FIG. 3.

Growth pattern of B. subtilis. For the sidewall, new layers of wall are added repetitively on the inside of the existing wall but just outside the cytoplasmic membrane. The wall is stretched as it is forced outward and as the cell grows longer. Near the surface it becomes partially autolyzed and finally discarded. At certain locations a septum forms, growing inward by the same process of addition of new wall in successive layers as that shown in the central part of the figure. This wall is compact and unstressed until a central split exposes the septal wall to stress because the pressure inside the cell is higher than that outside. Then the wall covers more area, and when the septum splitting is finally complete, the same amount of murein that would constitute a planar septum becomes expanded in area by 50%.