Bald cypresses and their knobbly knees

While I'm in an investigative mood, do Bald Cypresses breath through their knees? This was another question posed to me by our Melbourne Gardens Senior Curator of Horticulture, Peter Symes. Above is a cypress knee in Melbourne, and next up a collection (a 'knobble' perhaps) of knees at the South China Botanical Garden in Guangzhou (taken during a visit in 2009).

It's a common perception and two researchers at the University of Kansas, Craig Martin and Sarah Francke, believe so. In one rather simple experiment they have overturned years of alternate theory and folklore, to show that the knobbly 'aerial' roots of Taxodium distichum are, like those of mangroves, pneumatophores (literally, air stalks).

In nature, the Bald Cypress produces a lot of these woody projections from its roots, and while short and stumpy in the Melbourne Gardens they can be up to 'several metres' tall in the wild. The function of these cypress knees has been much debated although it's fair to say that until recently it has been assumed they have something to do with getting oxygen to roots in a sometimes waterlogged environment - i.e. a swamp.

While this sounds perfectly reasonable, there is actually little evidence to support the supposition or to discount alternative proposals. For example the knees might stabilise the tree in some way. They they might provide access to nutrients, perhaps by encouraging litter to accumulate and break down near the tree. Or could they be a storage organ, a place to hold excess carbohydrates for a less rainy day in the swamp.

Martin and Francke devised a rather simple experiment, measuring oxygen in the submerged (underground) roots when the knees were above water, and then when the knees were submerged. If there was less when the knees were entirely covered by water this, to their mind, proved that these knees were indeed air stalks (pneumatophores).

Happily, they found that oxygen was 'much higher' (about three times) in the roots when the knee was above water and therefore able to diffuse oxygen down into the soggy roots. Than ran various controls and concluded that there is now 'unequivocally strong evidence that cypress knees do indeed allow the diffusion of oxygen to the submerged attached root'.

Like any good scientist, they do qualify their result saying that the cypress knees may have evolved in response to things other than a need for oxygen by submerged roots. They are confident though that aeration was at least one major factor in their evolution.

You can see these knees in what we appropriately call the Taxodium Lawn, between the William Tell Rest House and The Terrace cafe, on Ornamental Lake at Melbourne Gardens. I'm not sure how much the knees are trimmed during mowing but it can't make that job particularly pleasant for our horticulturists.

There are other Taxodiums around the lake, mostly, I think, Taxodium mucronatum, the Montezuma Cypress. This species is less pyramidal in shape and tends to have weeping branches. It also lacks knees. Although it makes up for this by keeping its leaves in winter, unlike the Bald Cypress, named after this deciduous trait (in fact it's sometimes called more explicitly, the Deciduous Swamp Cypress). Both species come from swampy places in southern North America and Central America.

The taxonomy of the specimens in horticulture seems to be a bit of mess (according to Roger Spencer's Horticultural Flora of South-eastern Australia) but generally the Bald Cypress has longer leaves (about 10-15 mm), no leaves in winter and nice set of pneumatophores.

Images: Apart from the knobble photographed at South China Botanical Garden, all images are of the specimen of Bald Cypress on Taxodium Lawn, Melbourne Gardens, taken in March 2017.

Monster Swiss cheese plant in search of black holes

The leaf of the Swiss Cheese Plant, Monstera deliciosa, is nothing like a cheese of any kind when young. As it ages, perforations reach deep into the leaf and holes appear towards its centre. It becomes fern-like (pinnate), but with holes...

While lots of leaves have notches and crannies at their edges, its rather rare to find completely encircled holes. Interestingly, and pointing to some genetic trigger, most of the 40 species of Monstera have them, to varying degrees.

The question of how and why the holes form has taxed some of the greatest botanical minds, although mostly it has be said without much evidence. (Mostly too, at least in the nineteenth century, it taxed botanists with a venerable prénom...)

