Plants: where to draw the line? (or, ICBN rules, OK!?)

Image: Carl von Linnés, Species plantarum (1753).

For years we can be happy using a word and thinking we know what it means, and presuming that everybody else also understands it in the same way. Well, what about the word ‘plants’? I realise I’m probably addressing many botanists here so we should at least be able to agree on what a plant is: but do we – and can we? Take, for instance, the name of this very column, ‘Plant Cuttings’; what is ‘plant’ in this context? Or the name of the journal in which this collection appears, the Annals of Botany (AoB). Botany is the study of ‘plants’ – but, again, what are plants in that context? Surely it’s obvious: plants are green things that photosynthesise. Well, maybe. But not all plants are green and they don’t all photosynthesise. And what about those – such as carnivorous plants – that photosynthesise but supplement their diets with animals? OK, can we agree that a plant is a member of the Kingdom Plantae then? Well, yes, but what about those large aquatic plant-like macro-algae that photosynthesise and are major primary producers in marine habitats? In a more pragmatic, inclusive approach I like to broaden the concept of plants to autotrophs more generally (but also include non-autotrophic members of Kingdom Plantae). But, if macro-algae are included, we can’t exclude autotrophic micro-algae; that would be ‘sizeist’. And since there is a long and noble tradition of including cyanobacteria (‘blue-green algae’, Kingdom definitely not Plantae) alongside the algae (because of their plant-like photosynthesis – amongst other features), we have a very broad definition of plants indeed. But broad is good, and useful. And if phytoplankton – ‘micro-algae (including cyanobacteria)’ – is good enough for our sister journal, the tautologically entitled AoB PLANTS (, then who am I to be contrary. Well – and not wishing to make this long story overlong (although it is an important point I’m trying to make) – a pragmatic solution proposed by one of the Handling Editors of AoB was that a definition of plants could be organisms that are covered by the International Code of Botanical Nomenclature (ICBN – Cheers, Mike!). The ICBN is the rule book for botanical(!) names, binomials that are given to plants (great), fungi (which really aren’t plants, are they?) and ‘a few other groups of organisms’ [which includes cyanobacteria (yay!), chytrids, oomycetes, slime moulds, photosynthetic protists (micro- and macro-algae) and taxonomically related non-photosynthetic groups – hmm, where does it end?]. Well, that seems good enough as a working guide (and, pleasingly, means that all of the items – so far! – in ‘Cuttings’ are ‘legal’). Furthermore – and this is the real reason for delving into the semantic niceties of the term plants – this rather broad interpretation justifies me mentioning one of the most amusing plant stories of late, that of a new species of fungus named (and you really couldn’t make this up, though somebody clearly has!) Spongiforma squarepantsii. The fungus from the rainforests of Borneo – named ‘in honor of the famed cartoon character SpongeBob SquarePants, whose shape shares a strong resemblance to the new fungus. Moreover the hymenium when observed with scanning electron microscopy looks like a seafloor covered with tube sponges, reminiscent of the fictitious home of SpongeBob’ – was formally described by Dennis Desjardin et al. in Mycologia [2011; doi:10.3852/10-433]. Fun guys, these mycologists! (Mycologists that is who should really be botanists, per the ICBN…)