Friday, 17 June 2011

Lobelia physaloides

It's well known that New Zealand native flowers are, frankly, a bit boring.  Most are small, and white, green, or yellow.  There are a few red ones, like rata, pohutukawa, and kakabeak, associated with pollination by birds.  Blue flowers are very rare.

When it comes to our fleshy fruits, many are small and red or orange, associated with dispersal by birds.  There are small blue, purple or white fruits too, and some translucent ones.  Large fruits are quite rare, probably because our fruit-eating birds can't swallow anything larger than a karaka or tawa fruit.
That all serves to introduce the exceptional Lobelia physaloides, one of the most striking plants in the New Zealand flora.  Its flowers and its berries are large and purple.  The berries have fine white stubbly hairs on ridges running lengthwise, and at the apex where the remnants of the calyx are found.

Lobelia physaloides fruit; it's about 18 mm long.

This plant has usually been called Colensoa physaloides, or sometimes Pratia physaloides.  Botanists generally are adopting a wider view of Lobelia, which includes Pratia, Isotoma, and Hypsela.  But L. physaloides is so unlike the little creeping New Zealand Lobelia that many people still want to treat it as a separate genus.  How reasonable is this position?
First, there's good evidence L. physaloides isn't closely related to the other New Zealand species of Lobelia.  But is it still a species of Lobelia anyway?  This is one of those situations where looking at our flora in isolation from the rest of the world isn't a good idea. Overseas, many lobelias are large bushy plants with coloured flowers, some, like the spectacular species on African mountains or in the Hawaiian Islands, are small trees.
Antonelli (2008) showed L. physaloides belongs close to some African species; together they're sister to the rest of the genus in the expanded sense.   Lammers (2011) followed that up with a formal taxonomic treatment, and he puts L. physaloides on its own in its own section (sect. Colensoa) sister to a large section of species that are found in Africa and Asia.  If Colensoa is to be recognised as a genus, its large sister group would either have to be included in Colensoa too, or would have to be recognised as another genus.  These botanists with a global view haven't split Lobelia, and it seems New Zealand botanists might be influenced more by the differences than the relationships in this case, although they've been happy to include Pratia, Isotoma, and Hypsela in a broad circumscription of Lobelia.  
Divergent growth forms, flowers, and fruits in Lobelia seem to have evolved convergently many times.  Using such differences as the basis of a classification might risk putting unrelated plants together in a genus, or at least classifying plants in separate genera from their nearest relatives.  
Sectioned fruit of Lobelia physaloides.
More interesting is the question of what pollinates the flowers and disperses the seeds of L. physaloides.  Inside, the fruits are hollow and a bit like little bell peppers, with thousands of tiny seeds on a pithy white placenta.
What eats the fruits and what disperses the seeds?  Candidates might include birds, weta, and lizards.
Antonelli, A. 2008: Higher level phylogeny and evolutionary trends in Campanulaceae subfam. Lobelioideae: Molecular signal overshadows morphology. Molecular Phylogenetics and Evolution 46: 1–18
Lammers, T.G. 2011: Revision of the infrageneric classification of Lobelia L. (Campanulaceae: Lobelioideae).  Annals of the Missouri Botanical Garden 98(1): 37–62.

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