A View of Ultrastructure of Cucurbita xylem

Abstract

1. Certain features of differentiation of helically thickened vessel members were observed. Young vessel members contain cisternoid endoplasmic reticulum, mitochondria, and dictyosomes. During differentiation vesicles with single-layered membranes replace the cisternoid endoplasmic reticulum; mitochondria appear to degenerate; and the dictyosomes cease to be discernible. The changes in the endoplasmic reticulum and mitochondria of maturing vessel members resemble those previously described for maturing sieve elements. 2. Parenchyma cells containing chloroplasts are associated with the vessel members. Cytologically, they resemble phloem-parenchyma cells. Plasmodesmata occur in the primary walls separating parenchyma cells and the immature vessel members.