how to find a dispersal agent

As seed plants evolved, they invented many different ways to disperse their seeds away from the maternal plant to new sites. We observe various adaptations for dispersal by wind (maple seed wings, fireweed and dandelion fluff) or on the outsides of animals (stick-tights and burrs) or by bribing an animal with a food reward. The many varieties of bribery have evolved many times in separate lineages.

The food rewards for vertebrate animals that disperse seeds are typically what we call fruits, which vary in size and composition; they also develop in a variety of ways on the maternal plants. Quite different plant tissues can be turned into rewards for seed dispersers—seed plants have been quite inventive!

If you venture farther into this essay, be warned that our common terminology often differs from that of botanists. The common parlance has little relationship to botanical technicalities and often reflects culinary uses rather than biology (and we are not about to change, of course). Furthermore, botanical terminology itself is confusing; it’s meant to distinguish different anatomical relationships and developmental pathways, but these definitions are not always precise and vary even among botanists. Rather than try to untangle the semantic snarls in a way intelligible to readers, I’ve curtailed the elaborate descriptions.

In the angiosperms or flowering plants, the food reward is commonly a fruit of some type, formed from the seed-containing ovary and sometimes additional tissue from the parent. Gooseberries, currants, peppers, and tomatoes are fruits that are simply mature ovaries containing multiple seeds; cherries, plums, peaches, and olives are similar, but with a single hard seed inside. All of those could be called botanical berries, broadly defined. But sometimes considerable material from the base of the flower and the upper stem gets incorporated into the fruit. Thus we get apples, pears, rowan or mountain ash fruits, and serviceberries in which the ovary is now just the core and the co-opted floral base and upper stem are the edible part. And in pomegranates and rose hips, those upper stem tissues make the covering of the fruit.

What we call ‘berries’ of raspberry and blackberry are really aggregates of numerous small fruits borne on an enlarged base of a flower. Mulberries are multiple fruits from several flowers, all lumped together, and so is an ear of corn. Strawberries are not fruit at all (botanically); the tasty part is an expanded flower base, and the things we call seeds are actually tiny fruits.

All of this got started as I peeled a mandarin orange for lunch one day. I’ve peeled many of them over the years, but I suddenly got curious about its botanical description. The citrus fruits are actually specialized berries! They are, however, unique in having the juicy little sacs inside the segments; even other members of their taxonomic family do not have this feature. Most of the commercial varieties of citrus are said to be hybrids of various sorts, but mandarins are one of the original types, coming initially from the Himalayan region, spreading first to southeast Asia and eventually, via trade routes, to Europe, North Africa, and beyond. This made me wonder just how did that initial range expansion occur–Was it all due to human transport or did fruit-eating animals play a role too? In citrus orchards today, lots of different animals feast on the fruit, including rats, possums, raccoons, squirrels, parrots. Could similar animals have served as dispersal agents in Asia?

In some cases, the food reward for animal dispersers comes, not from the ovary and associated tissues, but from the maturing ovule. Again, definitions vary, but broadly defined, these outgrowths of the ovule are called arils. The seeds of pomegranates are covered by arils, as are those of the invasive Asian bittersweet vine and passion flower. 

Even the gymnosperms get into the aril story: juniper ‘berries’ and yew ‘berries’ are really made from modified cone scales that cover the seed like an aril, although the edible part arises from maternal tissue (the cone) rather than the ovule, as in angiosperms.

Many flowering plants make seeds with appendages called elaiosomes; these too have varied developmental pathways. This type of animal dispersal has evolved quite independently from those involving vertebrates. The appendages look like little lumps on the outside of the seed. They are usually full of oils and are attractive to ants and sometimes other insects that carry away the seeds and later eat the elaiosomes. Examples include at least some species of Trillium, Claytonia, Corydalis, and Dicentra.


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. 

