Fruiting karaka

Karaka (Corynocarpus laevigatus) is a small New Zealand tree with large fleshy fruits.  It's interesting for a number of reasons.  Its fruits were an important food resource for Māori but the kernels had to be treated to remove the toxins they contain.  It's become a weed in some parts of the world. Some botanists consider it a weed within New Zealand too, when it becomes invasive outside its presumed native range or habitats.

Right now, karaka trees are fruiting heavily.  But not all of them.  Some trees are covered in fruit and others have none or very few.  Some years ago, I wondered if this meant they had separate sexes, and was able to show that this is the explanation (Garnock-Jones et al. 2007).  Male trees do produce a few fruits, so the sexual system in karaka is best described as gynodioecy (some plants strictly female; others are inconstant males).

Here are the two trees that started this research off, photographed this month in Kelburn.

Karaka trees in fruit, Kelburn, Wellington, 2014

Here are the same two trees about 10 years ago.

Karaka trees in fruit, Kelburn, Wellington, 1998 (from Garnock-Jones et al., 2007)

On the female tree, the panicles fruit heavily, with many of the flowers (but by no means all) developing fruits.

Fruits on a female karaka tree

On males, usually a single fruit develops on each of a few panicles.

Fruits on a male karaka tree

Karaka flowers are small and white, but if you look closely you can tell the male from the female flowers.  The male flowers are actually about twice the diameter of females, open more widely, and have pollen in their anthers.  The male flowers in the photo have pollen on the stigmas, but only very few of them will produce fruits.

Karaka flowers.  On a female tree (left); male tree (right)

Reference.

Garnock-Jones PJ, Brockie RE, FitzJohn RG 2007.  Gynodioecy, sexual dimorphism and erratic fruiting in Corynocarpus laevigatus (Corynocarpaceae).  Australian Journal of Botany 55: 803–808.

Wednesday wildflower: old man's beard.

“Old man’s beard must go,” said the not-so-young bearded botanist David Bellamy some years ago on TV, and we all knew this was a serious weed that threatened to smother our native forests.  Since then, it’s not as common as it used to be.  The efforts of the Department of Conservation, local and regional councils, community groups, and individual landowners have largely seen off the worst infestations in many places.  But around Wellington city, and probably many other places, there are little pockets of old man’s beard everywhere.  My guess is it’s waiting for our vigilance to let up and it’ll be back with a vengeance.

Old man's beard fruits, Northland, Wellington.

Old man’s beard is called traveller’s joy in England, which goes to show that one country’s wildflower is often another’s weed.  The local name refers to the fluffy white plumes on the seeds.  More accurately (with my pedantic botanist’s hat on) these aren’t seeds but fruits, each bearing a single seed inside a loose-fitting fruit wall.  The flower, like many in the buttercup family, has multiple separate pistils, each with its own ovary, style and stigma.  After pollination, the pistils from a flower mature into a cluster of separate 1-seeded fruits, each with the fluffy remains of the style to form a plume that assists in wind dispersal.

One of the practical problems with a public eradication campaign is that people need to be able to recognise the target weed, and not try to eradicate look-alikes.  Many people worry that they might be pulling up one of the native Clematis instead, so I thought this week a few notes about these plants might be useful.

First, old man’s beard (Clematis vitalba) is one of just two species in New Zealand with once-pinnate leaves; they have a central axis with a terminal leaflet and two pairs of lateral leaflets.  The other species that’s characterised by once-pinnate leaves is C. maximowiciana, but its leaves are more leathery than the leaves of old man’s beard; also it has larger flowers, 30–50 mm diameter.  Most of the other species have three leaflets although these can be quite finely divided.  Two species, C. tangutica (yellow flowers) and C. flammula (white flowers), have twice-pinnate leaves (the pinnae [leaflets] are themselves pinnately divided).  The native C. afoliata has no leaves at all, just the leaf stalks that twine around supporting shrubs’ stems.

Old man's beard flowers, Kakariki, Manawatu.  They are mostly 12–25 mm diameter

Secondly, the introduced species mostly have 4 sepals in each flower, but many (not all) of the native ones have six.  All the introduced Clematis have hermaphrodite flowers (with functional stamens and pistils) whereas the natives all have unisexual flowers on separate plants (flowers have either stamens or pistils, but never both).

Clematis forsteri, a native species.  Pale yellow male flowers with 6 sepals.  If you see these, don't just look, sniff too: many are sweetly scented.

Most of the native Clematis flower in the springtime or even late winter (some plants of C. forsteri are in flower now in late July).  Old man’s beard is a summer-flowering plant, mostly from December to May.

Clematis paniculata, another native species, male flowers.

Old man's beard flowers are greenish white.  Most natives have pale or greenish yellow flowers, although the large (and unisexual) flowers of C. paniculata are pure white, C. marata and C. marmoraria are white or greenish, and C. marata quadribracteolata (corrected 4 September 2013) brown or purplish brown.  Finally, the introduced Clematis are all deciduous whereas the natives are all evergreen (except poor C. afoliata, which hasn’t got leaves to lose).

