Cocao,Theobroma cacao,Malvaceae

I like a nice cup of cacoa - and this is where it comes from. This is the flower of cacao, Theobroma cacao, source of chocolate. These flowers - only about a centimetre across - are pollinated by small insects - notably midges - and then the cacao pod develops (see below).

The tiny cacao flowers are produced in large numbers and are unusual in that they sprout directly from the bark of the tree - a botanical trait known as cauliflory. The plant was formerly classified in the family in the family Sterculiaceae but more recent systematic research has reclassified it as a member of the mallow family. It originates from the foothills of the equatorial Andes and is thought to have been first domesticated 3000 years ago in Central America, where it became the sacred beverage plant  of the Mayans, who believed it was a gift from the gods - a historical link commemorated in the Latin name that Linnaeus bestowed on the plant - Theobroma literally means 'food of the gods'. The Aztecs used cacao beans as a form of currency. 

To produce chocolate the cacao pods are split open and the seeds are extracted from the pulp, then allowed to ferment for several days and then roasted (when the chocolate flavour develops), then ground into a powder. The highest-valued Criollo cacao has relatively low levels of bitter substances but cacoa powder is usually mixed with milk to offset the bitterness. About 3.7 million tons are produced annually to satisfy chocaholic cravings, much of it in West Africa, but about 30 per cent of the crop is lost to pests and diseases. It's hoped that the recent sequencing of the cacao genome will speed-up the selection of disease resistant varieties.

This gent - Sir Hans Sloane, 1660-1753, commemorated with this statue in Chelsea Physic Garden , of which he was patron - is credited with bringing cacao to Britain from Jamaica.  Sloane was a physician and was interested in its medicinal properties but found that it was far more palatable if mixed with milk, inventing a patented recipe for milk chocolate that was eventually acquired by Cadbury. The original Cadbury's chocolate wrappers carried the inscription Sir Hans Sloane's Milk Chocolate prepared after the original recipe.

Sloane's belief in the medicinal properties of chocolate have been echoed for over a century in this illustration on the tin of Droste's cacao, a Dutch brand - where a nurse is shown delivering the drink to the patient. Notice how the image on the tin is repeated in the tin she is carrying on her tray - this cunning advertising image reinforcement, of an advert within an advert, has become known as the Droste Effect.

Adam's Laburnum, +Laburnocytisus adami, Fabaceae

In 1825 a French nurseryman called Monsieur Adam grafted a purple-flowered specimen of broom Chamaecytisus purpureus onto a yellow laburnum Laburnum anagyroides stem, hoping to create the broom equivalent of a standard rose. Instead he created a strange graft chimaera - a tree with the tissues of both parents intermingled, that produces three different kinds of flowers.  The core of the tree is laburnum, sheathed in layers of broom cells, and the whole arrangement is unstable. In the picture above you can see what appears to be a broom bush sprouting from the branch of the tree, looking like an unusually colourful mistletoe.

This picture, and the one below, show the profusion of purple broom flowers on slender, pliable stems......

... and is probably quite close to what Adam had in mind when he started his experiment, except that he would have hoped to produce this head of flowers on top of a single straight stem.

The whole tree, which looks like a rather lax, poorly growing laburnum in general shape, produces a second form of inflorescence .....

.... that looks like a dangling salmon-pink laburnum blossom - a blend of flower colours of both parents.

Look closely and you can see a hint of laburnum yellow towards the rear of these flowers ....

... but higher up on the tree, amongst the pink blooms, it also produces occasional pure yellow, typical laburnum flowers....

... which you can see here.

This example, of what is a rather rare tree, is currently flowering in Durham University Botanic Garden

In graft hybrids like this the cells of both parents coexist as separate genetic entities in a single organism. You could, at least theoretically, extract cells of each component, put them through sterile tissue culture and recover the two original parents. A similar graft hybrid between hawthorn and medlar, +Crataegomespilus, has also been produced but is rarely found in collections. The + before the Latin name indicates that the plant is a blend of cells from two parents that maintain their independent genetic identity, rather than being the result of sexual hybridisation.

