When is a yucca not a Yucca?

Notice the difference?  The italicisation and capital initial of the second Yucca.  That’s how the genus name of a species should be formally presented in a scientific paper, or in a newspaper article, or wherever.  Like Homo – the genus in which our own species (Homo sapiens) sits.

It might seem like a narrow and pedantic point, but it’s important.  Accurate and descriptive naming of species, genera, families and other taxonomic ranks is crucial to those of us who study biodiversity and is at the core of our science: without names for species, for example, we cannot make informed conservation judgements or comparisons between habitats in relation to which species are present or absent.  Names matter.

But it’s not just the names themselves, it’s also how they are presented which is important:  when I see the words yucca and Yucca in print, they signify two different things to me.  The word “yucca” is an informal name for a group of plants that is widely applied by gardeners and has no formal scientific status.  Yucca on the other hand refers to a very specific group of plants and has a clear meaning to a biologist.

To give you an example of this I’ll first have to introduce you to the Northamptonshire Natural History Society (NNHS) which was founded in 1876 and must be one of the oldest surviving local natural history societies in the country.  Some important 19th Century scientists were honorary members, including Charles Darwin, Thomas Huxley and Joseph Hooker.  This perhaps reflects Northampton’s proximity to London though there may be other factors: one could compile quite a long list of scientists with links to Northamptonshire.

The Journal of the Northamptonshire Natural History Society was first published in 1880 and continues to the present day.  Which brings us back to yuccas.  Last year a short article by a NNHS  member summarised the local weather conditions in Northampton for each month of 2010 (J. Northants Nat. Hist. Soc. vol. 45, no. 1).  December of that year was a particularly cold month and the author notes that “the species of Yucca trees planted in Northampton, which although thriving in recent years, were killed by the cold period”.

Strictly speaking Yucca refers to plants of a particular group which are endemic (i.e. only naturally occurring) to the New World.  The genus Yucca is a member of the asparagus family (Asparagaceae), subfamily Agavoideae.  The plants which suffered so much in the cold winter of 2010 are in fact New Zealand Cabbage Trees (Cordyline australis) which, as the name suggests, are endemic to New Zealand.  The genus Cordyline is also a member of the family Asparagaceae but belongs to the subfamily Lomandroideae and is therefore only distantly related to Yucca.

The leaves and stems of Cordyline and Yucca do look very similar, hence gardeners tend to use yucca as an informal name for both.  However when these plants flower it is clear that they are very different.  Flowers of the various species of Yucca are typically quite large and are adapted to pollination by a very specialised group of moths which lay their eggs within the flowers.  The reward for these moth pollinators is a brood site for their caterpillars, which feed on a proportion of the developing seeds of the Yucca plant.  In contrast the flowers of Cordyline australis are small and produced in very large numbers in dense inflorescences.  They are also highly fragrant, to which anyone who has grown one of these plants to maturity in their garden can testify.  The fragrance attracts a range of insects which feed on the nectar produced by the flowers and so pollinate them in the process.  It is these differences in flower structure, more than characters of stems and leaves, which taxonomists use to classify such plants.

Until recently large New Zealand Cabbage Trees were a feature of many front gardens across Northampton.  Some particularly fine examples were to be found along the Kingsthorpe Road between Osborne Road and Balmoral Road.  I suspect that the largest Northampton specimens were planted in the 1970s, perhaps because people wished to recreate something of the exotic feel of package holidays to Spain and Portugal.  Following the freezing weather of December 2010, the growing tips of most of Northampton’s New Zealand Cabbage Trees were killed and the top growth gradually withered and died.  I was sad to see this happen to my own plant, a medium-sized specimen that I had rescued from a skip at the University several years ago, and which had become well established in my garden.  However later in 2011 my plant, and those in neighbouring gardens, re-sprouted from its deep tap root and started to produce multiple rosettes of leaves around the base of its dead trunk.  Give it a few years and Northampton gardens will once again be crowned by these exotic imports from the Southern Hemisphere.  I moved house in early 2012 and wasn’t able to take my rescued plant with me, but I have a feeling it will survive many more cold winters to come.

Names matter to biologists, indeed to scientists of all types.  They signify and tell us things beyond just the words themselves.  To give a very personal example, a few people have asked me why I chose the title “Professor of Biodiversity” rather than “Ecology” (my main area of training, though confused in some peoples’ minds with New Age philosophies); or “Biology” (a much broader designation than I feel comfortable with); or even “Pollination Ecology” (narrow, to the point, but too restrictive).  After a LOT of thought I chose “Biodiversity” because it very broadly reflects my interests in the whole of Earth’s life forms, the interactions between these species, and how they come together as assemblages, communities and ecosystems.   But I’m also interested in the history of our understanding of biological diversity and this title gives me scope to pursue those interests too.

It’s all in the name.

9 thoughts on “When is a yucca not a Yucca?

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