Sunday, 24 July 2011

Scrub weeds

Yesterday was one of the coldest winter days in Wellington for many years, and one of the friends visiting us is as much of a weather nerd as I am, so in the afternoon we set off up nearby Johnston’s Hill to try to get a bit more altitude in the hope of seeing some snow.
A sleet shower approaches Karori from the south.
We were rewarded by a sleety shower on the way down, but the real cold set in overnight; however there has been little precipitation in Wellington so far.
The top of Johnston’s Hill is regenerating scrub on what once was sheep farming land.  Under the scrub are seedlings of native trees that one day will be a new forest canopy here.  Right now the scrub holds sway, and the best thing to promote forest on these hills is to try to make sure the scrub doesn’t get burned, which only sets the whole process back to the beginning.
The scrub is interesting, because unlike many places locally there are three species involved, all of similar abundance, but very different biology.

In this picture grey leaved tauhinu, yellow-flowered gorse, and orange-flowered Darwin’s barberry form a mosaic of textures and colours on Johnston’s Hill, with Wellington City in the background.
Tauhinu (Ozothamnus leptophyllus).
Tauhinu (Ozothamnus leptophyllus)  is the only native among this trio.  It’s a tall shrub, with small silvery leaves.  Being in the daisy family (Asteraceae) its seeds are dispersed by wind, each one in its own little fruit with a pappus attached that catches the wind.  I like it that tauhinu can hold its own against gorse and barberry; we even get seedlings coming up in the garden, testimony to the effectiveness of wind dispersal.
Darwin's barberry (Berberis darwinii)
Darwin’s barberry (Berberis darwinii) is a South American plant, first collected by Charles Darwin on the Beagle voyage.  It’s prickly, tall (it can be a small tree), and shade tolerant, so it can survive as the native canopy grows up around it.  Worse, it has fleshy fruit, so its seeds are dispersed long distances by birds.  It and its relatives are serious conservation weeds in parts of New Zealand, though they were originally introduced as ornamentals and hedge plants.
Gorse (Ulex europaeus)
Gorse (Ulex europaeus) is a familiar plant to New Zealanders and Europeans.  First introduced as a hedge plant here, it quickly spread and became established.  As a legume, it harbours nitrogen-fixing bacteria in root nodules, so it thrives under a regime of super-phosphate topdressing.  Sheep will eat the young shoots, but once the stems have hardened into prickles they’re inedible, so gorse can be hard to control.  Its seeds are dispersed explosively when the pods fly open on warm summer days, so they can disperse only 5–10m as a rule.  But those seeds are one of its real strengths because they can live in the soil for decades.  Our house was built in 1931, but whenever the ground is disturbed, gorse seedlings will pop up.  The big conservation value of gorse is that its litter breaks down to a nitrogen-rich mulch, it shelters regenerating natives, and eventually when the native trees or the gorse bushes grow tall it can’t regenerate in the shade.  Then the gorse is said to be a nurse crop for native forest regeneration, an attribute put to good use by Hugh Wilson at Hinewai on Banks Peninsula.

Thursday, 21 July 2011

Great season for hornworts.

I went looking for hornworts for my 2nd year botany class this week.  There are three local sites where I can pretty reliably find Phaeoceros carolineanus, and in all the plants were present in abundance.  At two sites there were no sporophytes, but at one of these places I've never seen sporophytes.  That's strange, because P. carolineanus is described as monoicous (has male and female gametangia present on the same plant), so it ought to be able to mate and produce sporophytes. 
Phaeoceros carolinianus at Old Karori Road.

At Old Karori Road (always the best site), P. carolineanus was more abundant than usual and there were plenty of patches with very dense sporophytes. I love these cool little plants, partly I guess because they're so different from the other two bryophyte groups (mosses and liverworts).  On some of the gametophytes that had sporophytes attached, we also found antheridial chambers, so these ones were indeed monoicous, or gametophyte cosexual in the terminology we're proposing. 
The students had plenty of material to work with.  They make a whole mount of the thallus and look at the distinctive single large chloroplast in each cell, and it's always nice to be able to show them the sporophytes too.  Unfortunately it wasn't such a good time for liverworts; it's been a bit dry for the last week or two and there were no sporophytes to be found.

Friday, 15 July 2011

A bluebell at the KT boundary.

