Splash power

using raindrops for dispersal

Some weeks ago, I wrote about spore dispersal in bird’s nest fungi, in which the mature spores are held in a small cup and when a raindrop falls into the cup, the spores are splashed out. I decided to learn more about what other species use raindrops for dispersal. It turns out that raindrops have been put to work, so to speak, to disperse spores, seeds, little asexual propagules, and even sperm.

Splashcups are apparently the most common means of using raindrops. Seed dispersal from splashcups has been reported for many genera of plants, including some that grow in our area: Veronica (brooklime or speedwell), Sedum (stonecrop), Sagina (pearlwort), and Mitella (bishop’s cap). However, I’ve not been able to confirm that our particular species of these genera have this adaptation. Something to look for!

Splashcups for seed dispersal are typically small, just a few millimeters across, and more or less funnel-shaped, with the sides of the funnel not too steep and not too spread out. Small seeds are splashed out at various speeds, up to a meter or so away from the parent plant. Splashcup plants are generally small, herbaceous species; they grow in a variety of habitats.

Some mosses use splashcups too, for dispersal of sperm from male individuals. The raindrops give the sperm a head start on their way to receptive females, but once started, they have to swim to their final destination (thus needing a film of water to complete the journey). The juniper haircap moss (Polytrichum juniperinum) here in Southeast is one of these, and other local haircaps may also do things this way. Other moss genera in Southeast with this adaptation for sperm dispersal include Atricum, Plagiomnium, and Mnium, but again I’ve not succeeded in confirming that our local species do.

Another moss uses splash cups made of modified leaves to disperse ‘gemmae’, which are asexual clusters of cells that can germinate and form new individuals. This moss is called pellucid four-tooth moss (Tetraphis pellucida); it disperses its sexual spores by wind. Although it is found in south-central Alaska, Yukon, and B.C.—that is, all around our area, but there’s apparently only one record, so far, for Southeast (in Sitka).

The lung liverwort (Marchantia polymorpha) occurs all over Southeast and belongs to a genus known for gemmae dispersal by splashcups and raindrops. In addition, males bear their sex organs on little, stalked ‘saucers’ (not the technical term!) and sperm are released onto these saucers. When a raindrop hits the loaded saucer, the sperm can be dispersed as much as sixty centimeters away. Then they have to swim, in a film of water, to a female.

A familiar lichen genus is Cladonia, some of which are known as ‘pixie cups’. These make stalked cups that contain little asexual granules (technically called soredia) composed of bits of fungus and algae that are enough to start a new lichen individual. These tiny granules can be splashed up to a meter away by a raindrop, but they may also travel by wind.

Another type of raindrop-assisted dispersal is thought to occur in Tiarella (foam flower), which we often see here. The seed capsule is shaped like an old-fashioned sugar scoop, with a long lower lip. A rain drop hitting the lower lip could flip out the seeds. This spring-board mechanism is also seen in some club-mosses (Lycopodium, including our local L. selago), which disperse little vegetative, non-sexual propagules (called bulbils) this way.

Fungi have a couple of other unusual ways of using raindrops or at least water drops for dispersal. Some of the puffballs release spores when the tender top of the mushroom is struck by raindrops. Certain fungi (not specified in the available literature) release spores by a water-drop-driven catapult: There is one drop (apparently produced by the fungal spore) at the base of a spore and one (source not given) along the side of the spore. These two drops merge, creating a big enough drop with reduced surface tension so it breaks open, pushing the spore from its attachment and popping it loose.

See this video by Bob Armstrong for an example of fungal dispersal by raindrop.

Raindrops even get involved in pollination of flowering plants. In some buttercups (Ranunculus) and marsh marigolds (Caltha), rain splashes pollen from the anthers (where pollen is produced) to the receptive stigma of the same flower.

All those splashcups and springboards and catapults are evolved adaptations of the respective species. They function to the benefit of the plant or fungus by increasing reproductive success via successful dispersal.

However, raindrops may also have non-adaptive effects, such as transmission of foliar diseases. If a raindrop hits a leaf infected by bacteria, viruses, or fungus, it bounces, potentially carrying the infective agents with it in a water drop. The distance carried depends on many factors, including water-drop size, leaf shape and orientation and its motion when hit by the rain drop, and location of the original infection on the leaf. A complicated matter, indeed, but of some importance especially in agricultural monocultures. 

Three fungal curiosities

a tree-swallower, a “bleeder,” and a “nest”

This essay is about an eclectic assortment of fungi, my choices being based entirely on happenstance and whim.

One September day on the Outer Point trail, we spotted a very large, yellowish patch on a tall alder snag. That patch covered most of one side of the snag; at a guess, it may have exceeded ten square feet. At first glance I thought it was a huge crustose lichen, but no, it was a fungus. On certain places on this expansive crust, there were small ridges like miniature conks, and on their undersides were numerous pores where spores would be produced. The crust has covered mosses and engulfed the stems of licorice ferns.

Photo by Jennifer Shapland

I went back to look at it about ten days later, and now the surface featured many shallow furrows that had turned brown. The furrows were four or five millimeters wide and several inches long, crisscrossing the flat surface of the big fungus. Were they the work of slugs or land snails, grazing their way along? No, there were no marks of the scraping radula or tongue. A few days later, another observer suggested that wind-bent branches hit the fungus. No, when I tried it, small branches striking the surface didn’t leave furrows like that. A few more days later, we finally noticed that all the furrows occurred between human knee height and head height. Then I found that I could mimic the furrows very well with the flat of a fingernail. So we—finally!– concluded that these furrows are just graffiti. Much less interesting than slug trails, but perhaps closer to truth. ‘Twas a shame to deface such a beautiful organism.

