Mosses stay close to the ground. Their humble habit belies their importance to evolution and ecology. For one thing, they have been on the banner of this blog right from the beginning. I think I instictively chose mosses as the essential combination of biology and physical geography that underpins all the topics dealt with in these pages. Writing in the American Journal of Botany, Shaw and Renzaglia called the evolution of the mosses and their relatives one of the major unresolved questions in plant evolutionary biology. At stake is the title of first land plant. Without this innovation much of our planet would remain bare and devoid of life. Beyond their evolutionary significance, mosses play an important role in ecology – particularly the single diverse genus we know as peat moss. This moss is the beaver of the plant kingdom, molding the landscape to its purpose and, incidentally, providing us with fuel and fiber.
The three contenders for the descendant of the first land plant are the hornworts, the liverworts and the true mosses; their leaves and spore capsules are tiny but quite distinct. Hornworts have green horn-like spore producing capsules (pictured), while liverworts have lobed leaves that reminded early naturalists of the shape of a liver. True mosses have well-shaped leaves with mid-ribs that look like tiny versions of vascular plant leaves. Depending on the morphology or genes that are compared, liverworts usually come off as the most distinct of the three and potentially the most ancient of land plants.
The Species 2000 treatment of the mosses (or more technically, Bryophytes) divides the group into eight classes, ponting to deep divisions in each of the three traditional groups. Of these,we will focus on the peat moss today. They are, unlike most mosses, built to float. They have many dead (or hyaline) cells in their leaves that contribute to their buoyancy and they have a branching pattern and reproductive structure unlike any of the true mosses. The class is almost entirely represented by a single genus of 380 species – another example of a major branching event in evolution. Their floating lifestyle allows them to colonize and dominate bogs in temperate and boreal parts of the world. Though peat bogs do not grow quickly, they decompose even less quickly. As a rule, bogs accumulate a millimeter per year of biomass – leading to a 10 metre thick mat of vegetation in a wet depression that is only 10,000 years old.
Wetlands – a good propotion being peatlands -cover 2% of the earth’s land surface but in places like Canada that can be as high as 15 %, representing 60% of the carbon stored in the country. Peatlands also change the chemistry of the landscape. At first, it was supposed that the ability of peatmoss to absorb calcium and magnesium in nutrient poor bogs causes their high acidity. Recent research has shown that peat moss is not unusually good at absorbing nutrients, nor producing acids. The acidic water to which peat moss is highly adapted may just be a byproduct of the buildup of organic matter in peatlands.
Mosses successfully challenge the common theme among plants and animals that a good proportion of their body plan be devoted to tubes and hoses (arteries, veins, xylem and phloem) that keep the individual cells supplied in the manner that they were accustomed to as single cell organisms. They do this by staying small and close to the ground. This has not prevented them from being one of the most diverse and consequential of life forms.