A Light and Electron Microscope Study of the Mature Central Cell and Egg Apparatus of Solanum nigrum L. (Solanaceae)

Abstract

The central cell and egg apparatus of Solanum nigrum L. were examined to determine the location and extent of wall ingrowth (WI) development as well as the type and distribution of storage products. Wall ingrowths are not found along the embryo sac wall but do project as a single, although interrupted, band of varying thickness from the synergid/central cell and egg/central cell walls. Wall ingrowths projecting from the surface of the synergids extend from just above the hook region to the level of the nucleus. A narrower band projects from the surface of the egg cell and is located opposite the egg vacuole. Mitochondria are closely associated with the wall ingrowths in both the central cell and synergids. The ultrastructural characteristics of the central cell and egg apparatus are similar to those reported for other species. However, the chalazal end of the egg cell is covered only by its plasmalemma: no islands of wall material occur. The plasmalemma of the central cell is consistently withdrawn from the chalazal end of the egg cell, forming one or two small electron-lucent spaces filled with small amounts of fibrillar material. The mature synergids resemble secretory cells and possess abundant dilated cisternae of rough endoplasmic reticulum and numerous mitochondria. No degeneration of the synergid cytoplasm occurred. In longitudinal section the filiform apparatus is anchor shaped and stains intensely for carbohydrates with an outer layer staining for proteins. The major repository for storage products is the central cell where numerous starch-filled amyloplasts accumulate. Small amounts of starch and lipid are present in the egg cell, but the synergids contain only minor amounts of storage lipid in the form of osmiophilic droplets. A hypostase with extensive wall ingrowth development surrounds the antipodal cells.