Urban Jungle

February 7, 2012

The primeval world of moss

Cool, moist months are green season for moss. That's when it catches an extended break from summer's heat, which can desiccate the plant, shriveling it into dormancy.

A velvet ribbon of moss can thrive in the protective recesses of the narrowest, most heavily trafficked crack in a sidewalk. Moss sets up shop when airborne spores settle in the crack and are activated by moisture. Initial algaelike filaments mature into male and female plants with tiny leaves only one cell thick.

Moss will accumulate windblown dust, building soil beneath it. If untrampled, a moss mound will rise and spread beyond the crack, generating an oasis that can sprout seeds of flowering plants.

Mature male moss releases weak sperm, which will fertilize eggs held by mature female plants if the sperm can swim to them. Without a watery path to the egg, the sperm stands little chance of accomplishing its mission — unless the right bug arrives.

In 2006, ecologist Nils Cronberg and colleagues demonstrated that moss can be fertilized by small arthropods (springtails and mites). These tiny, winter-hardy critters are drawn to fertile moss, which produces nutritious sucrose, starches, fatty acids and mucilage. That plant/animal relationship may stretch back almost half a billion years, during the early stages of life colonizing land.

But now, down in that sidewalk crack, moss fertilization has produced zygotes, which have developed into spore-producing stalks growing out of the tops of mother plants. To a fast-paced pedestrian, these sporophytes might look like whiskers poking out of a fuzzy green fissure in the pavement.

Sources: Science; Jon Shaw, Duke University; Ohio Moss and Lichen Association; University of Missouri Extension