種子傳播機制之探討—以風力傳播與開裂傳播為例

Investigation of Seed Dispersal Mechanism: A Case Study on Wind Dispersal and Ballochory

蔡青芳(Jing-Fang Cai)

指導教授 ： 莊嘉揚

國立臺灣大學/工學院/機械工程學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202203075

摘要

樹木和果實多樣的型態是為了維繫下一代展開異地求生的機會，而種子傳播是子代遠離母樹的主要機制。植物僅需靠改變型態便可與環境產生動態作用讓種子被動的傳播至數十公尺遠，固本研究旨在瞭解種子傳播的力學機制，以期提供與自然保育、生態演化與仿生設計等領域相關的深入解析。
本研究將以風力傳播與自力傳播種子做為主要探討範圍，而其中風力傳播又以直升機式種子「登吉紅柳桉」三翅果為主要研究對象，而自力傳播則以開裂釋放機制的「洋紫荊」果莢作為研究對象。利用摺紙技術自製剛性與柔性紙摺三翅果，以簡化真實翅果之幾何形狀。以高速攝影機紀錄紙翅果自由落下過程，分析在不同摺角角度下其終端速度與旋轉頻率之間的關係，並以翼片元素理論相互驗證。進一步以流固耦合模擬來分析紙翅果周圍流場結構，來探討翅膀柔性與否對紙翅果自由落下的飛行性能影響；而洋紫荊果莢的開裂傳播機制則是以電子顯微鏡、奈米壓縮試驗以及細胞收縮實驗來探討瓣的各層微結構與其機械性質，並以三維非線性有限元素模擬重現瓣的各層結構及變形過程，並分析各層細胞的收縮及層數是如何影響其變形型態。本研究進一步以高速攝影機拍攝洋紫荊果莢開裂過程以及瓣變形後的壓縮試驗以計算開裂機制的能量轉換效率，並以模擬來驗證其能量存儲與釋放的現象。
綜整理論、實驗與模擬結果，本研究發現紙翅果在自由落下時，時間－速度曲線可分為加速、減速以及穩態三個階段，是由於翼前緣渦漩的生成讓受重力加速的紙翅果開始減速，而因紙翅果的自轉讓翼前緣渦漩穩定貼附於翅膀上，使翅膀上產生高升力而達穩定落下，而紙翅果也隨著摺角角度越大其終端速度與旋轉頻率也逐漸增加。翅膀的柔性導致與在相同摺角角度的剛性紙翅果相比，其落下所需時間較長，且落下偏移量較小，在種子的傳播中若下落時間越長則越有機會被傳播的更遠。在洋紫荊果莢的研究中，本研究發現洋紫荊瓣的變形型態與其微結構的層數與各層細胞非等向收縮的收縮率有關。並以模擬驗證其開裂機制，當洋紫荊遇乾燥，其瓣內會有應力儲存的現象，當瓣與瓣之間達臨界應力，此時瓣內的應力會被釋放，並轉換給種子，使種子被傳播出去，而實驗發現洋紫荊開裂的能量轉換效率為32.8%，與其他種開裂傳播機制的植物相比，洋紫荊算是具有高效的能量傳播機制。透過本研究對於翅果與果莢的傳播機制分析與討論，將來可運用在微型飛行器的幾何設計或軟性機器人的材料等仿生應用。

關鍵字

仿生力學 ； 翅果飛行機制 ； 果莢開裂機制 ； 流固耦合 ； 有限元素法 ； 翼前緣渦漩 ； 能量轉換

並列摘要

The variety of tree and fruit forms maintains the chance of off-site survival for the next generation, and seed dispersal is the main mechanism for the progeny to stay away from the seed-tree. Plants only need to change their form to have a dynamic interaction with the environment, allowing seeds to spread over dozens of meters away passively. This thesis aims to provide a combined experimental and numerical study on the mechanism of seed dispersal and to shed new light on the fields of plant conservation, ecology, and evolution, as well as bioinspired design.
This thesis considers two types of seed dispersal: wind dispersal and ballochory. For the wind dispersal case, we focus on Shorea polysperma, which has “helicopter seeds,” whereas, for the ballochory case, we focus on Bauhinia×blakeana pods whose valves suddenly open to spread the seeds. We use origami techniques to prepare rigid and flexible three-winged paper seeds, which capture the major mechanical characteristics of actual three-winged Shorea polysperma seeds but have simplified and more controllable geometry. We used a high-speed camera to record the free-fall process of the paper seeds and analyzed the relationship between the terminal velocity and the rotational frequency at different folded angles. The experimental results were consistent with the blade element theorem.
Furthermore, the fluid-structure coupled simulation was used to analyze the structure of the flow field around the paper seed to explore the influence of the flexibility of the wings on the flight performance of the paper seed in free fall. For the ballochory case of Bauhinia×blakeanap pods, we carried out SEM, nanoindentation test, and cell shrinkage test to study the microstructure and mechanical properties of each layer of the valve. Moreover, we used three-dimensional nonlinear finite element simulations to reproduce the structure of each layer of the valve to analyze the shrinkage of each layer of cells and how the number of layers affects its deformation pattern. The explosive process of Bauhinia×blakeanap pods and the compression test after valve deformation were further photographed with a high-speed camera to calculate the energy conversion efficiency of the explosive mechanism and the phenomenon of energy storage and release was verified by simulation.
Based on the theoretical, experimental, and simulation results, this research found that the time-velocity curve can be divided into three stages: acceleration, deceleration, and steady-state. Due to the formation of the leading edge vortex, the paper seed accelerated by gravity begins to decelerate, and because of the autorotation of the paper seed, the leading edge vortices are stably attached to the wings, generating high lift on the wings and falling stably. The paper seed's terminal velocity and rotational frequency also gradually increase with the larger the folded angle. The flexibility of the wings leads to a longer time to fall and a smaller fall offset compared to the rigid wings at the same folded angle. The longer the fall time, the better the chance of seed dispersal. In the study of Bauhinia×blakeanap pods, this thesis found that the deformation pattern of the Bauhinia×blakeanap valve is related to the number of layers, the orthotropic shrinkage of each layer. Moreover, the explosive mechanism is verified by simulation. When Bauhinia×blakeanap is dry, the valves become stressed, and strain energy gradually accumulates. When the critical stress between the valves reaches the critical value, the valves’ stresses are released, and part of the stored strain energy is transferred to the seeds so that the seeds are dispersed at high speed. Our experimental results show that the energy conversion efficiency of Bauhinia×blakeanap explosive was 32.8%. Compared with other plants with an explosive seed dispersal mechanism, Bauhinia×blakeanap has an efficient energy transmission mechanism. Other than helping understand the mechanics of seed dispersal, our findings may also be applied to bioinspired applications such as the geometric design of micro air vehicles or the materials of soft robots in the future.

並列關鍵字

bionic mechanics ； wind dispersal ； explosive seed dispersal ； fluid structure interaction ； numerical simulation ； leading edge vortex ； energy conversion

參考文獻

[1] J. L. Harper, Population Biology of Plants. N. Wales, Bangor, UK., 1977.

[2] L. Van der Pijl, Principles of Dispersal in Higher Plants. New Jersey: Springer, 1982.

[3] H. Ridley, The Dispersal of Plants throughout the World. Kent: Reeve and Ashford (in English), 1930.

[4] 盧姿敏, 有趣到不可思議的樹木果實圖鑑. 台北市, 2022.

[5] R. Nathan et al., "Mechanisms of long-distance dispersal of seeds by wind," Nature, vol. 418, pp. 409-413, 2002