CYTOLOGY OF SIEVE ELEMENTS IN MINOR VEINS OF SUGAR BEET LEAVES

S ummary In view of some questions raised in the literature on assimilate translocation regarding the degree of differentiation of sieve elements in minor veins of Beta vulgaris L. these elements were examined with light and electron microscopes. The sieve elements throughout the minor veins in the sugar beet leaf, including those in the free endings of the veins, have typically differentiated sieve plates, with pores usually lined with callose and containing P‐protein. The sieve plates occur in the end walls and the side walls. The sieve elements are connected with adjacent parenchyma cells (including the companion cells) by branched plasmodesmata. The protoplasts of the sieve elements undergo ontogenetic changes common for these cells in most vascular plants. The one to several sieve elements seen in transection of a minor vein attain maturity before the xylem is completely differentiated. No sieve elements are added after this stage, and the earliest mature sieve elements are not obliterated. The parenchyma cells (including the companion cells) of the phloem are generally several times wider than the sieve elements and constitute the major part of the tissue. However, the width of a parenchyma cell may vary at different levels so that in some sections a given parenchyma cell may not exceed a sieve element in width even if it does so in other sections.