Plants

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. There is a monophyletic group of eukaryotes marked by the presence of primary plastids (plastids that are directly derived from cyanobacteria). This lineage, composed of glaucocystophytes, rhodophytes, and chlorophytes, including land plants, has been given several names (among them ‘Archaeplastida’ and ‘Chlorobiota’), but because it is the most inclusive monophyletic group of phototrophs that includes land plants, and because it accounts for the bulk of the organisms known both classically and informally as plants, we are comfortable with referring to this group simply as ‘plants’. This is generally consistent with the term’s usage colloquially and with the scientific literature going back to Ingenhousz and earlier. Some photosynthetic eukaryotes are not plants in the sense used here. These organisms rely on secondary plastids — plastids acquired either directly or indirectly by ingesting a rhodophyte or chlorophyte — and are not evolutionarily related to plants except in the context of this symbiosis. Plants, in the sense outlined above, are the dominant photosynthetic organisms in the terrestrial and some freshwater habitats, and they play a key role in both onshore and offshore marine environments. As a result these organisms are hugely important in the global carbon cycle and provide the basis for agriculture, timber, natural fiber, and other industries. Plants occupy a vast fraction of the Earth’s surface and have had a profound impact on the biogeochemistry and evolution of the planet (Figure 1). The colonization of the terrestrial environment occurred in the late Ordovician or early Devonian period, but red and green algae are far more ancient. The three ancient lineages of plants are glaucocystophytes, red algae, and green algae (including embryophytes).