  Plant morphology emerges from cellular growth and structure. The
turgor-driven diffuse growth of a cell can be highly anisotropic: significant
longitudinally and negligible radially. Such anisotropy is ensured by cellulose
microfibrils (CMF) reinforcing the cell wall in the hoop direction. To maintain
the cell's integrity during growth, new wall material including CMF must be
continually deposited. We develop a mathematical model representing the cell as
a cylindrical pressure vessel and the cell wall as a fibre-reinforced viscous
sheet, explicitly including the mechano-sensitive angle of CMF deposition. The
model incorporates interactions between turgor, external forces, CMF
reorientation during wall extension, and matrix stiffening. Using the model, we
reinterpret some recent experimental findings, and reexamine the popular
hypothesis of CMF/microtubule alignment. We explore how the handedness of
twisting cell growth depends on external torque and intrinsic wall properties,
and find that cells twist left-handedly `by default' in some suitable sense.
Overall, this study provides a unified mechanical framework for understanding
left- and right-handed twist-growth as seen in many plants.
