Tensioning the helix: a mechanism for force generation in twining plants

Abstract

Twining plants use their helical stems to clasp supports and to generate a squeezing force, providing stability against gravity. To elucidate the mechanism that allows force generation, we measured the squeezing forces exerted by the twiner Dioscorea bulbifera while following its growth using time-lapse photography. We show that the development of the squeezing force is accompanied by stiffening of the stem and the expansion of stipules at the leaf base. We use a simple thin rod model to show that despite their small size and sparse distribution, stipules impose a stem deformation sufficient to account for the measured squeezing force. We further demonstrate that tensioning of the stem helix, although counterintuitive, is the most effective mechanism for generating large squeezing forces in twining plants. Our observations and model point to a general mechanism for the generation of the twining force: a modest radial stem expansion during primary growth, or the growth of lateral structures such as leaf bases, causes a delayed stem tensioning that creates the squeezing forces necessary for twining plants to ascend their supports. Our study thus provides the long-sought answer to the question of how twining plants ascend smooth supports without the use of adhesive or hook-like structures.

High‐Power Actuation from Molecular Photoswitches in Enantiomerically Paired Soft Springs

“…[23,25] Cutting the bar-patterned film leads to av ariety of shapes depending on the cutting angle,t hat is the offset angle between the long axis of the ribbon and the orientation of the bars (Figure 2a), including flat ribbons and helicoids (Figure 2c). [15] In the absence of any bar pattern, the ribbons do not twist significantly ( Figure S5). Thec hirality of the shapes are determined by the details of the microfabrication process.I nt he absence of as ymmetrybreaking element, the formation of right-and left-handed springs is equally probable,a so bserved in achiral hydrogels where no difference in structure or composition was implemented across the thickness of the sheet.…”

“…The device is operated by isomerization of al ight-responsive molecular switch that drives the twisting of strips of liquid-crystal elastomers.T he strips twist in opposite directions and work against eacho ther until the pod pops open from stress.T his mechanism allows the photoisomerization of molecular switches to stimulate rapid shape changes at the macroscale and thus to maximizea ctuation power.Designing shape-morphing materials has become am ajor scientific challenge,w ith implications ranging from soft robotics [1][2][3] to realizing the full potential of artificial molecular machines. [4][5][6][7][8] Thevariety of movements seen in the plant and animal kingdoms have provided inspiration for the engineering of soft robots of all kinds, [9][10][11][12][13][14] and in particular, the realization that plant mechanics often rely on dynamic helical systems [15][16][17][18][19] has motivated the development of av ariety of chiral actuators where molecules were used either as active transducers of energy [11,13,20,21] or as relays for humidity or temperature changes. [22][23][24][25] These soft actuators have demonstrated reversible shape transformation, work, and motility, [26] but the response speed and power produced remain moderate,mainly owing to the lack of mechanisms to drive non-linear actuation.…”

Computational Morphodynamics: A Modeling Framework to Understand Plant Growth

“…Alternative models describe vein patterning due to the distribution of the phytohormone auxin in tissues (23, 112). The role of the mechanical properties of plant tissue has also been used to explain the rapid dynamics of the venus fly trap and twining of plants (33, 43, 54). …”

Substrate diameter and compliance affect the gripping strategies and locomotor mode of climbing boa constrictors

“…By using helical wrapping, twining vines grip their host plant by self-tightening under tension (Silk and Hubbard, 1991;Isnard et al, 2009). Self-tightening of a helix can produce greater normal force with increases in the curvature (decreased diameter), the pitch angle and the number of gyres (Silk and Hubbard, 1991).…”