Grafting, which has been most often used in viticulture, fruit tree and rose production was once viewed by many as being a very unnatural practice, comparable with the way many view genetic engineering today. In his poem The Mower, Against Gardens, Andrew Marvell (1621-1678) wrote:

Another world was searched through oceans new,

To find the marvel of Peru;
And yet these rarities might be allowed
To man, that sovereign thing and proud, Had he not dealt between the bark and tree,
Forbidden mixtures there to see.
No plant now knew the stock from which it came ;
He grafts upon the wild the tame,
That the uncertain and adulterate fruit
Might put the palate in dispute.


Genetic engineering has yet to produce anything in the horticultural world that's as extraordinary as Adam's Laburnum.

Aeonium arboreum,Crassulaceae

I can't remember how long I've had this Aeonium - it must be a decade at least - and throughout that time it has produced nothing but these geometrically attractive spiral whorls of succulent leaves on a woody stem that.....

... bears a spiral pattern of leaf scars left by the old leaves that die and fall off.

This year it has finally switched into flowering mode and produced a spectacular inflorescence. The whole plant is about a metre tall.

This will be its final hurrah - like some bamboos and the century plant Agave americana, Aeonium arboreum is monocarpic , gradually accumulating the stored energy required to produce its flowers and seeds and then dying.

Botanical trivia: Aeonium is the only genus I can think of that contains all five vowels in a single name - useful to contemplate, maybe, if you are compiling quiz questions for your gardening club?

Sundew, Drosera binata, Droseraceae

I've had this specimen of fork-leaved sundew Drosera binata for about a decade and it now fills a 10 inch pot and carries about 200 sticky-tentacled leaves. It's a death-trap for flies and would be quite capable of catching bumblebees if it had access to them.

Sundews face a dilemma - the need to capture and digest flies to supplement the meagre supplies of nitrogen in the soils in which they grow, while simultaneously using the services of insects to pollinate their flowers. The solution, as in many insectivorous plants, is to separate flowers and lethal leaves with a long flower stalk. It works pretty well in this case - the plant always sets plenty of dust-like seeds that germinate easily in surrounding flower pots in the conservatory, provided their soil surface is wet.

I particularly like the way the forked leaves unfurl - it's a bit like rolling out a deadly red carpet for passing insects ....

... and there is an air of menace about it in the final stages - like raised arms with clenched fists.  The common name of the plant is especially apt - it looks stunning when it catches the first rays of sun in the morning, when all those drops of mucilage sparkle like dew-drops.

Sundews were amongst Charles Darwin's favourite plants and he experimented with them in great detail, demonstrating that they released digestive enzymes when presented with animal protein (egg albumen in his experiments).

He noted that when a few tentacles captured a fly the surrounding ones responded by curling towards and then over it, often followed by the whole leaf folding over the prey, creating what he referred to as a 'vegetable stomach'. You can see how some of the longer hairs are curling towards this fly within a couple of minutes of it becoming stuck.

Each sticky-tipped hair is a complex strucure, with a glandular head that secretes mucilage (which is remarkably resistant to being washed away) and then enzymes that digest the prey after it dies of exhaustion during its struggle to escape. The products of digestion are transported down the stalk to the leaf in a double layer of cells in the hair stalk that pass the digestion products from cell to cell.

Darwin's book Insectivorous Plants contains detailed accounts of his experiments with this plant and can be downloaded here.

Bird of Paradise flower, Strelitzia reginae,Strelitziaceae

I've waited a long time for this. I bought this Strelitzia reginae as a small plant several years ago and made the fundamental mistake of planting it in a large pot, so it's been producing leaves rather than flowers until now. Back in November a flower bud appeared but its growth slowed right down in our unheated conservatory, but April's warmth has brought it into flower

There's no doubt that the Strelitzia bloom is very bird-like. It's also pollinated by birds - sunbirds - in its native South Africa

 

Hummingbirds in the New World hover in front of flowers and so burn up a lot of energy as they refuel with nectar, but sunbirds have a far more relaxed strategy, using the blue stamen of the flower as a perch

The stamen is a complex structure, formed by two elongated, fused blue petals and tipped with the white pointed stigma. The third blue petal that you can see here at its base is much smaller and conceals the nectar.

