SIEVE‐PORE DEVELOPMENT IN SOME LEPTOSPORANGIATE FERNS

Abstract

Sieve‐pore development was examined in four species of leptosporangiate ferns: Phlebodium aureum, Platycerium bifurcatum, Pityrogramma calomelanos, and Regnellidium diphyllum. In all four species, sieve‐pore development is initiated with the formation of a barrel‐shaped periplasmodesmatal region—bounded by a narrow, electron‐dense band—around the prepore plasmodesma. This phase is followed by the deposition of callose, or of a calloselike substance, around the prepore plasmodesma. With deposition of this substance, the periplasmodesmatal region is no longer discernible, but an electron‐dense band continues to border the pore site. Perforation of the pore site involves removal of the calloselike substance and widening of the plasmodesmatal canal, both processes occurring more or less uniformly along the entire length of the pore site. During widening of the plasmodesmatal canal, numerous membranous elements appear within the pore. The number of membranous elements within the pore appears to decrease with increasing age of the sieve element. The location of feeding aphid (Myzus persicae) mouth‐parts indicates that the conducting sieve elements in Pellaea viridis are mature cells interconnected by membrane‐containing, callose‐free sieve pores.