Ultrastructural aspects of cellular differentiation in the root cap of Zea mays

The root cap of primary roots of Zea mays is composed of five distinctive types of cells: (i) the calyptrogen, or cap meristem, (ii) columella statocytes, located in the center of the root cap and characterized by the presence of sedimented amyloplasts, (iii) peripheral cells (PC) I, cells adjacent to the columella statocytes characterized by a more random distribution of amyloplasts and the absence of mucilage, (iv) PCII, cells containing mucilage between the cell wall and plasmalemma, and (v) PCIII, highly vacuolate cells typically detached from the root cap proper. As cells of the calyptrogen differentiate into columella statocytes, proplastids differentiate into amyloplasts which sediment to the bottom of the cell. Sedimented amyloplasts do not contact plasmodesmata in the bottom of columella statocytes. The starch content of amyloplasts decreases as columella statocytes differentiate into PCI. As PCI differentiate into PCII, mucilage is secreted into the space between the outer tangential cell wall and plasmalemma. As PCII differentiate into PCIII, numerous small vacuoles fuse into a large central vacuole. This vacuolation may be at least partially responsible for the movement of mucilage through the cell wall.