Old man's beard still must go, but let's hope no native Clematis get pulled out instead by well-meaning weed-busters.  The Flora treatment for their identification is on line at Landcare Research's website and you can find pictures identified by botanists at the Naturewatch site..

Wednesday wildflower: winter heliotrope

Winter heliotrope isn't a true heliotrope, but a daisy, related to the senecios I've featured in a few other entries in this series (here and here).  But it is both a wildflower and a weed, a garden plant that has escaped.

Winter heliotrope, Petasites fragrans.

This patch was in Aro Valley, an old-established part of Wellington, originally working class but now a mix of gentrified old cottages and student flats, arty cafes and boutiques.  The plants were in a garden that had a rather wild appearance; I'm sure its owner was deliberately aiming for a wilderness look.

Winter heliotrope flower heads.

The flowers are pink, instead of the usual yellow for this tribe of daisies (Tribe Senecioneae, characterised by the involucral bracts being in a single row, not in overlapping rows like roof shingles).  Winter heliotrope is dioecious (has separate male and female plants), but all the plants in New Zealand are males.  Their outer ray florets are all sterile (they make neither pollen nor seeds); ray florets are female in most daisies (the general structure of daisy flower heads is explained here).  The inner disk florets are male (often hermaphrodite in other daisies).  Not being able to have sex doesn't deter this plant a bit, because it is able to spread vegetatively, and of course people deliberately and inadvertently help that process.

You might note the stigmas in these florets, the large white somewhat feathery things poking through the tube of purple anthers in the centre of these florets.  How come male florets have such a large stigma?  In this and several other families, the pollen is presented on the stigma, so even though the florets are male, they still need well-developed female parts for pollen dissemination.  In a hermaphrodite daisy floret, the stigma opens after the pollen has gone to expose the two receptive surfaces.  I didn't examine these closely at the time, but in the photo I can't see any that have opened.

Strange violets.

Māhoe (Melicytus ramiflorus) is one of the plants I love to show to visiting botanists from temperate Europe or North America.  They invariably ask, "What family is it?" and the answer always surprises because they're used to Viola as the representative of Violaceae.  Here's a violet that's a tree, has radially symmetrical flowers, and fleshy fruit; the world truly is upside-down in New Zealand.

Māhoe flowers on twigs of a male tree, Karori, Wellington

Well, not so fast.  It's true Viola is the type genus of Violaceae, but who said types have to be typical?  The type simply determines the application of the name according to the rules of nomenclature.  It's the circumscription—the definition of the membership of the group—that outlines its overall collective characteristics.  Violaceae is a largely tropical family, and they're woody except for Viola.  Quite a few have radially symmetrical flowers and fleshy fruits (Mabberley 2008).

A "normal" violet (Viola sp.), France.

Māhoe might be New Zealand's commonest tree.  There's certainly lots of it around Wellington, where it's an important component of the still low second growth forests that, over the last 50 years, have replaced the gorse on hills around the city.

Māhoe flowers in pulses, all the trees in a location flowering in synchrony several times a summer (Powlesland et al. 1985).  Coffee does the same thing.  The flowers are small and borne on the twigs, a form of flower presentation called ramiflory.  They're scented, especially at night, but it's not a very pleasant smell, in my opinion.

Melicytus ramiflorus, male flower.  Note the vestigial ovary in the centre and the swollen connective at the back of each anther that functions as a nectary

Melicytus ramiflorus female flower.  Note the large stigma and the nectar produced from the connective at the back of each staminode.  Female flowers are about 2/3 the size of males.

The flowers are unisexual and produced on separate trees, so a whole tree is either male or female.  The connective of the stamens and (in female flowers) staminodes is also the nectary, and each produces a glistening drop as a reward for the pollinators.

Māhoe leaf skeletons

Their leaf veins are pretty tough, but the rest of the leaf decomposes freely, to leave exquisite lacy leaf skeletons on the forest floor.

Viola cunninghamii, Hooker Valley, Mt Cook National Park.

New Zealand has three native violets (like V. cunninghamii above), plus some introduced species.  We also have quite a few Melicytus; some are small trees and others are twiggy shrubs.  Some used to be classified in the genus Hymenanthera, but that was merged with Melicytus in the 1980s.  Recent research (Mitchell et al. 2009) shows there are two pretty clear-cut groups within the genus, although the authors didn't draw attention to it, and it might make sense to recognise Hymenanthera again.

References

Mabberley, D.J. 2008.  Mabberley's Plant-Book (3rd ed.).  Cambridge.

Mitchell, A.D.; Heenan, P.B.; Murray, B.G.; Molloy, B.P.J., de Lange, P.J. 2009.  Evolution of the south-western Pacific genus Melicytus (Violaceae): evidence from DNA sequence data, cytology and sex expression.  Australian Systematic Botany 22(3) 143–157.

Powlesland, M.H.; Philipp, M.; Lloyd, D.G. 1985.  Flowering and fruiting patterns of three species of Melicytus (Violaceae) in New Zealand. New Zealand Journal of Botany 23: 581–596