Moss Is Cool

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I recently read Gathering Moss, by bryologist Robin Wall Kimmerer. I never intend any of my book review posts to substitute for actually reading the book, if it’s on a topic the reader finds interesting, but this post is particularly not a substitute. Gathering Moss is a lyrical, passionate collection of personal essays about moss. It’s my favorite nonfiction book I’ve read so far this year. If you love science and beautiful prose, I recommend it strongly.

Moss is really fucking cool.

Mosses are the simplest of all plants. They are small because they don’t have any support system to hold them up; a moss more than a few centimeters tall would fall over. Large mosses occur only in lakes and streams, where the water can hold up their weight.

The climate right next to the ground is different from the climate six feet above. You can see this yourself by lying down on a windy day: you’ll see that the air is more still and warmer. Obstacles create friction, which makes the air move more slowly. At the “boundary layer”, right next to the ground, the air is nearly motionless. The motionless air insulates the boundary layer, so it’s warm. The boundary layer traps water vapor and carbon dioxide. In fact, because decomposers like fungi and bacteria generally emit their carbon dioxide into the boundary layer, the boundary layer has up to ten times as much carbon dioxide as the general atmosphere.

Wet, warm, rich in carbon dioxide: the boundary layer is the ideal environment for photosynthesis. But the boundary later is also small—perhaps a millimeter high on a cliff face, maybe ten centimeters high in a moist forest.¹ Mosses, which can’t grow large anyway, have specialized in taking advantage of it.

Although small, mosses have their own ecosystems. One gram of moss contains about 150,000 protozoa, 132,000 tardigrades, 3,000 springtails, 800 rotifers, 500 nematodes, 400 mites, and 200 fly larvae. The moss ecosystem has its own epiphytes (plants which live on other plants) and predators. Early insect evolution occurred in moss mats, which provided a safe intermediate space between the water and the land.

Animals rarely eat mosses, because they are not very nutritious and have tough, indigestible cell walls. However, about-to-hibernating bears will eat a large quantity of moss, which binds up their digestive system so that they don't inconveniently shit.

Reproduction

Mosses are really bad at fucking. They are the incels of the plant world.

The smaller problem is that mosses reproduce by forming spores, which require wind to carry them to new places. But mosses have specialized in living in the boundary layer, where the air doesn’t move. They have to grow long setae (stalks) to get their spores to the part of the atmosphere with actual wind. This actually works out very well for them. Bryum argentum, the most common sidewalk moss, grows everywhere from Hong Kong to New York City to Antarctica. Its spores can travel half the globe before finally landing in a new home.

The larger problem is that mosses have yet to evolve the ability to have sex in any way other than expelling sperm into water and hoping it meets an egg. Unfortunately for mosses, they live on dry land, and also do not have legs that could take them to an ocean.

Mosses collect as much water as they can to try to ameliorate this problem, but it’s a race against evaporation. Within an hour, all the sperm a moss expels will die. The antheridium, which contains sperm, absorbs water until it bursts and squirts out all the sperm as far as it can. The moss sperm carries a soap with it to make the water easier to swim through. Some species have antheridia which are surrounded by a disk of leaves, so that if a raindrop falls on them it will go SPLASH and send the sperm as many as eight inches away. But even so, moss sperm are not very good at swimming: they seldom make it more than four inches away from the antheridium which produced them. Worse, moss sperm have no ability to swim towards an egg of a moss of the appropriate species; they just set off in a random direction and hope there’s an egg there.

As a consequence of this nonsense, many species of moss have given up on sex altogether and only reproduce asexually. Others are “bisexual” (simultaneously male and female), which makes sex easier at the cost of causing all your children to be inbred. In some Dicranum mosses, spores can become male or female. Spores that land on an unoccupied area will become female, while a spore that lands on a moss of the same species (and thus doesn’t have very far to travel) will become male.

