The very rapid release of reproductive bodies is perhaps most famous in the fungi, where several methods of flinging spores at high velocity have evolved independently.

Numerous organisms achieve explosive discharge, very often to propel reproductive bodies such as spores or pollen. As these are light and rapidly decelerated by air resistance, extremely high velocities are necessary, and in some cases permitting propulsion over considerable distances. Whilst the trigger is usually the release of stored elastic energy, the motor is typically assumed to be a rapid increase in the turgor pressure. This pressure increase is mediated by the movement of ions across the wall housing the material to be projected. However, it is worth noting that according to a recent spectroscopic analysis modest pressure levels might actually be sufficient for fungal spore discharge.

Particularly striking examples can be found in the fungi and plants, and in both cases there are important examples of convergence. These examples are given below, but there is a more general principle worth drawing attention to. As Jan Skotheim and “Maha” Mahadevan have pointed out, two broad fields of deformation in all biological materials can be recognised. One involves explosive release that depends on sudden elastic release and the other is slower and depends on the transport of a fluid to deform a structure. Effectively it is a balance between viscosity (looking to “soft” behaviour that involves deformation) and the elastic modulus (that if high will serve as a spring). The line between these two fields, the so-called poroelastic line, is well demarcated. Two points arise: first both fields will be occupied (and often convergent) and second in some cases it is clear that the design is close to the physical limits of what is possible: things simply can’t get any better.

Explosive discharge in fungi

Sudden and abrupt release of energy, either by turgor pressure or elasticity, has evolved innumerable times, but the fungi are especially interesting in this regard, both for the range of types and the fact that some of them appear to lie on the edges of physical possibility, with for example massive accelerations. “More g’s than the Space Shuttle”, as Nicholas P. Money puts it (1998, Mycologia, vol. 90, pp. 547-558). Needless to say explosive discharge amongst the fungi is convergent, but the most extraordinary examples are found in the basidiomycetes.

Basidiomycetes

In a typical basidiomycete life cycle, spores are released from basidia, microscopic structures found on the fruiting bodies. The image of gentle winds wafting away the spores as they drop from the gills is correct, but the actual release of the individual spores is little short of explosive and the term ballistospory is highly appropriate. The mechanism is as follows: Beside the spore a small droplet (often known as Buller’s drop after the scientist who pioneered the study of this extraordinary process) is secreted. The drop contains sugars and attracts water vapour; it then joins the spores and very rapidly sweeps over the spore. This leads to a decrease in the surface free energy and in a Newtonian fashion gain in the kinetic energy; in addition the movement of the liquid displaces the centre of gravity of the spore. The net result is the spore is catapulted free. Although all this happens on a tiny scale, the accelerations are quite extraordinary, with ultra-high-speed video revealing values in excess of 12,000 g – far faster than the famous strike of the mantis shrimp.

What, however, of the marine mushrooms, such as Nia? Living in water would seem to rule out ballistospory, but here we find the evolution of spore discharge mechanisms extraordinary similar to those seen in the aquatic hyphomycetes (ascomycetes).

Amongst the basidiomycetes, a highly convergent feature is the evolution of the gasteroid/gasteromycete form (e.g. puffball, earthstar), where ballistospory is lost. However in forms like Sphaerolobus, which has been referred to as the acme of fungal artillery, a remarkable osmotic mechanism in the complex fruiting body can hurl the spore mass considerable distances, sometimes in excess of 5 metres. So, despite the gasteromycetes abandoning ballistospory, some have independently re-invented a catapult mechanism.

Ascomycetes

Independently this group has evolved what are appropriately termed “fungal cannons”. The eponymous ascospores are stored in the asci and their explosive release is precisely controlled and entails a sudden increase in turgor pressure with the tip of the ascus opening and the ascospores blasting out. Here too quite extraordinary accelerations are achieved, with some literature quoting an initial velocity of 300,000 metres per second, equivalent to 30,000 g!!!

This mechanism is most likely primitive, but it may have been acquired independently, and it has certainly been lost independently several times. Forcible discharge also finds its uses in pathogenic ascomycete yeasts that are endoparasites and have an explosive discharge of the ascospores into the host tissue that is assisted by a needle-like extension. Ouch!

Zygomycetes

This, the third major group of fungi, has also independently evolved methods of explosive discharge, notably in Pilobolus, a member of the Mucorales that commonly grows on the dung of herbivores. A squirt gun mechanism propels the spores over distances of up to 2 metres. Interestingly, this is exploited by some parasitic nematodes that use the spores as a means of distribution.

Explosive discharge in plants

Flowering plants

In the flowering plants we find another kind of explosive release, again leading to very impressive accelerations, but this time catapulting pollen into the air. At least two examples are known, and they are convergent.

The first involves the flowers of the North American bunchberry dogwood (Cornus canadensis), where the filaments of the stamens are recurved and so store mechanical energy, which requires turgor pressure. As the petals rapidly separate and flip back they release the stamens, which fling out their pollen – the whole process takes less than 0.5 milliseconds. Evidently the time of release is precise, and nearly all the elastic energy is released, leading to an impressive acceleration equivalent to about 2,400 g! In fact the overall design is similar to the medieval machine for siege warfare, the trebuchet.

The white mulberry (Morus alba) employs a similar mechanism, with the energy stored in the filament of the stamen, and again the process involves turgor pressure increase, followed by very sudden release. The velocities achieved here are even more impressive – being some twenty times faster than Cornus and more than half the speed of sound, this is the fastest motion observed in plants so far (much faster than the snap of the famous venus flytrap).

Non-flowering plants

While the above examples of explosive discharge involve changes in hydrostatic pressure, some non-flowering plants employ a different mechanism. Moss spores are usually dispersed by wind, but in Sphagnum spore discharge is explosive and achieved by what has been described as an air-gun mechanism. It was assumed that as the spore capsules dry out and shrink air pressure builds up, which ultimately results in the lid popping off. However, a recent study, where spore capsules were punctuated (which obviously prevents a rise in air pressure), has shown that differential shrinkage of the capsule walls is actually sufficient to explosively discharge spores.

In the tropical spikemoss Selaginella martensii, spores seem to be ejected in a pretty similar way. When the spore-containing containers open slowly, they separate into two ovoid valves, which then expel the spores as they dry out and close in a sudden, rapid fashion. The “female” megaspores are shot over a distance of up to 65 centimetres with an initial speed of about 4.5 metres per second. However, the mechanism still needs to be analysed in more detail.