Aleksandra Rypień and Dorota Kwiatkowska examine colourful papery leaves, called scarious bracts, that surround the inflorescence of Golden Everlasting, the subject of their recent Annals of Botany paper Gradient of structural traits drives hygroscopic movements of scarious bracts surrounding Helichrysum bracteatum capitulum are adapted to move when environment humidity changes. Although all their cells are dead, these remain ready to bend toward the inflorescence centre to hide florets or outward to expose them.

Plants, despite the lack of muscles, evolved a wide range of mechanisms to generate motion. Movements of plant organs vary in speed: from very slow growth movements of shoots to snapping of carnivorous plant leaves or rapid explosive seed dispersal. Biologists have investigated how plant organs move in the absence of muscles since the pioneering work of Charles Darwin. Results of these investigations are crucial to understand plant propagation or reaction to changing environmental conditions. Moreover, we have adapted a number of movement mechanisms “invented” by plants to build modern constructions that are for example able to react by changing their shape in response to changing humidity or temperature.

Mechanisms of some movements seem incredible. This is the case for hygroscopic movements, where organs perpetuate reversible hygroscopic movements long after their separation from the mother plant or after plant death. Their mechanism is based on deformation of cell walls due to changes in water content, mainly in response to changes in humidity of surrounding environment. Because size, structure or composition of the cell walls in different organ portions are not the same, their wetting or drying generates compression in some organ portions and tension in others, which eventually leads to the movement. In its course the organ is often bending, twisting, rotating or curling.

Not the entire organ is involved in providing force driving the movement, but only its part called the actuator, which is built of an active and resistance part. Such construction resembles a thermally actuated bimetallic strip that curves in response to temperature changes, only the plant actuator responds to water rather than temperature. The active part is able to contract when drying or swell during wetting, while the resistance part stabilizes the developing forces. Hygroscopic movements contribute to flower protection, pollination, fruit or cone opening, or seed dispersal in numerous plant species.