Botanically, mosses are non-vascular plants in the land plant division Bryophyta. They are small (a few centimeters tall) herbaceous (non-woody) plants that absorb water and nutrients mainly through their leaves and harvest carbon dioxide and sunlight to create food by photosynthesis. They differ from vascular plants in lacking water-bearing xylem tracheids or vessels. As in liverworts andhornworts, the haploid gametophyte generation is the dominant phase of the life cycle. This contrasts with the pattern in all vascular plants (seed plants and pteridophytes), where the diploid sporophyte generation is dominant. Mosses reproduce using spores, notseeds and have no flowers.
Moss gametophytes have stems which may be simple or branched and upright or prostrate. Their leaves are simple, usually only a single layer of cells with no internal air spaces, often with thicker midribs. They do not have proper roots, but have threadlike rhizoidsthat anchor them to their substrate. Mosses do not absorb water or nutrients from their substrate through their rhizoids. They can be distinguished from liverworts (Marchantiophyta or Hepaticae) by their multi-cellular rhizoids. Spore-bearing capsules or sporangia of mosses are borne singly on long, unbranched stems, thereby distinguishing them from the polysporangiophytes, which include all vascular plants. The spore-bearing sporophytes (i.e. the diploid multicellular generation) are short-lived and dependent on the gametophyte for water supply and nutrition. Also, in most mosses, the spore-bearing capsule enlarges and matures after its stalk elongates, while in liverworts the capsule enlarges and matures before its stalk elongates. Other differences are not universal for all mosses and all liverworts, but the presence of clearly differentiated stem with simple-shaped, ribbed leaves, without deeply lobed or segmented leaves and not arranged in three ranks, all point to the plant being a moss.