They carpet shady forest floors and blanket old tree trunks and broad branches in spongy green. But they also persist in sidewalk crevices and cling to roofs, and in our gardens.
Mosses are an enterprising and persevering group of plants.
Carefully break open a vibrant green clump or feathery tuft of moss and you’ll see the individual plants, complete with stems, leaves, and tiny rootlets. Closer inspection will reveal how the minute, delicate leaves are arranged spirally around the stems. The arrangement of the leaves, as well as their size, shape, and color, vary a bit by species. On the order of 10,000 species worldwide, “moss” therefore shows a surprising variety of form and hue.
Think about where the bits will go when you’re done examining the moss colony: mosses will readily grow a new colony from shreds of an old one.
Later in the year, you’ll notice hair-like stalks holding tiny capsules high above the leaves. The capsules are full of moss spores; their shapes vary a bit by species, too.
Each capsule has a small opening at the far end, sealed by a cap that detaches when the spores inside are ready to disperse. Only dry spores disperse on the wind readily; moisture is kept out of the un-capped capsules by a ring of moisture-sensitive hairs or teeth around the opening. High humidity causes the hairs to close the opening and protect the spores; low humidity causes the hairs to relax and free the spores.
Sometime in summer, the countless spores will scatter on the wind. A few of them will settle in sites suitable for the new moss plants to thrive -- from branches to sidewalks, rocks to roofs. Not every moss thrives in every habitat, of course: the amount of direct sun and the amount of available water are key factors determining what lives where.
Like other plants we’re well-familiar with, mosses can reproduce sexually. In sexual reproduction, organisms split their genetic code material in half, sending their halves out (or making them available) to combine with the code-halves of others of their species. From a genetic perspective, there are two phases in this process: one phase where each cell has two sets of genetic material (called “diploid”) and another phase where each cell has one set of genetic material (called “haploid”).
In flowering plants and ferns, the cells in the phase we notice are the ones with two sets of genetic material. Plant lifecycles alternate between a phase with two sets of genetic material and a phase with one set of genetic material. In flowering plants, the one-set phase takes place inside the flowers and the one-set pollen and ovules combine (“fertilization”) to make two-set seeds that grow into two-set plants. In ferns, the one-set phase is a separate plant so small as to escape common notice.
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Mosses and their relatives are unusual in that the conspicuous, leafy plants are the one-set phase -- mosses can even be thought of as being male and female. Moss sperm swim in water to fertilize moss eggs to produce the two-set spore capsules that are raised above the leaves on thin stalks. The spore capsules make one-set spores that travel to make new one-set leafy mosses.
While mosses have leaves, stalks, and spore capsules, their water- and food-transport systems are not very efficient and they’re not well-protected against drying out: mosses need direct contact with a water source. However, most mosses can also survive dehydration and many seemingly dead mosses will resurrect with enough sustained moisture.
The moss you notice in sidewalk cracks when winter rains begin were almost certainly there in the summer, but dry and shriveled, waiting for the water. The rains revive the moss, making its glory stand out in places deciduous plants leave barren and drab in winter.
Here in the Pacific Northwest, many of our 900 species of mosses stray far from the proverbial north sides of trees. Because of their ability to shut down during our dry summers then revive during our wet winters, mosses can pop up everywhere, on trees, bare soil, and wet rocks -- including the cracks in our pavement and the tops of our buildings.
Mosses have experienced a bit of social revival in the last decade or so. Popular a century ago, “mossariums” or “mosseries” popped up again on-line in blogs, news services, and shopping sites. (A mossarium is a moist terrarium with a lid that features mosses instead of flowering plants or grasses.)
I made a few small moss gardens several years ago to try my hand at miniature horticulture. A tall glass cylinder, thrift-store stemware, old pie plates -- all served to house tiny experimental gardens. Now nine years old and five years old, most of those gardens thrive, although some species have muscled out others, according to their relative vigor and the characteristics of the tiny habitats.
The next moss gardening project? Encouraging a variety of local mosses to populate cracks in a newly laid used-brick path in the yard. That shouldn’t be too hard -- some of these intrepid plants started without my help within months of path construction, taking their rightful place in the shade.