One of the most spectacular plants to be found in the wet tropics zone of the Princess of Wales Conservatory at Kew Gardens is the titan arum.
The titan arum originates in the moist shaded rainforests of Sumatra. The first European botanist to encounter it was the Italian Odoardo Beccari, who was travelling in the region in 1878.
He sent back seeds to his patron in Italy and one of the young plants that germinated from them was subsequently dispatched to Kew, where it flowered in 1889, exciting great public interest. In 1926, when it flowered again, the crowds attracted by the phenomenon were so large that the police were called to control them.
About the plant
With its huge flowering structure (inflorescence) rising over 2.5 m above the ground and its single immense leaf, it certainly is a giant among plants, as its name suggests. Coupled with its characteristic foul stench, and the rarity of flowerings, this plant has always hit the headlines.
The massive inflorescence consists of a bell-shaped spathe, up to 3 m in circumference, with ribbed sides and a frilled edge, around a central spike-like spadix. On the outside, the enveloping spathe is green speckled with cream, but its interior is rich crimson.
At its base, the spathe forms a chamber enclosing the flowers which are carried at the lower end of the greyish-yellow spadix. The inflorescence arises from an underground tuber, a swollen stem modified to store food for the plant. This tuber, more or less spherical in shape and weighing 70 kg or more, is the largest such structure known in the plant kingdom.
After flowering, the flowering structure dies back and in its place a single leaf emerges. Reaching the size of a small tree, up to 6 m tall and 5 m across, the leaf consists of a sturdy glossy green stalk mottled with cream which divides into three at its apex and bears numerous leaflets.
Sugars made in the leaf are transported back to the tuber for storage as starch. Each year, the leaf withers before a new one develops, using the tuber’s energy stores. Eventually, the tuber becomes dormant for up to four months before another inflorescence emerges, growing upwards at a rate of some 10 cm per day.
Odours - foul and fragrant
Many of the 170 or so species of Amorphophallus produce a variety of obnoxious odours ranging from rotting meat, dung and rancid cheese to a nauseating gaseous stench. Size does not always equate to their ability to generate a stink. Relative to A. titanum, the inflorescence of A. bulbifer is small, yet the gaseous stench it produces can make working in a glasshouse with it a sickening experience, as staff at Kew can testify.
Some Amorphophallus species, however, produce pleasant odours; for example, A. haematospadix smells of bananas while A. dunnii has the odour of freshly chopped carrots.
Odours have been used to classify Amorphophallus species in the past, but the human nose can be deceived.
Studies at Kew
When plants in the Princess of Wales Conservatory flowered in 1996, 2002 and 2003, scientist Geoffrey Kite from Kew’s Jodrell Laboratory investigated the obnoxious smells that they produced.
The strongest smells occurred on two consecutive evenings, firstly when the female flowers were ready for pollination and then when the male flowers were ready to shed their pollen.
The major components detected in the carrion and gaseous odours are the sulphur-containing compounds dimethyldisulphide and dimethyltrisulphide. The banana odour of A. haematospadix appears to be due to isoamyl acetate while the carrot odour of A. dunnii consists almost entirely of 1-phenylethylacetate; the chocolate odour of A. manta has yet to be analysed.
One aim of the work is to compare the chemical nature of the odours with a modern classification of the genus based on DNA sequencing and a full scale morphological analysis. The chemical constituents of the odours might also provide clues to the pollinators of these plants since, for most species, the pollinators remain a mystery.
The few observations available are for foul-smelling species and these seem to attract carrion beetles. This is in accordance with chemical data on the odour; dimethyldisulphide is known to attract carrion feeding or breeding insects and is used in a commercial lure for screw-worm flies.
In 1996 a titan arum flowered at Kew for the first time for many decades. Six years later we saw an unprecedented three flowerings in as many months. This was the first evidence that Kew's horticulturists had finally cracked the secrets of cultivating this rare and unique plant.
Since then there have been several flowerings, including one in 2003, three in 2005, two in 2006 and three in 2009. Each event attracts vast crowds and intense media interest from around the world.
How Kew did it
The specimens which flowered in 2002 and 2003 were grown both from seed donated to Kew in 1995 and from tiny micropropagated plants received from the botanic garden in Bonn in 1995.
To build up the storage tubers, they were potted on into 750 litre containers in 1999 although they were in full leaf at the time. When the leaves died down, the largest tuber (weighing 75 kg) was placed in a 1,000 litre pot and the two smaller ones were put into new compost in 750 litre pots.
For the next 14 months, they were grown behind the scenes at Kew in our Tropical Nursery and fed regularly with a high potash liquid fertiliser, before being transferred to the Princess of Wales Conservatory.
In the Princess of Wales Conservatory during the day, the temperature is at least 24ºC and at night it drops no lower than 19ºC. Humidity is maintained at 70-80% – mimicking the conditions prevailing in the plant’s original rainforest habitat.