 

When the bird lands on the stamen its weight pushes it downwards, splitting the fused petals longitudinally and exposing the white pollen, which sticks to the bird's feet

When it visits another flower it lands on its stamen and while it's shuffling around probing for nectar it transfers pollen to the pointed white stigmatic surface that you can see above

A new flower rises from the inflorescence spathe every day. This is day two and I'm expecting three more

Image source: http://en.wikipedia.org/wiki/File:Charlotte1767Cotes.jpg

This is a flower deemed fit for a queen and was named by Sir Joseph Banks in honour of Charlotte of Mecklenburg-Strelitz (1744-1818), King George the Third's Queen-Consort, who was an avid botanist and great patron of Kew Gardens.

Puya chilensis, Bromeliaceae

A few years ago I was given some seed of Puya chilensis that germinated quickly and produced attractive small pot plants that had one particularly annoying trait. The arching whorls of leaves were fringed with vicious little recurved spines that were forever catching in my clothes or lacerating my skin, so one by one I gave them away to friends who had more space to grow them. On Tuesday, when I visited the National Botanic Garden of Wales I rather regretted that I hadn't been more patient, because their Puya chilensis had a magnificent three metre-tall flower spike. If I'd hung onto my plants for another decade, maybe............

.... or maybe not. I'd probably be permanently scarred by the Puya leaves by now, and you really need a glasshouse like that at the NBGW to flower this magnificent plant reliably. In the wild those recurved spines in the leaves have been known to impale birds and small mammals.

Puya chilensis comes from the coastal mountains of Chile and can be grown outside here in the mildest frost-free areas of Britain, such as the Tresco on the Scilly Isles. But what you really need is ......

........ a glasshouse like this - the magnificent structure designed by Sir Norman Foster for the NBGW.

This is the largest single-span glasshouse in the world, dedicated to displaying the flora of our planet's hotter, drier regions.

The stunning architecture of the building complements the natural architecture of the plants that it houses..........

........ while at the same time blending with its natural surroundings.

The glasshouse follows the contours of the land and reminds me of the carapace of a tortoise. A truely inspirational structure in an inspirational new botanic garden run by passionately committed, enthusiastic people.

Walnut, Juglans regia, Juglandaceae

When I was a kid my grandfather used to make little boats with half-walnut shells, fitting them with a matchstick mast and paper sail. Their association with childhood pleasures goes back a long way - apparently the Romans had a ritual at weddings whereby the bridegroom threw walnuts to children, symbolising the casting off of childhood preoccupations and the beginning of adult life.

One of the many botanically interesting aspects of walnuts is that they are not - botanically speaking - nuts. They are classified as drupes - fruits with a fleshy outer coating and a single hard seed inside, which groups them with peaches and plums whose outer coat is succulent. The tough outer coat of walnuts eventually splits to release the hard seed inside but it has its own special properties - it produces a deep brown dye. I discovered this for myself by accident when I was showing some visitors around Durham University Botanic Garden and picked up a fallen fruit and idly pulled it apart to show them the walnut seed inside - and it took three days for the brown stains to disappear from my fingers.

For about twenty years I had a walnut tree in my own small garden but it was foolish to plant such a fast-growing forest tree in such a small plot. It quickly outgrew the garden and I had to cut it down - and that revealed another interesting sensory aspect of the tree - its foliage and wood have a distinctive fragrance, that to me smells of apricots.

Walnut produces male and female reproductive structures separately on the same tree (monoecy) - the male flower being these fat catkins....

........... and the female being the ovary with a short style and twin stigmas that you can see between the leaf stalks at the top of the picture above. Solitary trees sometimes fail to produce nuts because there is a time differential between stigmas becoming receptive and the catkins shedding pollen (dichogamy).

Aside from its edible 'nuts', which were always an enduring feature of Christmas in our house when I was a child (no one could ever find the nut-crackers, which were not as elegant as the pair you can see here), walnut is famous for the beauty of its timber which is widely used for cabinet-making and for gun stocks. The tree originates from extreme south-east Europe and the Near-East (hence the alternative name Persian walnut) and can be killed in extreme winters. In 1709 there was a winter in Europe that killed thousands of walnut trees, which were felled and bought by Dutch merchants who foresaw an impending shortage and made a handsome profit by cornering the market for walnut timber in the years before new trees could be established.

There is an old saying that walnut trees are more fruitful if their branches are beaten. The likely origin of this is that walnuts used to be  harvested as soon as they began to ripen, while still attached to the tree, by knocking them down with long poles - otherwise birds and squirrels would get them first. This assault on the tree broke the tips of twigs and promoted the growth of fruiting spurs, producing a better crop of fruits in the following year.