The fern cavitation catapult: mechanism and design principles.

Leptosporangiate ferns have evolved an ingenious cavitation catapult to disperse their spores. 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. 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. Here, we study in detail the three phases of spore ejection in the sporangia of the fern Polypodium aureum. For each of these phases, we have written the governing equations and measured the key parameters. 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. Moreover, the measured parameters for the osmoelastic design lead to a near-optimal speed of spore ejection (approx. 10 m s(-1)). 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. 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 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.

The fern sporangium: a unique catapult.

Various plants and fungi have evolved ingenious devices to disperse their spores. One such mechanism is the cavitation-triggered catapult of fern sporangia. The spherical sporangia enclosing the spores are equipped with a row of 12 to 13 specialized cells, the annulus. When dehydrating, these cells induce a dramatic change of curvature in the sporangium, which is released abruptly after the cavitation of the annulus cells. The entire ejection process is reminiscent of human-made catapults with one notable exception: The sporangia lack the crossbar that arrests the catapult arm in its returning motion. We show that much of the sophistication and efficiency of the ejection mechanism lies in the two very different time scales associated with the annulus closure.

Perspectives on biological growth and remodeling.

The continuum mechanical treatment of biological growth and remodeling has attracted considerable attention over the past fifteen years. Many aspects of these problems are now well-understood, yet there remain areas in need of significant development from the standpoint of experiments, theory, and computation. In this perspective paper we review the state of the field and highlight open questions, challenges, and avenues for further development.

Cell surface expansion in polarly growing root hairs of Medicago truncatula.

Fluorescent microspheres were used as material markers to investigate the relative rates of cell surface expansion at the growing tips of Medicago truncatula root hairs. From the analysis of tip shape and microsphere movements, we propose three characteristic zones of expansion in growing root hairs. The center of the apical dome is an area of 1- to 2- microm diameter with relatively constant curvature and high growth rate. Distal to the apex is a more rapidly expanding region 1 to 2 microm in width exhibiting constant surges of off-axis growth. This middle region forms an annulus of maximum growth rate and is visible as an area of accentuated curvature in the tip profile. The remainder of the apical dome is characterized by strong radial expansion anisotropy where the meridional rate of expansion falls below the radial expansion rate. Data also suggest possible meridional contraction at the juncture between the apical dome and the cell body. The cell cylinder distal to the tip expands slightly over time, but only around the circumference. These data for surface expansion in the legume root hair provide new insight into the mechanism of tip growth and the morphogenesis of the root hair.

New Evidence for the Role of Mechanical Forces in the Shoot Apical Meristem.

The mechanism for initiation of lateral organs in the shoot apical meristem is still unknown. In this article we investigate one critical component of a buckling mechanism of organ initiation (that is, the presence and distribution of compressive stresses in the meristem). Direct evidence for compression in the sunflower capitulum was obtained from the gaping pattern of shallow cuts and the propagation of fractures. Cuts gaped widely in the central region of the capitulum but remained closed, or nearly so, in the generative and differentiation regions, suggesting the presence of circumferential compression at these locations. Fractures were initiated in the generative region and propagated circumferentially over most of their length. They did not cross the generative region perpendicularly, suggesting again the presence of compressive stresses in the circumferential direction. This conclusion was confirmed by the stress distribution computed from the geometry of the capitulum at three stages of development. One interpretation of these results is that the generative region corresponds to a zone of compression that could control the initiation of new primordia by means of buckling of the tunica layer.

Whorl morphogenesis in the dasycladalean algae: the pattern formation viewpoint.

The dasycladalean algae produce diverse whorled structures, among which the best known are the vegetative and reproductive whorls of Acetabularia acetabulum. In this paper, we review the literature pertaining to the origin of these structures. The question is addressed in terms of the necessary pattern-forming events and the possible mechanisms involved, an outlook we call the pattern formation viewpoint. The pattern-forming events involved in the morphogenesis of the vegetative and reproductive whorls of Acetabularia have been used to define five and six morphogenetic stages, respectively. We discuss three published mechanisms which account, at least in part, for the pattern-forming events. The mechanisms are mechanical buckling of the cell wall, reaction-diffusion of morphogen molecules along the cell membrane, and mechanochemical interactions between Ca2+ ions and the cytoskeleton in the cytosol. The numerous differences between these mechanisms provide experimental grounds to test their validity. To date, the results of these experiments point towards reaction diffusion as the most likely patterning mechanism. Finally, we consider the evolutionary origin of the vegetative and reproductive whorls and provide mechanistic explanations for some of the major evolutionary advances.

Pulmonary microsporidiosis in a patient with the acquired immunodeficiency syndrome. A case report.

BACKGROUND: Microsporidian protozoa are increasingly recognized as pathogenic organisms related to host immunosuppression. CASE: We report a case of pulmonary microsporidiosis detected in aspirated bronchial material and in bronchoalveolar lavage fluid collected from a 26-year-old, male acquired immunodeficiency syndrome patient presenting pulmonary symptoms and known to have intestinal microsporidiosis. The diagnosis was made on cytologic examination of the material processed according to the Papanicolaou technique, evaluated by Giemsa, Gram's, Grocott and chromotrope-2R modified trichrome preparations and by fungi-fluor fluorescent mycology stain. CONCLUSION: Our findings reveal an important fluorescent property of intracellular microsporidian spores, heretofore undescribed in the literature.