  Understanding how growth induces form is a longstanding biological question.
Many studies concentrated on the shapes of plant cells, fungi or bacteria. Some
others have shown the importance of the mechanical properties of bacterial
walls and plant tissues in pattern formation. Here I sketch a simple physical
picture of cell growth. The study is focussed on isolated cells that have
walls. They are modeled as thin elastic shells containing a liquid, which
pressure drives the growth as generally admitted for bacteria or plant cells.
Requiring mechanical equilibrium leads to estimations of typical cell sizes, in
quantitative agreement with compiled data including bacteria, cochlear outer
hair, fungi, yeast, root hair and giant alga cells.
