A seed flying like a bullet: ballistic seed dispersal in Chinese witch-hazel (Hamamelis mollisOLIV., Hamamelidaceae)

Abstract

The fruits of Chinese witch-hazel ( Hamamelis mollis , Hamamelidaceae) act as ‘drying squeeze catapults', shooting their seeds several metres away. During desiccation, the exocarp shrinks and splits open, and subsequent endocarp deformation is a complex three-dimensional shape change, including formation of dehiscence lines, opening of the apical part and formation of a constriction at the middle part. Owing to the constriction forming, mechanical pressure is increasingly applied on the seed until ejection. We describe a structural latch system consisting of connective cellular structures between endocarp and seed, which break with a distinct cracking sound upon ejection. A maximum seed velocity of 12.3 m s −1 , maximum launch acceleration of 19 853 m s −2 (approx. 2000 g ) and maximum seed rotational velocity of 25 714 min −1 were measured. We argue that miniscule morphological differences between the inner endocarp surface and seed, which features a notable ridge, are responsible for putting spin on the seed. This hypothesis is further corroborated by the observation that there is no preferential seed rotation direction among fruits. Our findings show that H. mollis has evolved similar mechanisms for stabilizing a ‘shot out’ seed as humans use for stabilizing rifle bullets and are discussed in an ecological (dispersal biology), biomechanical (seed ballistics) and functional–morphological (fine-tuning and morphospace of functional endocarps) contexts, and promising additional aspects for future studies are proposed.

NeogeneCorylopsisseeds from eastern Tennessee

“…Fruits and seeds of Hamamelidoideae exhibit a similar morphology across genera (Endress, 1989, 1993; Zhang & Wen, 1996; Benedict et al, 2008). Some authors have speculated that this morphological consistency is due to functional constraints dictated by the method of ballistic seed dispersal in the subfamily, in which pressure exerted by the base of the capsulate fruit forcibly ejects the seeds (Endress, 1989; Benedict et al, 2008; Poppinga et al, 2019).…”

“…Corylopsis belongs within the subfamily Hamamelidoideae Reinsch, the largest of the subfamilies of Hamamelidaceae with 22 to 23 extant genera (Endress, 1993; Stevens, 2001–2019; Magallón, 2007; Li, 2008). Hamamelidoideae are distinguished within the family by having one seed per carpel and ballistic seed dispersal (Tiffney, 1986; Endress, 1989, 1993; Stevens, 2001–2019; Poppinga et al, 2019).…”

High‐speed video and plant ultrastructure define mechanisms of gametophyte dispersal

“…For instance, many species across the angiosperm phylogeny employ ballistic seed dispersal, which also involves rapid movements, and can be documented with high‐speed video. Video can be used to document the movements and calculate energy transfer and ejection velocities, as has been done in Impatiens capensis Meerb., Hamamelis L., and Oxalis L. species, as just a few examples (Hayashi et al, 2009 ; Edwards et al, 2019 ; Poppinga et al, 2019 ; Li et al, 2020 ). Other plant motions related to nutrition have been documented, such as in the carnivorous plants Aldrovanda L. or Utricularia L. (Vincent et al, 2011 ; Westermeier et al, 2018 ), and additional mechanisms can be investigated in terms of their biomechanics and physics (Skotheim, 2005 ).…”

Functional packaging of seeds

“…Ibicella lutea ; Horbens et al ., 2014). Furthermore, combined systems of responsive and static encapsulations exist; that is, hard‐shelled seeds inside responsive fruits, such as in the Chinese witch hazel ( Hamamelis mollis ; Poppinga et al ., 2019), or hard‐shelled seeds (testa and pericarp fused/ caryopsis) with a responsive remnant of the carpel, known as awn (in the Geraniaceae family; Abraham & Elbaum, 2013). Awns are moisture‐responsive appendages that are attached to seeds and enable (self) dispersal and burial.…”