Plants have been recognized since ancient times as a distinct lineage of organisms, but the precise boundaries of the group have only recently come into sharp focus. Land plants, or embryophytes, are immediately familiar as the organisms that constitute the terrestrial vegetation, dominate agriculture, and play a wide range of cultural and economic roles in human society. It is more difficult to identify the boundaries of plants as a whole because there is a vast diversity of photosynthetic eukaryotes that have affinities with land plants but are less conspicuous or lack the structural features of land plants. These organisms, algae, were classically treated as a subset of plants, but include both close relatives of plants and quite distantly related organisms: the term ‘alga’ is a form-classification, not a natural one. Some algae are naturally classified with land plants, while others are not. Furthermore, the original diversification of many algal lineages greatly predates that of land plants, so although it is tempting to think of algae as a kind of plant, it is really more accurate to think of land plants as a kind of alga.