Tuesday, 7 August 2012

Wednesday wildflower: velvet groundsel, Roldana petasitis

The Asteraceae is a huge family (1535 genera/23,000 species, according to Judd et al. 2008) and Senecio is the largest genus, with 1250 species.  That's after some smaller genera have been segregated, like the mostly New Zealand genus Brachyglottis and the American genus Roldana.
The Asteraceae is characterised by having compound blossoms (capitula or heads) made up of many small florets.  In some, these heads are homomorphic, with just one type of floret (e.g., dandelions and thistles), but in most they're heteromorphic and have both ray florets and disk florets.  In some groups that normally have heteromorphic heads the ray florets may be lost to make a heteromorphic head; that seems to happen readily in some species of the Senecio tribe, Senecioneae.  Outside of the ray florets are the green involucral bracts, which in Senecio and its relatives are characteristically in one non-overlapping main row (D in the plate below).
Roldana petasitis, Kelburn, Wellington.
Today's wildflower is a species that's introduced to New Zealand and established mostly in parts of the North Island, probably as a garden escape.  The plants are softly woody shrubs, up to 2 or 3m tall.  These ones were collected in the Norway Street area of Kelburn, Wellington.  It's native to Mexico, but probably came here via Britain, where it is cultivated at least in the south, or perhaps via USA or Australia.
Roldana petasitis, an inflorescence of capitula.
The Flora of New Zealand (Webb et al. 1988, as Senecio petasitis) says it establishes locally from garden discards and from seed, and certainly this population looks like it has spread by seed to places where garden discards seem unlikely.  Yet the Flora doesn't describe the fruits, so I assume there were none available for study.  I'll be keeping an eye on these plants as the season progresses to see if fruits are produced.  The New Zealand Plant ConservationNetwork website says seeds are the main means of spread.  (Technically, in Asteraceae, seeds are dispersed inside 1-seeded fruits, called cypselas.)
Roldana petasitis, leaf.
The leaves are large, up to 20 cm diameter; the one in the plate below is a small one from just below the inflorescence.
The heads are produced in large branching inflorescences.  At first the five (occasionally 4 or 6) ray florets open and expand.  Then the outer disk florets begin to shed pollen.  In this male phase the stigma is pushing the pollen out through the anther tube, where it's presented between the projecting awns of the anthers (which are visible in the centre of D below).  Later, when the pollen has gone, the two arms of the stigmas open, ready to receive pollen.
Individual florets are also shown (C below), two rays on the left and three disk florets on the right.  The rays are female (they have no anthers) whereas the disk florets are hermaphrodite.
Roldana petasitis.  A, small upper leaf, adaxial surface; B, the same, abaxial surface; C, florets, two rays (left) and three disk florets (right); D, a capitulum, showing ray and disk florets and involucral bracts; E, a series of capitula at increasing time from first opening (see text for description); F, details of leaf surfaces, adaxial (left and abaxial (right).
Senecio has been a taxonomic problem for decades.  Most people regarded the genus as too big to make sense, but in the past it's been hard to achieve consensus among botanists about the various attempts to break it up.  Recently, the application of cladistic (tree) thinking and molecular data to this problem genus has yielded good results that seem to fit with morphology and biogeography (e.g., Pelser et al. 2007).  There's now good reason to treat this species as a member of Roldana, rather than Senecio.
Judd, W.S.; Campbell, C.S.; Kellogg, E.A.; Stevens, P.F.; Donoghue, M.J. 2008: Plant systematics a phylogenetic approach. Sinauer, Sunderland, Massachusetts.
Pelser, P.B.; Nordenstam, B.; Kadereit, J.W.; Watson, L.E. 2007 An ITS phylogeny of tribe Senecioneae (Asteraceae) and a new delimitation of Senecio L. Taxon 56: 1077–1104.
Webb, C. J.; Sykes, W. R.; Garnock-Jones, P. J. 1988: Flora of New Zealand. Vol. IV. Christchurch, Botany Division DSIR.

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