This month the Millennium Seed Bank's Seed Morphologist, Wolfgang Stuppy, introduces us to the freaky blue fruits of Decaisnea insignis. The taste is sweet and refreshing but this fruit is eerily known as ‘dead man’s finger’.
If you get excited by exotic fruits, here’s one that adds a little ‘creep factor’ to the experience. Best of all, you can even grow it in your own garden in the UK and use it as a party trick to freak your friends!
The fruit of Decaisnea insignis, also called ‘dead man’s finger’ (Photo: W. Stuppy)
Freaky blue fruit
The freaky fruit I am referring to here is kind of a secret delicacy. It’s been enjoyed for centuries by the Lepcha, the indigenous people of Sikkim, but outside its natural range the dead man’s finger is little known for its edible fruits. A member of the chocolate vine family (Lardizabalaceae), Decaisnea insignis is a shrub native to China, Nepal, northeast India (Sikkim), Bhutan, and Myanmar. At home in Asia it grows at altitudes between 900 to 3600 metres above sea level which is why it is frost-hardy in the UK where it is sometimes grown as an ornamental.
Left: a female flower of the chocolate vine (Akebia quinata), a popular garden plant with chocolate-scented flowers; right: a female flower of its close relative, the lesser known Decaisnea insignis (Photo: W. Stuppy)
Like a cold human finger
The greenish-yellow flowers of Decaisnea are between 3-6 cm in diameter and borne in drooping racemes. They come in separate male and female flowers but it is only the latter ones that give rise to Decaisnea’s most striking decorative feature, the very unusual finger-like blue fruits. About 7-12 cm long, soft to the touch and covered in an eerily skin-like peel, the fruit of Decaisnea does really feel like a cold human finger, hence it’s common name ‘dead man’s finger’.
The fruits of Decaisnea insignis are always borne in clusters of three (Photo: W. Stuppy)
One flower – three fruits
The fruits of Decaisnea are borne in clusters of three and this has got an interesting reason. Normally you would expect one fruit per flower but in Decaisnea, each female flower contains three separate carpels (the three straight cylinders in the centre; see photo above). Because the three carpels are free from each other, after successful pollination, each one of them turns into a fruit of its own. Actually, ‘fruitlet’ would be the botanically correct term for one of these fingers, the diminutive indicating the origin of a triplet of ‘fingers’ from one flower.
The fruit of Decaisnea insignis opens along a straight line to reveal the edible gelatinous pulp (Photo: W. Stuppy)
The taste of a dead man’s finger
Now comes the really interesting part. Almost as if opening with a zip, the fruits easily split along a straight line to reveal their translucent gelatinous pulp into which a large number of flat black seeds are embedded in two rows. The jelly-like pulp is the edible part, not the hard seeds. Having tasted my first dead man’s finger with great curiosity and excitement, I found its flavour very pleasant and subtle, mainly sweet, perhaps with a hint of melon or cucumber.
Unzipped – the fruit of Decaisnea insignis opened up along its ventral suture, like the carcass of a slaughtered animal, ready to be gutted. Yum! (Photo: W. Stuppy)
A bit of seed morphology
O.K. this is a bit nerdy but since I am a Seed Morphologist and information about the inner workings of the seed of a dead man’s finger is not something you can easily find anywhere else, I might as well... The seeds of Decaisnea insignis are about 1 cm long and about 3-4 mm thick. Inside the seeds bear a very small embryo embedded in copious nutritious tissue (endosperm) which the embryo consumes during germination.
A longitudinal section through the seed of Decaisnea insignis showing the very small embryo embedded in copious endosperm (Photo: Gemma Toothill)
The usual question: who is it meant for?
When I come across such weird fruits like the dead man’s finger I always want to know how they are dispersed naturally. Offering a sweet edible pulp, the fruits of Decaisnea insignis are obviously meant to be eaten by some animal but which one? Little can be found in the literature about Decaisnea’s natural dispersers. However, the size of the fruit (too big for most birds in its native range) and the fact that it obviously requires peeling (a bit like a banana) to separate the edible pulp from the thick leathery peel suggests that animals with a good degree of ‘manual’ dexterity are the co-adapted dispersers of Decaisnea insignis. Indeed, the only information in the scientific literature I could find about potential dispersers is that in China (Yunnan & Szechuan), snub-nosed monkeys (Rhinopithecus spp., Cercopithecidae) include fruits of Decaisnea in their diet [Dietary profile of Rhinopithecus bieti and its socioecological implications].
Whether snub-nosed monkeys are really the co-adapted dispersers of Decaisnea is hard to say because primates in general are intelligent and may learn to eat any fruit, even if is not ‘meant’ specifically for them. Also, this would not necessarily explain the fruits’ very unusual blue colour. Snub-nosed monkeys possess trichromatic vision like us humans and so the fruits could as well be some shade of red or yellow, which is a much more common fruit colour. If any readers would have more information towards solving the riddle of the natural disperser of Decaisnea, I would like to hear from them.
Just in case you wondered....
Some readers might be more familiar with the Latin name Decaisnea fargesii for the dead man’s finger. Indeed, on the basis of its blue fruits, D. fargesii from China (the most common form in cultivation) has been considered as a separate species from the Nepalese D. insignis with yellowish-green fruits. However, fruit colour alone is not considered a strong enough character to justify the separation into two species which is why some authors have combined both species into one, i.e. Decaisnea insignis (e.g. Flora of China, Mabberley’s Plant Book).
Danger! Don’t just eat any dead man’s finger!
Just to avoid any potentially dangerous confusion, it must be said that there are other species, unrelated and not all plants, commonly known as Dead Man’s (or Dead Men’s) Fingers, one of them deadly poisonous. Oenanthe crocata, a member of the carrot family (Apiaceae), is known under the common name ‘Hemlock Water-Dropwort’, but is also sometimes called ‘Dead Men’s Fingers’ (on account of the shape of its tubers). All parts of this plant are poisonous and occasionally lead to fatalities. Looking similar to Cow Parsley (Anthriscus sylvestris) or Comon Hogweed (Heracleum sphondylium), it should be hard to confuse Oenanthe crocata with Decaisnea insignis. However, since Oenanthe crocata is deemed one of the most poisonous plants in Britain, a word of warning seems appropriate here. After all, as my colleage Steve Davis pointed out, in recent years Kew has received frequent enquiries concerning Oenanthe crocata, including a case of someone being admitted to A&E, and several cases of livestock or pet poisonings. Other ‘Dead Man’s Fingers’ are Xylaria polymorpha (a fungus), Alcyonium digitatum (a species of soft coral) and Codium fragile (a seaweed).
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