Structural modifications in the extracellular investment ofNostoc commune Vauch. during the life cycle I. Motile and nonmotile normogonium, biseriate stage

Electron microscopic observations carried out onNostoc commune Vauch. in culture show that during development the extracellular investment undergoes evident modifications in size and structure. Since the investment of the motile hormogonium shows the characteristics of a “slime” and those of the nonmotile hormogonium and the biseriate stage show the characteristics of a “sheath,” the meaning of these two terms is discussed. In the slime, the existence of a reticular structure with polygonal meshes is recognized and is to be considered the structural basis of the organization of the investment. The transformation from slime to sheath takes place through a gradual diminution in the size of the meshes with a consequent increase of the fibril aggregation. The possibility is considered that slime-sheath reversibility as well as cessation of motility are related with modifications in the cellular secretion.