Sieve-element ultrastructure in Platycerium bifurcatum and some other polypodiaceous ferns: The nucleus.

Sieve elements of various ages were examined in Platycerium bifurcatum (Cav.) C. Chr. and Phlebodium aureum (L.) J. Sm., only older ones in Polypodium schraderi Mett. and Microgramma lycopodioides (L.) Copel. Early in sieve-element differentiation small crystalloids arise in the matrix of the sieve-element nuclei in Platycerium. As differentiation continues, the crystalloids increase in size and eventually may occupy up to a third of the cross-sectional area of the nucleus and extend almost its entire length. At the time of nuclear degeneration the crystalloids are liberated into the cytoplasm. Nuclear degeneration during sieve-element development in Phlebodium is essentially similar to that in Platycerium, with the exception that no nuclear inclusions exist in the sieve-element nuclei in Phlebodium. Stacking of endoplasmic reticulum against the nuclear envelope occurs in both Platycerium and Phlebodium. In the final stages of degeneration, the nuclear envelope ruptures and the contents of the nucleus mix with the cytoplasm. At maturity the sieve elements of all four species are devoid of nuclei, although occasional remnants of chromatin persist along the walls of some mature cells.