Jorge Luis Borges, in an essay commenting on the work of the seventeenth century philosopher John Wilkins, used the example of the (almost certainly fictional) “Celestial Emporium of Benevolent Knowledge” to make the point that all classification systems are on some level arbitrary and so inherently ridiculous. Wilkins, in his “An Essay Towards a Real Character, and a Philosophical Language” of 1668, had proposed a universal language in which the names of things were made up from components that logically defined what the named thing was, somewhat similar to Linnaean classification. The taxonomy from the Celestial Emporium, however, divided animals into fourteen categories, including “stray dogs”, “embalmed ones”, “those belonging to the Emperor”, “sirens”, “those that tremble as if they were mad”, “those drawn with a very fine camel hair brush” and, with inspired circular logic, “those included in this classification”.

Faced with the arbitrary and culture-dependent nature of classification, for practical purposes, what do we mean when we talk of “a plant”? Of course on one level it doesn’t matter. What we call things doesn’t change their nature: “that which we call a rose / by any other name would smell as sweet.” But as a journal called Nature Plants it is important that we have a clear view of what does and does not fall within our remit. We therefore read with considerable interest a recent essay in the American Journal of Botany1 that argues for a grouping of the organisms of the kingdom Plantae with green algae and other assorted organisms who share a common inheritance of photosynthesis.

The modern system of classification of life is based on the concept of phylogenies, with relationships between different organisms relying on their evolution from a common ancestor at some point in their past. This works well enough for animals, where horizontal gene transfer is very rare and so can either be ignored or regarded as a special case.

There is of course a large and significant example of non-parental inheritance common to all animals — indeed, to all eukaryotes — in the form of mitochondria. It is generally accepted that mitochondria are descended from some form of prokaryotic cell that became absorbed by the ancestral eukaryote around 1.7–2.0 billion years ago. Subsequent to this event there was exchange of DNA between the mitochondrial and eukaryotic nuclear genomes, in some lineages resulting in complete loss of the mitochondrial genome, but these events were so long ago that they don’t interfere with the phylogeny of animals. Also, the event seems to have occurred only once, or, if other prokaryotes became endosymbiotic with other early eukaryotes, their progeny do not survive.

Chloroplasts have a similarly endosymbiotic origin, but here things are not quite as straightforward. It seems clear that oxygenic photosynthesis evolved in cyanobacteria and the vast majority of chloroplasts trace their origin back to a single cyanobacterium that became endosymbiotic with a eukaryotic cell some time after the origin of mitochondria, but still over a billion years ago.

This organism is the common ancestor of embryophytes (essentially, land plants) and red and green algae, but the chloroplasts’ journey did not stop there. From the red line, chloroplasts have moved on into other algae, such as diatoms and haptophytes, while the green lineage has supplied chloroplasts to euglenids, among others. However, the amoeboid Paulinella traces its chloroplasts back to an independent capture of a cyanobacterium only 90–140 million years ago.

Because of the central importance of photosynthesis, Sanders has proposed grouping all organisms that perform photosynthesis using water as an electron donor and generate oxygen as a result, under the title “photoaerogens”1. This would chiefly consist of cyanobacteria, land and marine plants, and a selection of algae from a variety of clades.

It is an appealing notion and would overlap well with the range of organisms that have featured in our pages in the last seven years. But edge cases remain. Strictly speaking, the various plant species that lack chloroplasts would be excluded from this grouping. These include beechdrops (Epifagus americana), which is a parasite of beech trees; Indian pipe (Monotropa uniflora), which lives on mycorrhizal fungi; and a number of species of orchid, including the saprophytic phantom orchid (Cephalanthera austiniae). These are certainly plants with very close relatives within the photoaerogen club from which they would be excluded.

Conversely, there are organisms that engage in kleptoplasty. These quite literally steal chloroplasts by eating photoaerogens and, while digesting the rest of the organism, retain the chloroplasts intact. The ciliate plankton Mesodinium rubrum and several dinoflagellates use this strategy to benefit from second-hand photosynthesis, although such kidnapped chloroplasts tend to remain functional for only a limited time. Two species of flatworm from the order Rhabdocoela have also acquired the habit, as have a number of sea slugs, retaining the chloroplasts of their prey just beneath their skin. However, it is unclear whether they derive photosynthetic benefit from this behaviour.

Plants and algae, particularly the green lineage, have much in common, and research on one can inform our understanding of the other, but we will not be changing the name of Nature Plants anytime soon. Instead we will continue with our unscientific and imprecise aim to publish research that will be of high interest to plant biologists of all kinds.