Changes in the Nucleus and the Endoplasmic Reticulum during Differentiation of a Sieve Element in Mimosa pudica L.

Abstract

The differentiating sieve element in stem and leaf phloem of Mimosa pudica L. was examined with the electron microscope. The nucleus of a sieve element at first resembles that of a companion cell, except that its nucleolus is smaller. The first indication of approaching disintegration is a change in the appearance of the nuclear envelope. With the fixatives used, its profile becomes wider and clearer, giving the impression of having been dilated. Subsequently, the nuclear contents disintegrate and the envelope breaks apart. No nuclear remnants are discernible in mature sieve elements. The endoplasmic reticulum (ER) of young sieve elements consists of cisternae-bearing ribosomes. At the beginning of nuclear disintegration ER cisternae become stacked, with electron-dense material accumulating between the cisternae. The stacking assumes various forms. Some of the stacked ER is independent of cell organelles and occurs next to the plasmalemma along the cell wall. Part of the ER becomes stacked against the plastids or the nucleus. There is also unstacked ER which forms an anastomosing system along the plasmalemma. All of the modified ER bears no ribosomes. The anastomosing ER appears to persist longer than the stacked ER.