Structural principles in the design of hygroscopically moving plant cells

Plants do not have mineralized skeletons. Instead, each of the plant's cells has an envelope of a cellulose-based wall, which provides a mechanical support to the organism. This stiff wall enables plants to assume flexible body shapes. However, the wall interferes with proteinous muscle-like movements of cells and organs because it is too stiff to yield to forces generated by motor proteins. Nevertheless, plants move constantly. The movements rely on water translocations, which result in the swelling (or growth) of cells located strategically. Water may swell protoplasts in movements that require live cells, like tip growth, tropism, and gas exchange. Other movements are initiated by the swelling of cell walls. These occur in dead tissues that can afford drying. The hygroscopically based movement is very common in seed dispersal mechanisms. The seed that detaches from the mother plant is carried by a cellulosic device. This device was synthesized by the plant and programmed to do some mechanical work, like jumping, crawling, and sowing, in order to deliver the seed to a germination location. This nonliving device provides the seed with means to move away from its mother and siblings. The movement may utilize several types of cells, which differ in the arrangement of cell wall cellulose microfibrils. I present here three types of contracting cells that, together with stiff fiber cells resisting any contraction, create a variety of hygroscopic movements.