Sieve Element Cell Walls

Abstract

Sieve elements typically have non-lignified cellulosic walls similar to those of parenchyma cells. The walls are often thickened and in most instances this thickening is described as “nacreous” thickening or the “nacre” wall. The term “nacre” was introduced because of the refractve properties of this wall and its characteristic lustre when viewed with the light microscope. Cell walls are usually classified as either primary or secondary but in the case of the nacreous wall of the sieve element there is no general agreement as to the use of these terms.(Esau, 1969). The term primary wall is used for the wall which is first formed around a developing cell and which surrounds the protoplast during growth in surface area. The term secondary wall is used for wall layers that are laid down after growth in surface area has ceased. Although it has not been formally accepted that the nacreous wall is a secondary wall, it is best considered as such in order to clearly distinguish between the major wall layers. Between the primary walls of continguous cells is the intercellular substance.