When the Swiss Cheese Plant made it to Europe from its home in the tropical rain forests of southern Mexico to Central America, it was quite a sensation. Fittingly, it was the Swiss botanist Augustin de Candolle who in 1827 first commented on their possible origin. A sign of weakness in the leaf, he said, a debilitation.

French botanist Augustin Saint-Hilaire disagreed entirely. It must be excessive vigour he exclaimed in 1840, In 1854, fellow countryman, Auguste Trécul, actually examined the leaves as they grew and found that the holes were caused by cells dying in the effected areas and that the leaf continuing on beyond them. This didn't solve the 'why' but helped with the 'how'.

In 1969, two botanists from Royal Botanic Gardens Kew, Robert (Melville) and 'F.A.' (Wrigley), confirmed that the holes were a 'necrosis' (a dying of cells). The compared them to colour blotches on the leaves of the Zebra Plant, Calathea, finding the patterns quite similar and more or less explained by the Diffusion Reaction Theory proposed by the great, and scandalously treated, mathematician Alan Turing. The theory appears describes the pattern caused by the interaction of growth promoting substances interacting with some kind of growth inhibitor. Again it's more to the how than the why.

Over the years a few whys have been suggested: for example, allowing water to drip to the ground and get closer to the roots, keeping the leaf cool, camouflaging the leaf to protect it from from animal grazing, and in recent years (2013) a rather complicated (to me) hypothesis where growth of the leaf is kept more consistent by having holes in it resulting in an advantage for a plant growing in a largely shaded habitat with dappled light.

Personally I'm attracted to the idea that holes might allow leaves to better survive hurricanes or similar dramatic wind events. I'm familiar with the punching of holes in large signs made of cloth for the same reason, and can see this being an adaptive pressure (i.e. a thing that allows the plant to live and reproduce longer).

Kit Wignall, touring with me to Spain in May, suggested it might be a way to reduce the weight of the leaf while extending it as far as possible. Which does make good sense. So perhaps I favour the combination of weight reduction, light penetration to lower leaves and a bit of hurricane protection thrown in for good luck.

For now though, it seems no-one really knows. And that's just one of the intriguing characteristics of the Swiss Cheese Plant. In case you are intending to eat the fruit, don't. At least not without some planning.

The unripe fruit, as pictured here, contains irritating chemicals such as oxalic acid. After a year or so, the hexagonal scales will dry out to reveal the white (edible) pulp of the fruit below. Alternatively, I gather, you can pick the fruit when the base starts to wrinkle and wrap it in a bag for a few days until the scales have separated.

The University of Connecticut, my source of the fruit information, also lets us into an intriguing fact about its seedlings. When they germinate the first shoot seeks out the darkest place it can. Not, just away from bright light but to where things are at their blackest. Once it finds suitable substrate - for example the base of a tree - it then sends a shoot light-ward, towards the canopy. In this case the why is pretty clear but the how I'll leave for another day.

Notes: All the pictures are from The Ian Potter Foundation Children's Garden at Melbourne Gardens, Royal Botanic Gardens Victoria, except the picture of the fruit, which was taken in my backyard at Sydney a few years back. And thanks to Peter Symes for suggesting this fascinating but still unresolved topic!

Texan dichondra a welcome mirage in summer

Talking Plants is resuming normal transmission! I'm back in the seat at Royal Botanic Gardens Victoria and back on the look out for flowers, gardens and other botanical treats to serve up each week.

Today, we'll ease back into things with a plant from the desert regions of southern USA and Mexico, reaching Europe (at least in dried, pressed form) in the early nineteenth century. I see it every day, in my backyard, but until I sat down next to it in a low chair, in late February, I hadn't noticed the flowers.

The silvery leaves of our Dichondra argentea 'Silver Falls' tumble from an old colander (competing with the rather rampant Ivy-leaf Toadflax, Cymbalaria muralis), and they look their best when our climate is a little more like tropical America, in our Melbourne summer.

