By Paula White
Long before flowering plants existed, mosses and other nonvascular plants (i.e. plants that lack nutrient transport mechanisms such as veins and stems) paved the way for subsequent life forms, including humans, to evolve. Though oxygen was present in the earth’s atmosphere 2.4 billion years ago, it took many more eons and the continuous production of oxygen by mosses and similar plants, known as Bryophyta, to create atmospheric oxygen levels similar to today’s. Mosses share space in this plant class with lichens, algae, liverworts, and hornworts, and common names tend to lump these small and similar looking plants together. Common names, while often descriptive, also create confusion. Take for example Spanish moss, which isn’t really a moss at all–it’s a flowering plant native to the oak forests of the southeastern United States. I’ve seen something that looks similar to Spanish moss draped over oak trees in the golden state, but they’re most likely lace lichen, common throughout California, or other lichen species.
With the recent rains, now is a good time to encounter mosses. Growing up I remember hearing that mosses grow on the north side of trees. They might, and then again, they may grow on the south side, if that side for whatever reason happens to retain moisture more than the north side. Forests are (usually) moist environments and provide good moss habitat. Trees in mixed oak/bay forests may appear to be carpeted with velvety green clumps of moss. Mosses are pioneer plants that help break down dead plant matter and even rocks and create niches for other species to thrive. Mosses also grow on soil, leaf litter, concrete, and even asphalt. Lacking roots, they instead attach themselves firmly to surfaces with web-like structures called rhizoids. Mosses absorb many times their weight in water and moss covered slopes resist erosion from rainwater. And they are beautiful!
You may be wondering how mosses get nutrients without a root system. Their leaves are only one cell thin, and thus absorb nutrients directly from the air. Mosses are highly sensitive to air quality and for this reason have been used as indicator species to detect pollutants. In Portland, OR, researchers used moss to detect elevated levels of cadmium, allowing them to pinpoint a previously unknown source of this heavy metal. A Berlin-based firm called Green City Solutions has gone a step further, installing 4 meter square moss walls fed by rainwater to filter out airborne pollutants. They claim that 80% of pollutants are removed by these moss walls. Historically mosses have been used in cold climates such as Scandinavia, Alaska, and parts of Canada to insulate walls and to cover the spaces between the doors and windows to prevent heat loss. Moss fibers from the common haircap (Polytrichum commune) make durable braiding materials and were also used to fill mattresses. And during the early 20th century soldiers’ wounds were packed with peat moss compresses–not only was the moss extremely absorbent but its antiseptic properties also relieved pain.
Mosses have a fascinating reproductive cycle. In phase one, gametophytes produce egg and sperm cells. The sperm cells swim across moist surfaces to find a structure called the archegonium which contains a single egg. During phase two, mosses produce sporophytes on a stalk (seta) with a capsule containing millions of spores on the tip. These capsules are often very colorful. Mature spores are generally released by wind and rain, though a visit from a pasing animal such as a squirrel or a bee could also trigger spores to disperse. Spore dispersal usually happens when the plants are dry. The next time you are walking in the woods, bring a magnifying glass and take a closer look at the mosses, and take a moment to appreciate these small plants that make such a big impact on our world.
Sources: New Scientist and Press Association . “Without oxygen from ancient moss you wouldn’t be alive today.” EARTH 15 August 2016. https://www.newscientist.com/article/2101032-without-oxygen-from-ancient-moss-you-wouldnt-be-alive-today/
Scully , Ruby Prosser. “Cities are using walls of moss to tackle air pollution from traffic.” TECHNOLOGY 20 August 2019. https://www.newscientist.com/article/2213755-cities-are-using-walls-of-moss-to-tackle-air-pollution-from-traffic/#ixzz7Bjos3U1x
Nordstrom, Ulrica. 2018. Moss: From forest to garden: a guide to the hidden world of moss. New York : W.W. Norton
Harthill, Marion P. and Irene O’Connor. 1975. Common mosses of the Pacific Coast. Healdsburg: Naturegraph Publishers.