Wahlenbergia matthewsii
The Geological time scale is divided up into Eras, Periods, and Epochs, and their boundaries are marked by evidence of rapid, even catastrophic, change.  The most famous is probably the one formed by the mass extinction at the end of the Mesozoic, known as the KT boundary (K for Cretaceous, to avoid confusion with the Cambrian; T for the Tertiary, nowadays called the Cenozoic with a slightly different definition).  This mass extinction is famous because it included the extinction of the dinosaurs, about 65 million years ago (I should say the non-Avian dinosaurs though, because small dinosaurs with wings, beaks, and feathers are very common today, only we call them birds because they seem so different from the lumbering dinosaurs that were their ancestors).
Wherever the boundary between Cretaceous and Tertiary rocks (we should perhaps say Cretaceous and Paleocene) is exposed on the Earth’s surface, it’s a place of great interest to geologists who want to examine fossils to see what was living in that location on either side of this mass extinction.
Students examining the KT boundary, Marlborough, New Zealand

In New Zealand there’s an accessible location on the Kaikoura coast where the KT boundary can be seen, and geologists have drilled cores out of the rocks on both sides of the junction.  The exposure is in a narrow limestone gorge that’s home to some of the iconic plants of the Marlborough region.

Wahlenbergia matthewsii and Pachystegia insignis on a
limestone cliff near the KT boundary in Marborough,
New Zealand
Among them is Wahlenbergia matthewsii (towards the top of the picture), a large-flowered native bluebell, and Pachystegia insignis (near the middle of the picture), the Marlborough rock daisy.  Not so long ago we used to think that many of our distinctive native plants were as old as the KT boundary.  It was thought that they, or at least their ancestors, had been in New Zealand ever since it broke off from Gondwana, 80 million years ago.  The opposite view has developed in recent years, and we now have good evidence that many of our plants arrived here very recently.  This is especially so for our alpine plants, which makes sense because alpine environments are relatively young here.  These newly arrived plants have come from all over, but by far the commonest source is Australia.  Some have come from South America or Africa, or even the northern hemisphere, but often these have established in Australia first, then crossed the Tasman to New Zealand.

The native bluebells are a case in point.  The genus Wahlenbergia seems to have originated in Africa about 30 million years ago.  Their relationships and evolution have been studied recently by Christopher Cupido in South Africa and Jessie Prebble in New Zealand using DNA sequences to trace their history.  From Africa, the group arrived first in Australia, where from a single ancestor it evolved into quite a large group over almost 5 million years.  Then, between 1.6 and 0.7 million years ago, two Australian bluebells crossed the Tasman to New Zealand, establishing two species groups in this country.  Another Australian species even seems to have found its way home to Africa.  At about the same time, some iconic Australian plants like Eucalyptus, Acacia, and Casuarina went extinct here in New Zealand.
Figure from Prebble et al. (2011) that shows the evolution and dispersal of southern bluebells.


Prebble, JM; Cupido, CN; Meudt, HM; Garnock-Jones, PJ. 2011.  First phylogenetic and biogeographical study of the southern bluebells (Wahlenbergia, Campanulaceae).  Molecular Phylogenetics and Evolution 59: 636-648

Sunday, 10 July 2011


I made marmalade today, something I’ve never done before.  I used mandarins from Johanna Knox’s garden.  Marmalade is citrus jam and the word marmalade comes to us from the Greek melimelon (a quince jam made with honey; the Greeks didn’t have cane sugar) via Portuguese.
Clockwise from top left: mandarin, navel orange, lime, lemon.
Citrus is a genus in the family Rutaceae, and the particular kind of fruit they have is a hesperidium.  Slice it cross-wise and you’ll see radiating walls like the spokes of a wheel; these are the walls of the individual carpels that make up the ovary and fruit.  Outside the carpels is the rind, made up from the ovary wall.  The rind is rich in flavorsome oils, which will burn dramatically if you squeeze lemon peel next to a lighted match. 

The inside of each locule (cavity) is packed with juice-filled hairs, and a few seeds along the central core of the fruit.  Each of those juice-filled bags within a carpel contains lots of juice-filled cells, and the cell walls have a special role in the making of marmalade.
Sliced lemon, showing 8 carpels, pith, and rind.
I’m indebted to a wonderful book for much of what follows: On food and cooking, by Harold McGee.  I have the 2004 edition.  I can’t recommend this book highly enough: food, botany, and chemistry—who could ask for more!
Plant cells are enclosed in relatively rigid cell walls made of cellulose.  Each cell secretes its own cell wall, by transporting vesicles of cellulose to be extruded through the cell membrane. The individual cells are stuck together by a thin layer of pectin, the middle lamella.  Pectin is an essential ingredient for marmalade, because it makes the gel that gives solidity to the jam. 
Pectin is a polysaccharide, a long chain of sugar molecules joined end-to-end.  The pectin chains are attracted to each other and form the glue that binds the cells tightly together.  But when fruit pulp is boiled in water, the pectin strands separate and they can’t come back together to find each other among all that water.  So the first step in making marmalade is to boil up the fruit in water.  I boiled mine for 2 hours.
Once the pectin is boiled out of the middle lamellae into solution, the jam-maker has to reassemble the pectin gel to set the jam.  Three properties of pectin are useful here.  First, adding sugar to the solution gives the pectin molecules something to coalesce around; they’re attracted to the sugar molecules.  Secondly, an acid environment helps the pectin gel to form.  I beefed up the acidity by adding the juice of a couple of lemons.  Thirdly some of the water has boiled off by now, and more will boil off from the sugar solution because adding sugar raises the boiling point to about 110C.  When these conditions are right, the pectin forms a gel as the solution cools, trapping water in the network of pectin strands: marmalade.
Four and a half jars of Theobrominated marmalade.