Tetraphis Moss

A Tetraphis moss colony begins on the bare wood of a log. At first, Tetraphis reproduces asexually. It grows gemmae cups, which look like miniature birds’ nests, at the end of its shoots. Gemmae are clusters of ten to twelve cells which are already photosynthesizing. When it rains, a raindrop falls on the gemmae cup and splashes the gemmae up to fifteen centimeters away.

In a new colony, there’s plenty of space. The gemmae land near their parent, in a place that they know is good. The new plants are clones of their parent in order to double down on what works. Gemmae are large, which costs the plant more to make but increases the chance of success when the gemmae land somewhere.

As the colony gets more crowded, Tetraphis mosses switch to being mostly female (with a few males) and produce sporophytes. Tetraphis sporophytes are a capsule shaped like an open jar and ringed with four teeth (hence the name Tetraphis, or “four-toothed”). When the capsule is ripe, it releases millions of spores. A gemma would likely just land on another Tetraphis moss, not on good wood. It’s time to send their spores off in every direction to discover somewhere new; it’s time to have sex and take a roll of the dice to see if a new variation will do better than the current one. Because spores are a high-variance strategy, they are small and require little investment; Tetraphis mosses produce as many of them as they can.

When the colony is at a very high density, Tetraphis mosses become more and more male, because it’s more effective for an individual to be male the closer they are to other, potentially female mosses. Eventually, the male colony is spent and becomes brown and dry. The colony dies and is replaced with other mosses. But at about this time the log decays sufficiently that some of it falls off and fresh new wood appears, and the cycle begins again.

Sphagnum Moss

Globally, there is more living carbon in the Sphagnum genus of mosses than in any other single genus.

Sphagnum mosses are bog mosses. When you walk on a bog, you’re actually not walking on the ground—you’re walking on water held together by Sphagnum. In a bog, the pools of water are eerily still. The only source of water is rain, trapped by the Sphagnum moss, so there are no currents and no streams going in or out.

Only about one in twenty cells in a living Sphagnum plant is alive. Each leaf has lines of living cells circling patches of dead cells. The dead cell walls’ job is to absorb water. Lots and lots and lots of water. You can wring a quart of water from a handful of Sphagnum.

Because the soil is saturated in water, bogs are anaerobic. Because there is no oxygen for the microbes, decomposition in a bog is very slow, which is why we can find bog mummies.

Only the top layer of Sphagnum is alive at all. Peat is partially decomposed Sphagnum remains, crushed by the weight of the plants and water above it. Peat can be burned for fuel. Scotch whiskey tastes “peaty” because the malt is dried over a peat fire. The peat is up to fifty degrees colder than the surface. Historically, people who lived in the taiga would use cold peat as a refrigerator.

In the past, Sphagnum was used for diapers and menstrual pads. It can absorb can absorb twenty to forty times its weight in water, which is comparable to a modern disposable diaper. Its air spaces, astringency, and mild antiseptic properties prevented rashes and other unpleasantness. It was also used for bandages during World War I, when cotton was scarce.

Splachnum ampullulaceum, which grows in bogs, is perhaps one of the most specific mosses. It grows on deer feces. White-tailed deer feces. White-tailed deer feces which have laid on the peat for four weeks. In July.² Splachnum ampullulaceum has yellow and pink sporophytes which look like flowers in order to trick flies into landing on them. The flies collect the spores and transport them to nutrient-rich dung, which Splachnum ampullulaceum uses to grow as quickly as possible.

These are only a few of the interesting moss facts contained within the book Gathering Moss, which I recommend if you have gotten to the end of this post and therefore are probably unusually interested in mosses.

Gathering Moss: A Natural and Cultural History of Mosses, by Robin Wall Kimmerer. Published 2003. 168 pages. $12.

1. A moss itself can make the boundary layer larger by creating more obstacles and thus more friction, and many mosses do through evolving dense hairs, spines, long upright leaves, etc.

2. Its cousin species specialize in moose and in wolves and coyotes.