Ultrastructure of phloem in palms

Summary

Organelles in a differentiating sieve element of a palm undergoe structural changes or dissolution prior to the maturity of the element. Plastids with electron dense inclusions and the mitochondria undergo some structural modifications and persist even after the sieve element matures, whereas microfilaments, microtubules, Golgi bodies, and most of the ribosomes disappear. P-protein filaments, if present during the earlier stages of differentiation, either disappear or apparently are incorporated with the aggregated ER. The nucleus becomes lobed and often shows a close spatial association with the Golgi bodies and smooth ER. A loss of chromatin matter of the nucleus followed by the invasion of cytoplasm in the space originally occupied by the nucleoplasm spells the complete disintegration of the nucleus except for the nuclear envelope which becomes folded or stacked, and subsequently is indistinguishable from the aggregates of smooth ER. Details of sieve plate pore formation and the modified ER that assumes a parietal position in the sieve element are described. A membranous structure, presumably ER, traverses the plasmalemma-lined sieve plate-pores in recently matured protophloem sieve elements. The sequential changes in the sieve element structure from the time of its inception to its near (morphological) maturity are summarized.