My marmalade turned out pretty cloudy, because I got lazy and simply put all the fruit through the food processor to make a pulp; then I sieved a lot, but by no means all, of the solids out of it. The left-over pulp will be great stuff for cleaning the glass of the shower; citrus juice is so acid that it’ll dissolve the scale (calcium carbonate) that forms as water dries on the glass.   The marmalade also set pretty solid; perhaps I over-sugared it.  

Tuesday, 5 July 2011


Before European contact brought a wealth of Eurasian and American temperate crops to New Zealand, Maori had to find their food among the native flora and fauna and the few crop plants they brought with them from the tropical Pacific.  New Zealand lowland plants are often woody and there are not many suitable green vegetables.  One of the best is puha, sometimes spelled puwha.
Puha seedling
Puha is classified in the genus Sonchus, which is part of the daisy family Asteraceae.  Within Asteraceae, puha belongs in the tribe Lactuceae, along with lettuce, dandelion, chicory, and salsify.  The name Lactuceae refers to the milky juice that's characteristic of this tribe.  This juice contains sesquiterpene lactones, whose bitter tastes probably protect the plants against grazing.  These have been reduced in commercial lettuce varieties by plant breeding.
Native species.
New Zealand has between three and five native species of Sonchus, plus three introduced ones.  Few New Zealand botanists would agree with me about the number of native species; most would say there's only one.  In fact, I'm the author of the most recent Flora treatment of Sonchus (in Webb et al., 1988) that says there's only one, so I'll explain the reasoning behind my change of heart. 
First, there's Sonchus kirkii, which was probably the original Maori puha.  It's a coastal plant with rather thick and glaucous (bluish-green) leaves.  The New Zealand Plant Conservation Network notes that it appears to be declining in abundance.
Sonchus grandifolius, cultivated at Otari–Wilton's Bush

Secondly, there's S. grandifolius, confined to the Chatham Islands.  It was separated from Sonchus by Egyptian botanist Loutfy Boulos in 1965 and placed in a new genus, Embergeria.  It differs from most Sonchus in being much larger in all its parts and lacking barbs on the pappus hairs at the tips of its fruits.  This is another of those cases of classifying plants separately if they're different from their close relatives.  However, large size and loss of adaptations for dispersal are common features of island plants worldwide, and I expressed doubt about this classification back in 1988.  Since then, molecular systematics (Kim et al. 2007) has shown S. grandifolius clearly is a member of the Sonchus lineage; to treat it as a separate genus would disguise its true relationships.
Finally, another 1–3 species should be added to New Zealand Sonchus.  The endemic genus Kirkianella was described in 1961 because the plant previously known as Crepis novae-zelandiae clearly didn't belong in Crepis.  In the 1980s I formed the view that it might rather belong in Sonchus.  I wish I'd said so at the time, because recent molecular systematics research has borne this out (Kim et al., 2007).  Its DNA sequences and its fluffy pappus are evidence of a close relationship to S. grandifolius.  It looks as if the great British botanists Bentham & Hooker came to the same conclusion; the International Plant Names Index lists Sonchus novae-zelandiae attributed to them, but I haven't checked it in the original yet to see if they formalised the name.  At the moment there's only a single species recognised in Kirkianella, but there are two different chromosome numbers and a range of leaf shapes, which are strong clues that there could be more than one species involved.
Introduced species.
Of the three introduced species, S. arvensis is a large creeping perennial with large flower heads and conspicuous yellow glandular hairs on the outer bracts.  It's known from a few locations in Hawke's Bay, Canterbury, and Otago. 
The two common wild species are S. asper and S. oleraceusS. asper is an annual or biennial with softly prickle-edged leaves and smooth fruits, whereas S. oleraceus is an annual with toothed leaves and wrinkly fruits.  
Puha, Sonchus oleraceus
 The name S. oleraceus gives it away as a plant that's good to eat, and this species has largely superseded S. kirkii as the edible puha.  Moreover, Kevin Gould and colleagues found it had antioxidant levels several times higher than blueberries (Gould et al. 2006).  But S. kirkii is said to be easy to grow from seed, so it'd be an interesting addition to the vege garden, and growing it could help conserve the species (but don't collect plants or seed from the wild; try a good native plant nursery).
So what's puha like to eat?  I have to admit I haven't tried it, and I'll instead refer you to Johanna Knox's blog "Wild Picnic" and her tempting recipe for puha pakoras at "Wild Concoctions".  Incidentally, the latter recipe featured recently on TV2's Erin Simpson Show, unfortunately without attribution, although it appears to be no longer on the show's website.  That's a serious breach of netiquette, or worse.