E. coli cell wall is organized with the stiffer components
(glycan strands) oriented along the circumferential direction
[12]. It will be interesting to discover whether there
is mechanical anisotropy in plant cell walls, or whether
they are more like the fission yeast cell wall.
It is important to note that the anisotropy of growth
(elongation along only one axis) can occur using either
anisotropic or isotropic wall material; in fact, isotropic
material can be used to construct virtually any cell
shape. Furthermore, the mechanical properties of the
cell wall can be far more complex than the simple scaling
relationships we have described above. For example,
the relationship between stresses and strains will no longer
follow equation 1 at sufficiently large strains; recent
atomic force microscopy measurements indicate that the
E. coli cell wall exhibits nonlinear properties in its pressurized
state that may help the cell resist expansion during
hypoosmotic shock [13]. The assumption of a
constant thickness across the thin shell may also break
down, particularly during septation due to differences in
the mode of wall construction at the septum [14]. Ultimately,
these mechanical characteristics must be integrated
with the patterns of insertion and remodeling of
the wall, which can both alter cell-wall thickness and
lead to a viscoelastic response in which the wall material
flows like a viscous liquid when stressed. This produces
a diverse array of potential growth mechanisms in walled
cells. Biophysical models can provide testable predictions
for the relationships among turgor pressure, growth patterns,
and the distribution of strains and growth rate
across the cell surface [15,16].