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Not every seed will find a fissure, so not every seed will be buried

Common stork's bill or redstem filaree, Erodium circutarium, is flowering now on the plains and in the foothills. Its blooms are small, just 10 to 15 mm across, but such a striking shade of deep lavender that they catch the eye. Each bloom has 5 petals, 5 stamens with anthers, and 5 styles. After the flowers are pollinated, a long, pointed fruit develops, encasing the 5 persistent styles which adhere together to form the stork's slender, pointed bill, 2.5 to 5 cm long.

Stork's bill is native to Europe, eastern Asia and northern Africa. But it is a very successful immigrant and is now established around the world, from north latitude 70 to south latitude 70, with the sole exception of Southeast Asia.  It was introduced to Mexico in the early 1500s during the Spanish invasion of Mexico and conquest of the Incan Empire. Ecologists assumed that stork's bill was introduced in 1769 to California when Spanish priests established a mission in present-day San Diego. But stork's bill pollen was found earlier, further north, in sediment cores taken offshore between Santa Barbara and the Channel Islands. It is now suspected that stork's bill dispersed by natural means from earlier missionary settlements in Baja, moving faster than the plodding pace of the missionaries. 

In 1852, the eminent botanist John Torrey, while commenting on botanical collections around the Great Salt Lake, noted that common stork's bill was so widespread across the west that it must be indigenous. Perhaps he did not take into account the 12 expeditions by Spanish conquistadors searching for gold between 1594 and 1787. Clearly, from its spread through Mexico and North America, stork's bill is an efficient invader. But why is it so successful?

Common stork's bill seeds explode from the plant, find a fissure, and screw themselves underground. Photo by Jeff Mitton.

Stork's bill seeds have elaborate excrescences (described below) that allow seeds to cling to the coats of horses, cows, deer and buffalo, providing them with the potential for long distance dispersal. But that is just part of the explanation.

Species in the genus Erodium share two features contributing to seed dispersal and establishment: ballistic dispersal of diaspores and self-burial of seeds. The diaspore is a seed and its elaborately shaped awn, a modified hair or bristle. The portion of the awn attached to the seed is tightly coiled (awn coil), and it has stiff hairs sticking out in all directions. The more distal portion of the awn (awn tail) is attached at a right angle to the awn coil, and it is gently curved. The awn, by virtue of its shape, holds the seed against the substrate. The awn is not living tissue, but it is hygroscopically active, meaning that it assumes different shapes depending on whether it is wet or dry. 

Common stork's bill seeds explode from the plant, find a fissure, and screw themselves underground. Photo by Jeff Mitton.

When tissue is very dry, the awn coil is a tight spiral and the awn tail is held at a right angle and has a gentle curve. But when the awn tissue is wet by a passing shower, the tissue soaks up water and relaxes--the full length of it might be straight. It regains its coiled and bent shape as it dries.

As the fruit of the stork's bill matures and turns from green to brown, the five straight awns, lying together inside the bill, build up tension as the tissue dries. Finally, the tissue of the stork's bill ruptures and the 5 awns coil and bend, exploding the fruit casing and ejecting the 5 diaspores through the air. If they do not hit adjacent plants, they will travel an average of one meter.

The next wetting will cause the awn to relax and straighten and if the end of the awn is lodged in the soil, tension will build up until the seed hops or slides along. Through successive wetting-drying cycles the seed is pushed along until it lodges in a fissure. Rain showers drive the cycles, but if relative humidity varies from low during sunny days to high during chilly nights, the coil will tighten and flex, moving the seed along. If fissures in the soil are common, most seeds find a fissure within 5 to 8 cycles. 

A seed in a fissure will allow the intricate awn structure to screw the seed deeper into the fissure, effectively burying it. Time lapses of this process shows a seed in a fissure, and the distal tip gripping the ground. By holding the distal end stationary, drying or wetting of the awn coil will cause the seed to spin as the coil relaxes or winds tighter, screwing the seed into the ground.

Not every seed will find a fissure, so not every seed will be buried. But buried seeds are less likely to be eaten by birds and mice and less likely to be damaged by a surface fire. In addition, comparison of the fates of seeds that buried themselves versus those that did not, showed that buried seeds had higher germination probabilities, higher growth rates, and superior survival. Stork's bills do not have brains, but nevertheless, natural selection favors, meaning that reproductive success is higher, for plants whose seeds "behave" well by searching for fissures and then burying themselves.