Cellulose fibrils direct plant organ movements.

The secondary plant cell wall is a composite of cellulose and a water-swelling matrix containing hemicelluloses and lignin. Recent experiments showed that this swelling capacity helps generating growth stresses, e.g., in conifer branches or in the stem when subjected to side loads. A similar mechanism also provides motility to wheat seeds. Here we study a simple mechanical model for the cell wall which--in contrast to earlier models--considers extensible cellulose fibrils in an isotropically swelling matrix. Depending on the detailed architecture of the cellulose fibrils, the model predicts that swelling may lead either to significant compressive or tensile stresses or to large movements at low stresses. The model reproduces most of the experimental observations in the wood cells and in the awns of wheat dispersal units. It is also simple enough to provide general guidelines for designing the architecture of fibres in an isotropic swelling medium to generate movements and forces of various kinds and directions.