Gathering Moss: A Natural and Cultural History of Mosses

Algae (singular: alga) are primitive, mostly aquatic, nucleus-bearing plants that have the capacity for photosynthesis. They lack roots, stems, leaves, and the specialized reproductive structures found in other plants. Algae range in size from microscopic organisms to some growing more than 200 feet in length. They play an important ecological role as a source of oxygen and food. Kimmerer focuses on algae in “Back to the Pond,” where she explains how mosses evolved from algae. She offers this explanation in part to demonstrate how fundamental algae are to all plant life on land and partly to make it clear why moss reproduction requires a wet environment.

The term “boundary layer” is introduced in the collection’s third essay, “The Advantages of Being Small: Life in the Boundary Layer.” The boundary layer is the space where air and land come together, and it is where mosses are particularly well-adapted to thriving. The boundary layer is a unique microenvironment where the small size of mosses is not a liability, but an advantage. The boundary layer is warmer than higher layers of air, and wind speeds are greatly reduced. This traps water vapor and carbon dioxide, enhancing the mosses’ ability to photosynthesize.

Bryology is the study of bryophytes: nonvascular plants including mosses, liverworts, and hornworts. It is a branch of botany, and those who practice bryology are called bryologists. Robin Wall Kimmerer’s specialty within botany is bryology, and this is the subject of Gathering Moss. Kimmerer first introduces this term in “Learning to See,” when she is discussing her work teaching students about mosses. Her introduction of this term at this point is not a coincidence: She has already discussed her teaching earlier in the text, but at this point she is talking about the importance of terminology. As she explains why it matters for her students—and for people in general—to know the specific terms used to describe various aspects of the natural world, she finally introduces the specific term used to describe what she teaches. She uses this as a springboard for her explanation of what mosses actually are and to correct some common misuses of the term “moss.”

Ecological succession is the process through which the species inhabiting an area change over time. It tends to unfold in a predictable pattern, as initial species interact with the environment and create favorable conditions for new species, these new species create new conditions, and so on, until a “climax community” is formed. The process then halts until a disturbance—such as a forest fire—alters conditions once again. Kimmerer introduces this concept in “Binding up the Wounds: Mosses in Ecological Succession.”

As Kimmerer observes in “In the Forest of the Waterbear,” mosses are epiphytes: nonparasitic plants that grow on other plants for physical support, deriving their nutrients and moisture from the rain, air, and debris from other plants and animals. Other examples of epiphytes are liverworts, lichens, bromeliads, orchids, algae, and some species of cacti and ferns. Within the ecosystem of a clump of moss, many other epiphytes usually make their homes.

In “Kickapoo,” Kimmerer introduces the intermediate disturbance hypothesis. This principle states that species diversity is greatest in areas that experience a moderate level of stress or disturbance. Where there is not enough disturbance—no fire, flood, or other source of environmental stress—highly competitive species will crowd out less competitive species. Where there is too much disturbance, all species are at risk. Moderate levels of disturbance keep dominant species in check and provide competitive opportunities for other species—as Kimmer observes after a flood pulls patches of Conocephalum away from the cliff, opening up space for less-dominant species to colonize.

Mosses and lichens are sometimes confused, as Kimmerer points out in “Learning to See.” Unlike a moss, however, a lichen is not actually a plant. It is a symbiotic relationship between a fungus and an alga, and as such it does not have leaves and stems like a true plant. Most of the lichen’s body is made up of a fungus, and its alga component provides the organism with the ability to photosynthesize. As an example of how mosses and lichens can be confused, Kimmer gives the example of “reindeer moss,” which is called a moss even though, in actuality, it is a lichen.

Like moss, liverwort is a primitive epiphytic bryophyte. It produces spores and lacks a vascular system, and so—again, like moss—it is dependent on a moist environment. Liverworts get their name, as Kimmerer explains in “Kickapoo,” from the shape many of them take. Their three-lobed structure reminded Medieval people of the shape of the human liver. Some liverworts are easily distinguished from mosses by this shape: these “thalloid” liverworts lack separate leaves and stems and instead show a single structure called a “thallus.” The only universal difference between mosses and liverworts is that liverworts have single-celled rhizoids—a feature that must be examined under a microscope.

In “Learning to See,” Kimmerer explains that lycophytes are a group of plants that are often confused with mosses. “Club moss,” for example, is not a moss at all—it is a lycophyte. Lycophytes are, like mosses, small, spore-bearing plants. Unlike mosses, however, they do have vascular systems. In fact, they are the oldest plants with vascular systems, and millions of years ago there were actually giant lycophytes that grew to over a hundred feet tall. Today, these larger varieties are long extinct.

Mosses—their nature, Robin Wall Kimmerer’s approach to studying them, and her beliefs about why they are worth studying—are the subject of Gathering Moss. Kimmerer gives a definition of what mosses actually are in the essay “Learning to See”: They are “the most primitive of land plants,” lacking most plant structures aside from stem and leaf (13). They reproduce through spores and have no vascular systems, no fruits, flowers, seeds, or roots. Part of Kimmerer’s fascination with mosses is that they are so small and primitive and yet so successful and diverse. These are motifs that she returns to in several of the collection’s essays.

Poikilohydry is the inability of certain plants to regulate their own internal moisture content. When conditions are wet, poikilohydric plants swell up with water, and when conditions are dry, they become desiccated. As Kimmerer points out in “An Affinity for Water,” mosses are poikilohydric; although this may seem to be a disadvantage on the surface, during times of extreme drought it is an advantage. Higher plants, with their more complex structures for regulating moisture content, can survive small swings in environmental moisture—but large swings often overwhelm them. Mosses, by contrast, can simply wait, dormant, in their dried state until there is more water available. Then, they rehydrate and resume their active state.