developing Fibre-tracheid Pit Membranes of Sorbus aucuparia L.

The distribution of plasmodesmata in cambial pit fields anddeveloping fibre-tracheid pit membranes in Sorbus aucupariaL. has been studied using conventional and high voltage transmissionelectron microscopy. In cambial pit fields, plasmodesmata arewidely separated while, during cell enlargement and the stageof secondary wall formation, they occur exclusively in a denselypacked cluster, eccentrically located on the pit membrane ina thickening of primary wall material. It is not clear at themoment whether the cluster arises by aggregation of the cambialpit field plasmodesmata, or by de novo formation in a localizedregion coupled with loss of plasmodesmata outside this region.The significance of these changes is discussed. Sorbus aucuparia L., plasmodesmata, pit membranes, xylem differentiation