Ultrastructure of metaphloem sieve elements inZea mays

Abstract

Metaphloem sieve elements from various parts of the plant body ofZea mays L. were examined with the electron microscope. No qualitative differences were found among sieve elements from sources, sinks, and intermediate regions of the plant. At maturity all sieve elements are lined with a parietal layer of cytoplasm, consisting of a plasmalemma, an anastomosing network of endoplasmic reticulum (ER), occasional stacks of ER, mitochondria, and plastids with protein crystalloids. Many mature sieve elements contained persistent, although apparently degenerated nuclei. In addition, in many mature sieve elements the vacuoles apparently continued to be delimited from the parietal cytoplasm by the tonoplast. In most relatively uninjured sieve elements the plasmalemma-lined sieve-plate pores contained little callose and cytoplasmic components and, hence, were largely unoccluded. P-protein was not encountered in any sieve elements at any stage of development.