The Ultrastructure of the Secreting Ligule of Selaginella kraussiana

Cultured stem tips of Selaginella kraussiana had mucilage on the surface of leaves and in the interstices between the ligule tip and developing leaves. Polysaccharide bodies, similar in structure to those in the mucilage, also were observed within the apical cells of the ligule. The apical cells of the ligule formed an elevated and distended cuticle at the interface between them. The space between the cuticle and cell walls contained small electron-dense granules or amorphous deposits. No pores or breaks in the cuticle were observed. The cytoplasm of the apical cells contained irregular and dilated endoplasmic reticulum (ER), numerous vesicles and small vacuoles, mitochondria, and plastids without starch grains; dictyosomes were not detected. The cuticle of the basal portion of the ligule was not distended. The dense cytoplasm of these basal cells contained parallel arrays of rough ER, dictyosomes, plastids, and numerous mitochondria. The ligule is specialized for the production and secretion of extracellular mucilage.