Dilated Endoplasmic Reticulum Cisternae in Differentiating Xylem of Minor Veins of Mimosa pudica L. Leaf

The differentiating tracheary elements in the xylem of minorveins in Mimosa pudica L. contain, as is usual, complete nucleateprotoplasts. Within the latter, dictyosomes with associatedvesicles and endoplasmic reticulum (ER) are prominent components.The ER cisternae show an uncommon feature of developing voluminousdilations accumulating finely fibrous material. Similar dilatedER cisternae occur in parenchyma cells associated with the trachearyelements. Most of the dilated cisternae are elongated, taperingat both ends, and almost circular in transections. They varyin size. The largest measured was 10 µm long and 3 µmwide. In the tracheary elements the cisternae break down asthe protoplast disintegrates. For a time, mature cells containfibrous material, apparently the product of the dilated cisternae,at least in part. In the parenchyma cells the dilated cisternaeare released into the vacuoles after the associated trachearyelements reach maturity. They become structurally modified anddisintegrate. The timing in the appearance and disintegrationof the dilated ER cisternae suggests that these structures havesome function with reference to the differentiation of trachearyelements in the xylem.