We’ve been transforming a small wilderness behind our house into a rock garden. Clearing away litter revealed a tiny, unusual fungus I’ve wanted to find for years.
The Greek word for cup, kyathos, describes this little cup-shaped fungus, called Cyathus. Dozens were growing on a stump between stalks of moss, each looking just like a miniature cup nestled in the moss.
Cyathus’s design is near-perfection, if one takes into account the laws of physics involving hydraulics, acceleration, compression, and gravity.
The design is a splash cup. The idea behind a splash cup is to disseminate seeds or spores by the force of falling water as far away from the parent as possible. There are other organisms with splash cups, including other fungi, lichens, and seed plants, but this small fungus is so efficient, it’s difficult to find a more sophisticated example.
You can demonstrate a splash cup’s effect with a few dry peas placed in the bottom of a wine glass. If a mere ounce of water is dashed into the open glass, it accelerates while entering the narrowed bottom with so great a force that peas are shot out all over the place.
Now compare a wine glass to the tiny splash cup of Cyathus. A cross-section reveals parabolic, fluted sides that serve to direct a large raindrop down toward the constricted bottom. Increasingly confined within the narrowing cup, the water accelerates, then lifts the tiny, egg-like spore cases and ejects them out of their container. Cyathus spore cases travel three or four feet through the air, a remarkable distance considering the little cup is only a quarter of an inch in diameter and a spore capsule no more than a millimeter. (A different, larger kind of splash cup fungus shoots its bigger spore capsules over 20 feet.)
My backyard stump supported an entire sequence of Cyathus growth shapes, everything except the invisible web of underlying fungal filaments, or hyphae, that absorb nutrients deep within the decaying wood. The earliest visible stage is a tightly compacted, little brown ball poking above surrounding green moss. Next, a split across the ball’s top widens into a yawning opening sealed by a white membrane stretching from rim to rim like a tiny kettledrum. A central hole develops in this and widens as the membrane shrinks toward the sides. Finally, the wide-open cup reveals several oval spore cases nestled in the bottom, thereby earning the organism its other common name: bird’s-nest fungus. Now the cup is ready for the next rainstorm and a chance of being hit by a raindrop.
The splash cup is only the exterior of the fungus’s spore-producing organ. The spores themselves are encapsulated within oval “eggs,” each held inside the cup by a coiled thread. When a raindrop scores a direct hit, the thread breaks at its lower end and trails behind the ejected egg-like spore case as an extended tail. When the airborne thread comes in contact with nearby vegetation, its adhesive surface sticks to a leaf and wraps around it like a spring. There it waits for the plant to be eaten by a grazing mammal, after which the entire spore case travels intact through the animal’s intestine. Eventually, the little container is deposited along with dung so its far-flung spores can germinate under fertile conditions.
Energy in rainfall is a factor in the distribution of spores from yet another fungus we discovered as we cleared away leaf litter. Ordinary puffballs of field and forest are affected by blows to their papery, spherical spore cases. Poke one and a puff of brownish “smoke” shoots up from a central hole. The cloud is composed of hundreds of thousands – millions, even – of near-microscopic spores forcefully expelled into the air by sudden compression within the puffball.
What we uncovered was more than an ordinary puffball: it was an earthstar, Geaster. Under dry conditions, earthstars are puffballs tightly wrapped by five or six thick, triangular rays resembling “petals.” These protective flaps discourage spore discharge between rainstorms A finger-touch upon a dry, curled-up earthstar won’t evoke much of a response.
But when it rains, or even if there is only an increase in humidity, each earthstar ray absorbs moisture and peels back until, together, they form a star-shaped corona around the swollen puffball in the center. Now raindrops are able to hit the unprotected, thin-walled sphere; spores are ejected with each strike and fall to the wet ground several feet away. Or they may be caught by wind and carried around the world. When the inner ball is finally emptied of spores, it collapses and the entire structure, rays and all, rots and disintegrates.
The elegant, specialized adaptations of earthstars and bird’s-nest fungi soundly refute the mistaken impression that fungi are primitive and simple.