The Ultrastructure of the Regenerating Root Cap of Zea mays L

Removal of the cap from the primary root of Zea mays activates cell division in the quiescent centre. It is the descendents of these cells that eventually regenerate a new cap—a process that is complete in about 4 days at 23 °C. The ultrastructure of the cells of the regenerating cap was examined at daily intervals. During the first day after decapping the dictyosomes in the exposed outer layer of cells change from a relatively quiescent state to one where they are secreting a polysaccharide slime which accumulates between the plasmalemma and the outer cell wall. Amyloplasts grow in size and appear to divide, and the endoplasmic reticulum proliferates. Many different cytoplasmic features that are normally characteristic of cells in distinct locations within the undisturbed cap occur, at first, all together within the few cells that are the source of the new regenerating tissue. Regeneration of a normal structure in the new cap is achieved by progressive changes in the structures of the cell organelles, apparently in response to the position that the cells containing them occupy within the growing cellular ensemble at the root apex.