How do mushrooms shed their spores? The capillary catapult

Author: Chloé Gerin Co-author: Marc Joos Discipline:

Fluid Mechanics, Biomechanics

Abstract

Our experimental project takes place between physics and life sciences: the discharge of mushroom spores. In a catapult process using a water drop, the spores (about 10 µm seed-like particles) are shed at up to several meter/s away from mushroom gills. The understanding of this catapult mechanism could be applied to other cases, in a biomimetic approach. We propose to study this process in weightlessness, with a large drop size range. Summary

Our experimental project takes place between physics and life sciences: discharge of mushroom spores. This subject is about chemical engineering, uid mechanics, capillarity and biomechanics. Many mushroom species uses uid mechanics to ing their spores away, in order to insure their reproduction. Fungi such as mushrooms have a mechanism called ballistospore discharge that has interested mycologists for a long time [1, 2]. The discharge of ballistospore by basiodiomycetes is an elegant example. It uses uid mechanics at low-scale. A drop will merge with the spore to launch it away. The purpose of our experiment would therefore to study the discharge mechanism in weightlessness, with a large drop size range. What is precisely the fusion process? How does the system behave with a few centimeter drop? Better understood, this catapult mechanism will be hopefully used in a biomimetic approach for applications in engineering. This phenomenon mixes streight reproduction -that is, life sciences- and uid mechanics -that is, macroscopic physics-. It is interesting to note that this mushroom succeeds in using a certain force (capillarity) to foil another one (organism cohesion and air viscosity). After discussions with physicists working on this subject, it appears that it is dicult indeed impossible to reproduce this model in the laboratory. Only small drop and spore can be mimicked because of gravity. If gravity can be neglected at the scale of the mushroom, it can not be neglected anymore at the scale of the laboratory. In the International Space Station, it would be possible to reproduce

this phenomenon and study capillarity more precisely without gravity eects. The spore size allows to have a favourable surface force/gravity ratio. During the spore y, we have a favourable Stokes' force/gravity ratio. On a scale adapted to the laboratory, we lose these favourable ratios: gravity becomes more important. Working on Earth does not enable us to correctly identify the phenomenon, and modify important parameters in a suciently large domain for model validity. On weightlessness, it would be easier to increase diameter of water drops to a radius about one centimeter. It would therefore allow the study of the discharge mechanism. The spore is asymmetrically placed on the sterigma -the stem supporting the spore. Shortly before discharge a droplet of liquid appears on a site close to the point of attachment of the spore to the sterigma. This grows rapidly at rst by atmospheric condensation, and then less rapidly. After 20-30 s, the drop is about 10 µm in diameter. The spore and the drop then snap together, like an elastic band. The acceleration of the droplet is provided by the spore pressing on its point of attachment to the sterigma. When the mass of moving water moves past the sterigma, the recoil leads to the detachment and launch of the spore, as with a catapult. An acceleration of 250 km.s−2 takes place on a distance of 5µm. Without atmosphere, that is without viscous drags, what would this distance be? Moreover, we make the assumption that the spore speed is proportional to R−1/2 (R the radius of the drop), the speed decreases therefore when the radius increases, while acceleration increases as well. The spore will be launch farther because of the viscous braking characteristic timescale. This model can be validated on Earth only for a very restricted size range (drops about few microns). The experiment would be to change parameters like drop size or position, possibly to change liquid (i.e. density). We would use an adaptable articial system. Thanks to a high-speed camera, it would be possible to visualize the whole ejection process and to better understand the drop-support fusion. Doing quantitative experiments to centimeters would be helpful to understand these processes. Getting spore initial acceleration would enable us to go back to force that was needed to move the spore and to verify that the only present force is the result of surface tension. It would be interesting to observe a lack of force and nd out his origin.

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how do mushrooms shed their spores?

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