Root cap structure in the fern Ophioglossum petiolatum: light and electron microscopy

A correlated light and electron microscope study of the root cap of the fern Ophioglossum shows that a central and peripheral region can be distinguished. Central cells develop large distally displaced amyloplasts, somewhat thickened cell walls with plasmodesmata mainly in the transverse walls, vacuoles (many of which contain dense osmiophilic inclusions), and proximally located endoplasmic reticulum. Young peripheral cells have randomly located amyloplasts, vacuoles (some of which contain osmiophilic deposits), and rather uniformly and slightly thickened cell walls with a few plasmodesmata. With age, the peripheral cells undergo many changes including a loss of starch and the appearance of many plastoglobuli in the plastids, a thickening of cell walls (particularly on the outer tangential wall), and an eventual breakdown of the cytoplasm. During wall thickening, dictyosomes which pinch off two types of vesicles, electron-dense vesicles and those containing fibrillar material, are abundant in the cytoplasm. Both types of vesicles fuse with the plasmalemma and apparently release their contents into the wall. The root apical cell and its immediate derivatives are characterized by a large nucleus, plastids with small starch grains, many small vacuoles mostly containing dense osmiophilic deposits, and a cell wall with a large number of plasmodesmata.