The stem reaches a support
In Brazilian coastal restinga forest, Selenicereus setaceus grows among trunks, branches and rocks. Its young stems can extend beyond the canopy and span open spaces. Soffiatti and Rowe observed support-searching stems reaching approximately one metre. Supporting such a segment requires an appropriate combination of mass, shape and rigidity. [Soffiatti and Rowe 2020, Soffiatti i Rowe 2020, Results, Ecology and Phases of Growth]
The cross-sectional structure changes during development. Young segments have wing-like ribs and a star-shaped outline. Climbing stems are usually triangular, while older basal segments become circular or elliptical. Placing tissues towards the perimeter helps resist bending, while the older axis can bear the load of a stem already attached to its support. [Bastola et al. 2021, Bastola i wsp. 2021, pp. 2–3, Soffiatti i Rowe 2020, Results][Soffiatti and Rowe 2020, Bastola i wsp. 2021, pp. 2–3, Soffiatti i Rowe 2020, Results]
Soft tissues contribute to support
The stem consists of an outer skin, a succulent cortex and a central wood cylinder. Bastola and colleagues tested whole-stem bending and the properties of individual tissues. In support-searching stems, the cortex and outer skin together accounted for 60–94% of flexural rigidity. Water stored in the cells therefore also contributes to the mechanical performance of the stem. [Bastola et al. 2021, Bastola i wsp. 2021, pp. 1, 5–8]
In the measurements, the mucilaginous cortical tissue behaved as a viscoelastic solid. It stored part of the deformation energy and dissipated another part. The authors compared this organisation with a hydrogel enclosed within a skin. Its properties changed with tissue age. The central wood, in turn, made a substantial contribution to resistance to compression. This division of functions accommodates both water storage and support of an elongated organ. [Bastola et al. 2021, Bastola i wsp. 2021, pp. 5–13]
A spine stabilises the first contact
Areoles, short shoots that produce structures including spines, occur along the ribs. At the developing tip, spines point forwards and lie close to the surface. As they mature, their tips become recurved towards the base or directed sideways. These small hooks can catch a surface irregularity immediately upon contact. [Rowe et al. 2023, Rowe i wsp. 2023, section 3.1, Fig. 4][Soffiatti and Rowe 2020, Rowe i wsp. 2023, section 3.1, Fig. 4]
Rowe and colleagues separately pulled individual spines and roots connected to the stem. The spine test assessed the strength of its connection to the areole in a specified loading direction. Observed failures included bending and detachment of the spine and separation of soft areolar tissues. This yielding of the connection can limit transmission of excessive force to the whole succulent stem. [Rowe et al. 2023, Rowe i wsp. 2023, sekcje 2.2, 3.2, 4.4]
A root strengthens the connection
Adventitious roots develop after rapid attachment by spines. They connect to the internal wood cylinder and emerge through the soft tissues to the stem surface. Root attachment requires time for growth. The initial contact maintained by hooks therefore stabilises the organ while the later, stronger connection forms. [Rowe et al. 2023, Rowe i wsp. 2023, sekcje 3.1, 4.3, Fig. 5–6]
In tensile tests, the root–stem connection withstood a greater mean force than the spine connection. However, the authors tested the strength of the root and its anchorage inside the stem. The force of root adhesion to tree bark requires separate measurement because it depends on the surface and root development. The two-step mechanism describes the sequence of initial hooking and reinforcement of contact while retaining these distinct measurement boundaries. [Rowe et al. 2023, Rowe i wsp. 2023, sekcje 2.3, 3.3, 4.2]
Growth in a moving environment
Wind moves branches and changes the distances between attachment points. A system combining a compliant stem, numerous spines and roots that develop later can distribute loads among several sites. Restinga observations show that the same plant uses trunks, small branches, rock surfaces and the ground. The findings for S. setaceus provide a detailed model of this species. Other climbing cacti have their own combinations of traits that require separate investigation. [Rowe et al. 2023, Rowe i wsp. 2023, sekcje 4.1–4.3, Soffiatti i Rowe 2020, Discussion][Soffiatti and Rowe 2020, Rowe i wsp. 2023, sekcje 4.1–4.3, Soffiatti i Rowe 2020, Discussion]
The research attracted the attention of engineers developing robots that grow by extending their bodies. A lightweight winged cross-section, layered material organisation and the combination of a rapid hook with slower anchorage provide design principles. Cactus biomechanics thus traces a path from living tissue structure to a technical concept. Developing such a device still requires its own measurements of materials, forces and reliability. [Bastola et al. 2021, Bastola i wsp. 2021, pp. 11–14, Soffiatti i Rowe 2020, Discussion][Soffiatti and Rowe 2020, Bastola i wsp. 2021, pp. 11–14, Soffiatti i Rowe 2020, Discussion]
Failure without Tears: Two-Step Attachment in a Climbing Cactus
Rowe N. P., Cheng Clavel L., Soffiatti P., Failure without Tears: Two-Step Attachment in a Climbing Cactus, Biomimetics, 2023, 8(2):220, doi: 10.3390/biomimetics8020220
Two-step attachment of Selenicereus setaceus stems by spines and roots, with mechanical tests of failure at their connections to the stem.
Structural performance of a climbing cactus: making the most of softness
Bastola A. K., Soffiatti P., Behl M., Lendlein A., Rowe N. P., Structural performance of a climbing cactus: making the most of softness, Journal of the Royal Society Interface, 2021, 18(178):20210040, doi: 10.1098/rsif.2021.0040
Layered organisation of the Selenicereus setaceus stem, contribution of soft tissues to bending, rheological measurements and compression tests.
Mechanical Innovations of a Climbing Cactus: Functional Insights for a New Generation of Growing Robots
Soffiatti P., Rowe N. P., Mechanical Innovations of a Climbing Cactus: Functional Insights for a New Generation of Growing Robots, Frontiers in Robotics and AI, 2020, 7:64, doi: 10.3389/frobt.2020.00064
Growth form, variable cross-sections and two-step attachment of a climbing cactus as biological models for robotics.
