Self-burial Mechanics of Hygroscopically Responsive Awns

Abstract

We present the results of a combined experimental and theoretical investigation of the mechanics of self-burial of some plant seeds whose morphologies respond to environmental changes in humidity. The seeds of Erodium and Pelargonium have hygroscopically responsive awns that play a critical role in their self-burial into soil. The awn, coiled in a dry state, uncoils to stretch linearly under highly humid condition because of a tilted arrangement of cellulose microfibrils in one of the layers of the awn's bilayered structure. By measuring the mechanical characteristics of the awns of Pelargonium carnosum, we find that the extensional force of the awn can be aptly modeled by the theory of elasticity for a coiled spring. We further show that although the resistance to the seed-head penetrating relatively coarse soils without spinning is large enough to block the digging seed, the rotation of the seed greatly reduces the soil's resistance down to a level the awn can easily overcome. Our mechanical analysis reveals that the self-burial of the seed is a sophisticated outcome of the helically coiled configuration of the awn.

Moisture-responsive films of cellulose stearoyl esters showing reversible shape transitions

“…Moisture-responsive behaviors are widely spread in nature, such as moisture-responsive events of plants and fungi during the dispersion of seeds and spores 1 2 , the opening of pine cones, twisting and bending of wheat awns ( Triticum turgidum ) 3 4 5 6 . During the alteration of the environmental moisture content, i.e.…”

Potential Effects of Awn Length Variation on Seed Yield and Components, Seed Dispersal and Germination Performance in Siberian Wildrye (Elymus sibiricus L.)

“…This indicated that awn length interacts with traits affecting seed dispersal in Siberian wildrye. Besides, some studies have demonstrated that environmental factors, such as wind, may affect seed dispersal under the aid of seed appendages, such as awns [23,41], which implies that awns may affect seed dispersal, especially under wild conditions. In our study, seed dispersal distance showed a negative correlation with seed length, indicating that longer seeds are not easily transported by seed dispersal agents.…”

“…It is reported that larger seeds are disadvantageous during dispersal but contain more carbohydrates reserves, which facilitate seed germination and give rise to more vigorous seedlings than smaller ones [20,21]. A number of studies have shown that long and barbed awns assist seed dispersal by adhesion to animal fur [22][23][24]. However, reports on the contribution of awns on germination behavior remain scarce.…”

Pneumatic Coiling Actuator Inspired by the Awns of Erodium cicutarium

“…That is, the walls of the Erodium awn cells have reinforcing cellulose fibers arranged in a tilted helical pattern ( Figure 1B) so that the longitudinal axis of the helical fibers does not align with the cell axis. As a result, when the ambient humidity drops, plant tissues in the Erodium awn would shrink in volume due to hygroscopy, and their tilted helical fibers can direct this shrinking into a coiling motion Aharoni et al, 2012;Abraham and Elbaum, 2013;Elbaum and Abraham, 2014;Jung et al, 2014;Zhao et al, 2017). Similarly, the awn would uncoil when humidity increases, essentially creating a "drill bit" in the plant kingdom.…”