Kimmerer introduces the term “adaptive radiation” in “Sexual Asymmetry and the Satellite Sisters.” The term refers to the evolution of a large diversity of species from a common ancestor, with each new species adapted to a different biological niche. In “Sexual Asymmetry,” Kimmerer uses the term in her discussion of Dicranum 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.