What is this magnificent, hall-of-fame specimen? It seems that nobody really knows! It might be X or maybe Y, or something else altogether. Several mycologists have been consulted, with no concrete results. A sample sent to a lab for DNA analysis might yield a solid answer.

A strange fungus sometimes seen near the visitor center, among other places, is called the bleeding tooth fungus (Hydnellum peckii). Its name is not about bleeding teeth; it’s a tooth-fungus (with spore-bearing ‘teeth’ instead of gills or pores) that seems to ‘bleed’. The immature cap is white, but sometimes there are little pools of red fluid on the surface. Another name, perhaps for the more squeamish observers, for this critter is strawberries and cream. As far as I can tell, nobody really knows why those pools form on the cap. The present idea is that it’s a way to get rid of excess moisture, as some other fungi and some plants do by exuding droplets of water. But why are the pools red? Is the color just an incidental by-product of some metabolic process?

Photo by Jos Bakker

This fungus is mycorrhizal, forming connections with the roots of conifers and exchanging nutrients. The cap turns dark as it matures and produces spores. The stalk is thick and short, so short that the cap is often semi-buried in moss and debris. That raises the question of how the spores get dispersed from under the cap.

Yet another curiosity is a bird’s nest fungus (Nidula candida), a decomposer living on dead wood and rotting vegetation that is quite common around here. It earned its name from its appearance: the mature, reproductive form features a small (roughly one cm) cup containing several little round (somewhat flattened) objects. So it resembles a miniature bird’s nest with eggs. The ‘eggs’ contain spores. The cap is ingeniously built so that a raindrop can splash the ‘eggs’ out of the cup to a distance of a meter or more. The ‘eggs’ of some bird’s nest fungi (but not ours) are ejected with long, sticky threads, which catch on vegetation as they fly off. Eventually the ‘eggs’ deteriorate, releasing the spores. The spores germinate, producing stands called hyphae; two hyphae of the right mating type can merge to develop a nest-like fruiting body. Splash-cup dispersal mechanisms are uncommon means of spreading offspring around, but they are known also from some plants.

Photo by David Bergeson

Late October in Juneau

“little finds” in a tough time

Fall is rather a tough time for a curious naturalist here. Most of the songbirds are gone, the flowers aren’t blooming any more, the days are short and getting shorter, and most of the time it’s raining. Furthermore, it is hunting season, so deer, and ducks, and even gulls are rightfully wary and shy.

It’s not a great time for hiking either, although Parks and Rec always goes, anyway. A few days ago, we hiked up at Eaglecrest in a howling gale that drove celestial shrapnel in our faces. The next week, Mt Roberts offered strong winds, rain, and lots of mud. Although a few sections of the trail have been much improved, it is apparently a ‘work in progress’.

Not long ago, I walked with two photographer/naturalist friends down the trail to Bridget meadows and along Cowee Creek. We wistfully recalled the tiny toadlets of spring and the excited greater yellowlegs defending its territory in a muskeg. But ‘more eyes’ was useful, expanding our little finds and promoting fruitful—or at least entertaining—speculations.

In the forest and forest edge, birds were few: a Stellers Jay, a raven in the distance, a flock of crossbills checking out the spruce cone crop, and two male pine grosbeaks resting in a tall tree. Along Cowee Creek, there was more avian life: one eagle, a flock of mergansers and a group of bufflehead—all of which reversed their direction of swim when they spotted us, and a gang of gulls loafing on the riverbank across the way—and even they took off immediately. A vagrant seal, no doubt chasing in-migrating coho, was equally shy.

As we strolled across the meadows, we noticed many spots where some critter had been digging. Normally, we would assume that bears had been grubbing for tubers of riceroot (a plant that we call usually call chocolate lily, which is closely related). But there was little other bear sign out there, just one track and no scats. So we wondered if the horses that roam over the meadows, creating trails and pock-marks as they walk and leaving lots and lots of digested evidence, could possible have added root-grubbing to their repertoire. Hmmmm, a long shot, that!

I learned that the streams flowing into the big slough harbor cutthroats in spring. I also learned that some determined researcher in the Interior had found about sixteen thousand cones in the winter cache of a red squirrel.

Furthermore, I found out that one can distinguish poison-hemlock from its similar-looking but nonpoisonous relatives, not only by its chambered roots, but more easily by examining the leaf veins: a poison-hemlock leaf has veins that run from the midrib out to notches at the edge of the leaf, but the related species have leaf veins that go to the points on the serrated leaf edges. It is worth knowing the differences if you like to try tasting wild plants: Poison-hemlock is exceedingly poisonous; if you eat any part of the plant, you are likely to get very sick indeed, and possibly dead. Note: this plant is not related to our hemlock trees at all, although you probably don’t really want to eat those, either. Poison-hemlock is what Socrates used to commit suicide in ancient Greece.

Photo by Bob Armstrong

I found a twig that sported a row of distinctive fungi. These were bird’s nest fungi, so called because they have the form of a cup with several ‘eggs’ or spore capsules. But these were the smallest ones I’d ever seen, only about three millimeters across. They were holding spores, awaiting a direct-hit raindrop to splash them out of the cup.