( b) Opening of the sporangium in response to evaporation at the surface of the annulus. ( a) Closed sporangium with its annular cells filled with water. Schematics of the sporangium action as a cavitation catapult. Without a continuous column of water to sustain the elastic forces in the annulus walls, the elastic energy is quickly released, leading to fast closure and ejection of the spores as in a catapult ( figure 2).įigure 2. When the water tension is too large, cavitation occurs, and bubbles are formed within several cells. The force required to bend the annulus walls during opening is balanced by the negative pressure or water tension that develops inside the cells. It is this rotation of radial walls that drives the opening of the sporangium. The geometry of the cells is such that the decrease in cell volume forces the thick radial walls to rotate towards each other. Upon reaching maturity, the sporangium is exposed to air allowing water to evaporate through the thin outer walls of the annulus cells. The sporangium catapult mechanism is illustrated in figure 2. The annulus and spores are highlighted in blue and yellow, respectively. ( b) Scanning electron micrograph of a partially opened sporangium of Polypodium aureum. Two close-ups show an individual sorus and the sporangia within it. ( a) A fern leaf shows multiple sporangial clusters (sori) on its underside. In the case of the leptosporangium, a row of 12–25 cells known as the annulus is responsible for spore ejection ( figure 1 b).įigure 1. However, the same smallness prevents the spores from detaching easily from the mother plant hence, the requirement for an active ejection mechanism. The small size of the spores (less than 50 µm) allows them to be carried by air currents over great distances. įerns reproduce by means of spores that develop within a spherical sporangium ( figure 1). Considering that these favourable features are all encapsulated within a simple structural design, the fern leptosporangium offers exceptional potential for biomimetic applications. Among these mechanisms, the cavitation catapult of leptosporangiate ferns stands out on several counts: (i) it is the fastest plant movement on record, (ii) it is independent of cellular metabolism, and (iii) it is fully reversible. Plants have evolved a wide variety of mechanisms to achieve motion and have applied them to numerous functions such as dispersal, nutrition, support and defence. Our results highlight the importance of the precise tuning of the parameters without which the function of the leptosporangium as a catapult would be severely compromised. Finally, using high-speed imaging, we elucidated the physics leading to the sharp separation of time scales (30 versus 5000 µs) in the closing dynamics. Our analysis of the trigger mechanism by cavitation points to a critical cavitation pressure of approximately −100 ± 14 bar, a value that matches the most negative pressures recorded in the xylem of plants. Moreover, the measured parameters for the osmoelastic design lead to a near-optimal speed of spore ejection (approx. For the opening of the sporangium, we show that the structural design of the annulus is particularly well suited to inducing bending deformations in response to osmotic volume changes. For each of these phases, we have written the governing equations and measured the key parameters.
Here, we study in detail the three phases of spore ejection in the sporangia of the fern Polypodium aureum. The confluence of these three biomechanical functions within the confines of a single structure suggests a level of sophistication that goes beyond most man-made devices where specific structures or parts rarely serve more than one function. The mechanism relies almost entirely on the annulus, a row of 12–25 cells, which successively: (i) stores energy by evaporation of the cells’ content, (ii) triggers the catapult by internal cavitation, and (iii) controls the time scales of energy release to ensure efficient spore ejection. Leptosporangiate ferns have evolved an ingenious cavitation catapult to disperse their spores.
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