The silver sheen is due to a covering of downy hairs which also covers the flowers, which along with their tiny dimensions (less than 5 mm across) makes them, as the Missouri Botanical Garden Plant Finder puts it, 'insignificant'. They are 'not showy', as the same site explains. All true but they are rather cute if you can get close enough.

In the following picture, you can just see the two capsules that follow the flower, providing the genus name 'dichondra', which means two grains (the grains being the ovaries or resulting capsules in fruit). 'Argentea' means silvery and I'm presuming I should use the cultivar name 'Silver Falls' for our specimen, a variant happy to dangle from a basket, or colander. 

The species Dichondra argentea was collected and named by the famous German ecologist, Alexander Humboldt and French botanist, Aimé Bonpland, after their journey through Latin America between 1799 and 1804 (the German botanist Carl Wildenow formally described the species, from their information, in 1809).
It was a good find. The genus Dichondra is a small one, currently thought to have only 15 species, most of which have creeping stems that can send down roots at the point where the kidney-shaped leaves arise. The distinctions between species can be fine but presumably the silvery sheen made this one stand out from the (small) crowd. 

Dichondra species are mostly found in the tropics but here in Victoria we have Dichondra repens, Kidney Weed, a species found naturally in I think every State in Australia (including the even more temperate Tasmania). There is possibly a second species in the north of Victoria and in other States, called 'sp. 1' in VicFlora and 'sp. A' in the Flora of New South Wales, plus the sometimes weedy Dichondra micrantha from North America and east Asia.

Interestingly, the rest of the genus is found in the Americas: North America has eight species, South America about seven. It's an odd world distribution but that's the way it is. Dichondra is in the family Convolvulaceae, with Blue Morning Glory (Ipomoea indica) and Silverbush (Convolvulus cneorum), as well as the sweet potato (Ipomoea batatas). The family as a whole is spread all over the world, apart from the colder extremities in both Hemispheres.

Dichondra argentea is a good tough plant for our impending climate in Melbourne but I do note that it didn't survive a trial on an un-irrigated roof garden in Texas. It also gets a little shy during winter but should pull through.

Conservatoire et Jardin Botaniques Genève CC

Geneva's botanic garden turns 200 this year! A year younger than Sydney's Royal Botanic Garden, it was conceived in 1816, the year 'without a summer' in the Northern Hemisphere (due to the recent eruption of Tambora in Indonesia). In Geneva's case the garden started near the university in Bastions in 1817, moving to its current location in the north of the city in 1904.

To ease my way back into proper blogging here is a image heavy, text light, reflection on the Conservatoire et Jardin Botaniques Genève. At 30 hectares with 8,000 plant species it is a little smaller than Melbourne and Sydney's city botanic gardens, but with a similar number of species represented.

The first Director is one my botanical heroes, Augustin-Pyramus de Candolle (1778-1841). Not only did he create and head up a major botanic garden, he was an enthusiastic scientist with an interest in algae (describing in 1801, when he was 23 years old, the genus Vaucheria on which I did my PhD, and the species Batrachospermum gelatinosum, the representative - Type - species of the red algal genus I've worked on for most of the rest of my research life).

Today the botanic garden is part beautiful landscape and part explication of botany. Generally it all holds together and there are some lovely interpretation ideas and creative plantings.

This use of a retaining wall beside the adjacent railway line works very well.

The interpretation is a little busy at times but grabs your attention and makes you realise how important plants are our lives. And that's point right?

Sometimes it gets serious. This display on the wall of the glasshouse devoted to the plant family Gesneriaceae (e.g. African Violets) is bold and clear. The plants themselves were a little tired on my visit but that might be due to the heat - it was in the mid-30s today.

There is plenty of art. Here is an artist, Sylvain Meyer, doing what artists do (I think checking emails), followed by what he does at other times - a very nice installation beneath an Oak Tree.

And then there is some more colourful children's art, on the underpass between the main garden and its isthmus beside the lake.

With Conservatoire in its name, you'd expect a few glasshouses. This is the temperate house, flagging the 200th anniversary on it's top...