Meandering in the meadows

musings about seed dispersal

One fine day in late September—in between other days of slatting rain and wind, Parks and Rec hikers headed up the ridge behind Cropley Lake. It was a good day for that hike. However, I was feeling considerably less ambitious, so I chose another way to spend the lovely day. Oh, I started at Eaglecrest, all right, as did the main group, but I elected to meander about the mid-elevation meadows with a friend.

The first treat was a migrating red-tailed hawk soaring over the ridges, calling to another one half a mile away. This bird had very dark plumage, as is common in western populations, but the red tail was clearly evident.

That was a good start to our little explorations. We soon found another prize: a stand of alpine blueberries, which (maybe along with the other low-bush species) beat the tall-bush species for flavor. We browsed for a while and then figured out the best situations in which to look for more. Sure enough, the good ones were definitely concentrated in certain kinds of places (a secret, of course), so naturally we searched them out and browsed some more. Very satisfactory! We could have harvested a good bucketful, if we hadn’t eaten so many.

As we cruised around, we also encountered several stands of the high-bush blueberries, both the ‘early’ one and the ‘alaskan’ one, with good crops of berries still on the bushes. So all those folks who fussed about a poor berry crop maybe just did not go far enough.

We found the seed heads of leatherleaf saxifrage, with seed still in the capsules. I’m ashamed to say that I’d never looked closely at these before, but we did so now. And that led to a discussion of the various means by which seeds disperse from the parent plants. Here’s a quick synopsis with selected local examples:

Colorful berries are usually sweet (and a few are rich in oils), and both color and content are adaptations to attract vertebrate consumers that eat the fruit and pass the seeds through the digestive tract. The seeds survive gut passage and often germinate in a handy little pile of manure. If a bear is the consumer, the manure pile is quite sizable and competition among the hundreds of germinated seeds is fierce. When an urban human is the consumer, however, the usual dispersal pathway generally fails. I have seen animals eating all of our local kinds of berries except bunchberry (a.k.a. dwarf dogwood), which seems to remain in place until the berries just fall off (unless a mouse has opened the fruit and eaten the seed).

After a meal of Viburnum. Photo by David Bergeson

We’ve all seen the seeds of fireweed, cottonwood, and willow floating through the air on their fluffy, white ‘parachutes’. Lots of other plants disperse their offspring in similar fashion: goldenrod, aster, dandelions, to name a few. Still other plants put little wings on their seeds: all of our conifers, for example, and maples and alders. Both wings and parachutes are adaptations for wind dispersal.

The seed pods of lupine open explosively on warm, dry days, scattering the seeds. You can hear the pitter-patter of falling seeds if you listen for it. The dwarf mistletoe that parasitizes hemlock trees also disperses its seeds explosively. And the plant known as jewelweed or touch-me-not vigorously pops open its seed pods at the slightest touch.

Some plants make seeds with hooks that latch onto fur (or socks) and get carried some distance from the parent plant. Examples include bedstraw, avens, and some grasses. And plants that grow in water sometimes have flotation devices (but how do pond lilies and buckbean seeds get from one muskeg pond to another??).

There are many plants, however, whose seeds have no evident adaptation for dispersal; leatherleaf saxifrage is one such, and shooting stars, wild iris, and chocolate lily fit this category. In some cases, the stem that supports the seed capsule is moderately tall, and seeds may simply get shaken out at a short distance from the parent. In other cases, the entire plant may be eaten by a large animal and the seeds passed through the digestive tract. A few plants, such as the swamp gentian, put their seeds in splash-cups to await a raindrop that will wash them out. But in many cases, we are just left to wonder how the seeds get to new sites for establishment.

I have not mentioned a means of seed dispersal that occurs in other North American forests, namely dispersal by ants. The seeds of these plants have an oil body at one end of the seed; the ants collect and eat the oil body, and then dump the seed in their midden. Very neat! But, alas, I don’t know of any ant dispersed plants around here, and there may not be any, given our paucity of ants.

However, for many local species I do not know the most likely means of seed dispersal. Folks often go on walks to survey the variety of wild flowers that bloom earlier in the year, so why not try a walk to survey the variety of seed capsules and likely dispersal mechanisms?