Spine and root anchorage in a climbing cactus: A two-step attachment strategy and its relevance for technical bio-inspired devices

International audience

anglais. Climbing plants have countless strategies for anchorage and safe access to the canopy.Selenicereus setaceusis a climbing cactus from dry, lowland forests of Southern Brazil. Stems have “light” compositions, with limited stiff tissues, relying on turgescence and cross-section geometry for rigidity. It develops two types of attachment, via spines and roots in a combined “2-step” system. Spines are pre-formed on the edges of triangular stems, in groups of 3-5, pointing in different directions like grapnels. Spines are rigid with thick-walled cells guaranteeing instantaneous attachment with surrounding vegetation. Roots are formed later and emerge through the stem surface. Unlike spines their deployment is slower requiring growth and tend to only anchor to solid supports like tree trunks. To measure the strength of roots and spines, simple tensile tests were carried out in the field with a portable Instron device. One single spine can sustainca.2.6 N (corresponding to an equivalent stem length of 2.6 m and a mass of 285g. One root can withstandca.11.8 N (corresponding to a stem length of 10.5 m and mass of 1200g). This “2-step strategy” is efficient in guaranteeing anchorage in widely different environments; in moving (wind swept) shrubby habitats and in sheltered understory on massive tree trunks. Furthermore, one mechanism (spines) can facilitate the steadying or bracing of the cactus stem during the slower root growth. The mechanisms are of particular interest for the development of technologies applied to anchorage, in complex combinations of soft and hard tissues for soft-bodied artefacts.