The need for a constraining layer in the formation of monodomain helicoids in a wide range of biological structures.

1. To be mechanically effective, supporting structures which are helicoidal need to be monodomain, with planar or concentric layers. 2. To achieve this in cholesteric liquid crystalline chemical models, a constraining surface is required. 3. The prediction which logically follows from this is that natural helicoidal systems in plant cell walls, spores, animal eggshells and cuticles need to be secreted within an initial constraining layer. 4. Evidence in support of this prediction is presented for a wide range of living systems, by reinterpretation of published work. This helps, at least partly, to explain the profusion of different kinds of layers in skeletal structures. 5. By contrast, systems lacking constraining layers have polydomain texture. 6. In plants, normal turgor pressure appears to be required for the deposition of monodomain helicoidal wall layers: reduced pressure leads to polydomain helicoid.