Structural performance of a climbing cactus: making the most of softness

Abstract

Climbing plants must reach supports and navigate gaps to colonize trees. This requires a structural organization ensuring the rigidity of so-called ‘searcher’ stems. Cacti have succulent stems adapted for water storage in dry habitats. We investigate how a climbing cactus Selenicereus setaceus develops its stem structure and succulent tissues for climbing. We applied a ‘wide scale’ approach combining field-based bending, tensile and swellability tests with fine-scale rheological, compression and anatomical analyses in laboratory conditions. Gap-spanning ‘searcher’ stems rely significantly on the soft cortex and outer skin of the stem for rigidity in bending (60–94%). A woody core contributes significantly to axial and radial compressive strength (80%). Rheological tests indicated that storage moduli were consistently higher than loss moduli indicating that the mucilaginous cortical tissue behaved like a viscoelastic solid with properties similar to physical or chemical hydrogels. Rheological and compression properties of the soft tissue changed from young to old stages. The hydrogel–skin composite is a multi-functional structure contributing to rigidity in searcher stems but also imparting compliance and benign failure in environmental situations when stems must fail. Soft tissue composites changing in function via changes in development and turgescence have a great potential for exploring candidate materials for technical applications.

Failure without Tears: Two-Step Attachment in a Climbing Cactus

“…None of the failure modes appear to involve tearing or opening of the cactus water-storage system. This is consistent with ductile kinds of failure observed for stems failing under self-loading in this cactus species [8].…”

“…This requires integration of tough, stiff resistant materials with a “soft” water-storing body. Previous studies have underlined the “soft” construction of this species [ 8 ] and how it can form the basis for developmental, bio-inspired stem movements that respond to changes in the environment [ 7 , 9 ]. This current study has shown that despite a soft body organisation, the cactus is capable of producing two distinct and complementary attachment mechanisms.…”

“…Many of these functional aspects of climbing plants are becoming of interest for developing new technologies and new materials for soft robotics and other bio-inspired applications [ 1 , 2 , 3 ]. Aspects such as additive growth mimicking growth in length of climbing plants [ 4 , 5 , 6 ], adaptive orientated stem movements [ 7 , 8 , 9 ], hook and micro-hook attachment [ 10 , 11 , 12 ], and mobile searcher artefacts [ 13 , 14 , 15 ] are all examples of climbing-plant-inspired technologies that have co-opted specific biological traits for technical functionalities. Some of these technical innovations have not only reproduced specific tasks but have also included an adaptive or strategic functionality that can behave variably according to a changing local environment.…”

“…In this paper, we explore the two-step attachment system in the climbing cactus Selenicereus setaceus (Cactaceae) from dry coastal forests of eastern Brazil [ 8 , 22 ]. Cacti have highly adapted stems for water storage and photosynthesis, and this species shows a number of traits related to this condition: lightness of construction in terms of biomass, integration of a hydrogel-like material in soft tissues, and a simple layered organisation that has been the basis for new bio-inspired multi materials actuators [ 7 , 9 ].…”

Anatomical and Biomechanical Properties of the Junction between Stem and Aerial Roots of Selenicereus undatus

“…Additionally, the terms "nomadic vines" and "aerial hemiparasites" have been proposed for plants such as lianas and mistletoes, respectively [1][2][3][4]. All these habits have in common that they need some kind of anchorage to their host such as winding, root climbing or, in the case of some Cactaceae, spines [5,6]. In the case of attachment via roots, distinct anatomical specializations can be found, depending on whether the roots have an anchoring, nutrition conducting or combined function.…”