Bryophytes are small, herbaceous plants that lack any vascular tissue, so they rely on simple diffusion to distribute water to their cells. They require a fairly damp environment to reproduce, as the sperm of one plant must be able to swim to the egg of another through a water layer.
All plants undergo alteration of generations, where a gametophyte plant reproduces sexually to produce a sporophyte plant that reproduces asexually to produce a gametophyte. In most plants, the gametophyte is small compared to the sporophyte, but in bryophytes, the gametophyte stage is much larger. The sporophyte generation is tiny, lives only a short time and is dependent on the parent gametophyte.
The three major types of bryophytes are mosses, liverworts and hornworts. Mosses are the most diverse group, and display the two general forms of cushiony masses of straight stalks or highly branched mats. Liverworts are the simplest of all living plants, and these look like flat scaly leaves with relatively large lobes. Liverworts are unusual among plants in that they store food as oils rather than as starches. Hornworts resemble liverworts in overall appearance, but their anatomical details imply a closer relationship to mosses. They exist in partnership with cyanobacteria, also known as blue-green algae.