The best bed I ever had was a deep carpet of moss under a big spruce tree in Berners Bay. Although I was certainly very interested in the project we were working on up there, it sure was hard to roll my carcass out of that cushy bed every morning.
That was about the extent of my interest in mosses until recently, when a friend coaxed me (it wasn’t hard) to try to learn to identify a few common species of moss. Little did I know what I was getting into!
I’m used to using field marks for birds — white outer tail feathers, a yellow wing stripe, maybe a black blotch on the belly. That approach seldom helps at all with mosses. Many of them look quite similar macroscopically and they can change their appearance as they dry. Some are very hard to distinguish from certain liverworts, which comprise a totally different group of plants.
So out came the hand lenses, especially the nifty (and inexpensive) ones with a little light. Now we can begin to see a few details of leaf structure and arrangement. That helps — and also sometimes confuses. However, many important diagnostic characters must still be discerned with a real microscope. So learning to identify most kinds of mosses more than superficially will remain a challenge.
In the meantime, I’ve reviewed some background about the biology of mosses, helped along by a charming book called “Gathering Moss” by B. W. Kimmerer.
Mosses are small, lacking both roots and vascular tissue to conduct water and nutrients around the plant.
Their leaves area very thin, getting water and carbon dioxide for photosynthesis by simple diffusion through the cell walls. Clinging to surfaces of trees, logs, and rocks, these short-statured plants live in a “boundary layer,” where the air is relatively still, evaporation is reduced and humidity is maintained. When mosses get too dry, they shut down metabolically, but they often can wait out a dry spell and be rehydrated, resuming normal functions.
When sexual reproduction is part of the life cycle (it isn’t always), that process depends on water. Sperm cells usually have to swim to reach fertilizable eggs, which are often produced on a different stem. Water is held among the stems and leaves by capillary action and surface tension, and the weak-swimming sperm have to negotiate a complex waterway on their way to an egg. Sperm cells don’t live long, so they have to get there quickly if they are to do the job. Most of them die en route. Recent research has shown, however, that sometimes moss sperm are carried around by tiny invertebrates, such as springtails, that live in the moss.
Mosses generally are either male or female, although some are hermaphroditic (both male and female). The green plant that we see so commonly is called the gametophyte generation, because it produces sperm and eggs. When a sperm has fertilized an egg, a shoot called a sporophyte grows from the embryo. At the top of the sporophyte is a capsule that contains spores, which will disperse to start the next generation of green gametophytes. These capsules come in a varied array of shapes and sizes that are sometimes useful for identification and are often aesthetically pleasing too.
Mosses host a mini-world of small creatures that fill all the roles of animals in the bigger world with which we are familiar — there are vegetarians, scavengers and predators. A fascinating community of tiny invertebrates commonly inhabits a single handful of wet moss: many thousands (!!!) of minute nematodes, rotifers, protozoans, springtails and tardigrades feed on bacteria, algae, fungi and each other. All these microscopic creatures would have interesting stories to tell, but tardigrades or “waterbears” are really quite captivating. They are only a fraction of a millimeter long, rather barrel-shaped, with eight stubby, hooked legs. They feed by sucking the juices out of plant cells or small organisms. Some apparently don’t reproduce sexually, but for those that do, fertilization is external and the eggs incubate in the shed exoskeleton of the female. When the moss dries out, tardigrades shrivel up and become hyper-dormant, reducing metabolic activity to almost zero. In this condition they are nearly immortal and can survive all sorts of normally lethal events, reviving when the wet returns.
Beyond the cushy bed in my camp and the sciences of moss biology is found a more artistic kind of appreciation. A visiting viola player took a walk in our forests on a nice day this past year. He reported being visually enchanted by the rich diversity of green hues and textures of the mosses (and ferns) that carpet the understory and drape the trees. He found that this visual orchestra connected in his mind with a certain piece of music he was playing during his stay here. As a professional musician, he knows music well, of course. Now, for him, there are visual colors and textures of the forest forever associated with his music. A nice example of syn-esthetics — the synthesis of different aesthetic senses. Oh, the fine art of “seeing” with multiple “eyes”!
• Mary F. Willson is a retired professor of ecology.