Ultrastructure of phloem in palms

Abstract

Ultrastructure of mature phloem was studied from eighteen species in eleven genera of palms. Mature sieve elements are typically devoid of nuclei and contain only a thin layer of parietal cytoplasm. Plastids and mitochondria are present in various stages of degeneration depending on the age of the sieve element. Tubular structures apparently unrelated to P-protein are present in close association with the ER in many mature sieve elements. Membranous material comparable to the ER traverses the sieve-plate pores of mature protophloem elements in some smaller roots. However, sieve-plate pores of mature metaphloem elements are normally free of ER. Sieve-plate pores contain variable amounts of callose and P-protein-like filaments in most sieve elements of material that was cut into small slivers prior to fixation. However, sieve elements located near the middle of long slivers of material often have sieve-plate pores that are free of filaments after fixation. Sieve elements estimated to be several decades old are basically similar in structure to recently matured sieve elements except for the apparent decrease in the amount of ER, and increased degeneration of mitochondria and plastids in the older elements. Structural similarities and differences of sieve elements in palms are compared with those of other angiosperms.