Ultrastructural study on the formation of sclereids in the floating leaves of Nymphoides coreana and Nuphar schimadai

Abstract

The formation of star-shaped sclereids in the floating leaves of Nymphoides coreana and Nuphar schimadai was studied microscopically. These foliar sclereids were associated with the aerenchyma and found as the form of idioblast. The outer surface of mature sclereids was smooth in Nymphoides, but with many prismatic calcium oxalate crystals in Nuphar. However, the early morphogenesis of these two kinds of sclereids was similar. The sclereid initials were distinguished from the neighboring cells by their distinctly large nucleus. The expanding sclereid initials were constrained by the neighboring cells. Crystal formation in young sclereids of Nuphar started near the cessation of sclereid expansion. The crystals were bounded by crystal sheath and located in crystal chambers between the primary cell wall and plasma membrane. Calcium antimonate precipitates were found, especially on the crystal sheaths as well as between the plasma membrane and the primary cell walls. The crystal chambers have a paracrystalline appearance connected with the crystal sheath and the plasma membrane. After formation of crystals, the secondary wall was deposited and then the crystals became embedded between the primary and secondary walls. The possible functions of the foliage sclereids and the plans for further investigation are discussed.