Mosses are the most widely distributed of the bryophyte groups and the most diverse, with about 15,000 species. You may have seen moss growing on a shady log or on top of rocks by a stream. However, mosses grow in all terrestrial environments, from deserts to tropical rain forests. Liverworts (about 8000 species) and hornworts (about 100 species) are less widespread and also less diverse. In considering the diversity of these three groups, let’s start by reviewing some of their features that allow them to carry out photosynthesis on land.

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Bryophytes have small, simple bodies (Fig. 33.3). Some produce only a flattened photosynthetic structure called a thallus. Others consist of slender stalks and have a leafy appearance. However, these leaflike structures are quite different from the leaves of vascular plants in that they are only one to several cells thick and lack internal air spaces. Both thalloid and leafy species are found in the liverworts, whereas all mosses are of the leafy type and all hornworts are of the thalloid type.

Because bryophytes do not produce lignified xylem conduits, they cannot pull water from the soil. Instead, they absorb water, nutrients, and CO₂ through their surfaces. For this reason, bryophytes have little or no waxy cuticle to protect their photosynthetic tissues. Bryophytes can absorb enough water to remain metabolically active when the environment is wet, but they must be able to tolerate desiccation when the environment is dry. Thus, while the small size and lack of differentiated structures might suggest that bryophytes are delicate, nothing could be further from the truth. Bryophytes can be found from the equator to both the latitudinal and altitudinal limits of vegetation, and from swamps to deserts.

Before the evolution of vascular plants, bryophytes likely covered much of the land surface. However, because bryophytes are small and they can only photosynthesize when surface water is present, they are poor competitors for light and space with vascular plants. Today, bryophytes thrive in local environments where roots do not provide an advantage—for example, on the surfaces of rocks. Many bryophytes live on the branches and trunks of trees rather than on the ground. Plants that grow on other plants are called epiphytes (from the Greek words epi, “on,” and phyton, “plant”). Bryophytes are well suited to growing on other plants because they do not depend on the soil as a source of water.