Inside some interesting collections. The cacti from the Americas, the lime-green Euphorbia from Africa or nearby islands.

At first I thought I'd found the world's first outdoor herbarium (the library of preserved plants we usually keep well away from light, heat and moisture) but it turns out this is an artistic wrap around the botanic gardens' shop.

The real herbarium is a little more difficult to find. I think it's partly in the building on the right of this next picture and partly underground. When you peer through the glass covering that concrete bunker structure what you see is in the second picture... Something like we plan to replace our existing herbarium in Melbourne - an underground bunker. We have 1.5 million specimens, arguably the most in an Australian herbarium, Geneva has around 6 million (although some of these - the 'cryptogams' are stored in another building, coming up soon).

Every now and then the plants are interrupted by animals, in zoo-like exhibits. I love flamingos, and not just because their pink colours come from algae and the various bugs that eat algae.

I promised you a Cryptogam Herbarium, and here it is. It's called the Console, and it sits on strangulated piece of land between the highway and the lake. It contains preserved specimens of algae, fungi (and lichens), bryophytes and ferns. More than a million of them. The size, importance and heart of this herbarium are due to my old friend Augustin-Pyramus (the last of these next three mages is the one being used to promote a horticultural tribute to the 200 years 'A.-P. de Candolle: une passion, un Jardin').

Note: I was visiting the botanic garden in Geneva while attending the Sixth Global Botanic Gardens Congress. This meeting of all the world's botanic gardens is held every three to four years, and the next is to be held in ... [drum roll] ... Royal Botanic Gardens Victoria! More on that, you can be sure.

Iberia VIII

My last post from the Iberian Peninsula, with a bit of Portugal and Spain, plus the Basque Country of France as an honorary part of Iberia. Just one more blogging-lite (from a botanic gardens meeting in Geneva, Switzerland) before normal transmission resumes in July...

Starting with the north of Portugal, there were some pretty gardens around the Bom Jesus do Monte, not far from Braga, but it's the ornate stairs that everyone goes there to see (and descend and ascend).

Nearby is Braga (the third largest town in Portugal, after Lisbon and Porto), a town of buildings rather than parks, but some gorgeous constructions such as this.

Then on to a smaller university town, Guimarães. Again more about the buildings than plants but there are some trees around them (including a few giant blue gums - a species that dominates the west of the Iberian Peninsula).

Miño, in northern Spain, has a good looking sandy beach and a few of the common natives and weeds growing on it, including this Sand Stock (Malcolmia littorea) a local to the Mediterranean.

This Wild Carrot (Daucus carota or thereabouts) could be anywhere, and along many of the Iberian roadsides. I can't resist it.

A big surprise was the botanic garden just out of Gijón, in northern Spain. Jardín Botánico Atlántico de Gijón is a 25 hectare mix of local flora displays, formal 19th century gardens and some more natural areas (including a collection of old oak trees - Quercus robur - that have been managed for wood and charcoal for up to four centuries). First up though, a nice display of edible plants, and then some closely planted plane trees.

And in town, some very attractive buildings, yet again...

Near to Gijón (30 km away) is the town of Oviedo with the stunningly coloured (honey yellow) cathedral and other colourful buildings (some even with flowers...). Oh, and someone made a film there recently and he said how much he liked the place so much the locals put up a statue of him.

Then in the foot hills of the Picos de Europa, mist... So not so many views but plenty of eucalypts (the first picture shows seedling Blue Gums - Eucalyptus globulus), the second, a heath (Erica) in the limestone. 

Oh, and some wildlife. This cow is resting in the early morning not far from Hasparren, in the Pays Basque region of Spain.

And to finish, the Limes or Lindens (Tilia) have been stunning on our trip. This is probably Tilia cordata, but it also reminds me of a display at the Gijon botanic garden, which reminded me that the Swedish inventor of our botanical name system, Carl Linnaeas, got his family name (later Latinised) came from this tree!

Time now to return the hire car, and pack away the 'Iberian Peninsular Tour, June 2017' number plate accessory...