Brightly coloured or fleshy fruit are not what you would associate with nettles. Indeed neither would most botanists who study them in a herbarium where once brightly coloured intricately shaped structures are reduced to congealed dark brown blobs. It is thanks to field work, and the enthusiasm of many amateur naturalists and their cameras that their beauty and complexity is becoming better known. Fleshy-fruited nettles are found across the family, comprising probably 1/4 of the genera. Nettles are very inventive in producing these fruits, with stalks, petals, fruiting branches or bracts being re-purposed following flowering. Although I work on nettles and so am naturally biased, I can’t think of another plant family that has come up with so many ways to produce a tasty morsel for a bird or mammal, or which produces such complex shaped fruits.
In the case of the tree from Taiwan and the Philippines, Discocnide meyeniana, the three tiny and unequal green petals around each ovary swell up into a ghostly white cradle which likely flags the exposed seed to potential dispersers. In this case likely a small bird. Another species with unusual fleshy fruits is a small tree from the Dominican Republic, Gyrotaenia microcarpa. In this species it is the fused flowering branches which become fleshy, expanding in fruit to leave the seeds exposed in small clusters on the outside, a bit like a misshapen strawberry.
In several species fruits designed to attract consumers also have stinging hairs. This is the case of the Latin American shrub Urera baccifera. In this species, not only do the tiny green petals become white and swell up to conceal the seed, but their flowering branches and stalks become bright magenta and fleshy, curling protectively over the white berries brandishing hypodermic stinging hairs. This suggests that not any animal is welcome to take the fruit. Whatever feeds on the berry will need to delicately free it without getting stung.
Probably the strangest looking fruit is that of Procris, a group of succulent species from Asia, the Pacific and Africa. In these plants the flower stalks all fuse to form a swollen foot. In fruit, together with the reduced petals at the ovary’s base, this becomes fleshy and when wet the whole structure is covered by a thick slime. It is hard to imagine what kind of animal this fruit is aiming to attract!
I hope that this very brief and subjective survey of nettle fruits demonstrates how innovative and surprising plants (and nettles in particular) can be.
For over a hundred years the genus Myriocarpa in the nettle family which comprises ca 15 tree species in South and Central America has been impossible to place within the family. This is largely because of the very unusual shape of the part of the female flower that receives pollen, known as the stigma (see image below) and the fact that neither of the two great experts could agree over whether the petal-like structures at the base of the flower were petals associated with the flower, or bracts associated with the stalk. Whilst this might not seem like the stuff to keep a botanist awake at night it has become of interest again as using DNA data we have identified as sister to another small group of trees, Gyrotaenia, found in Cuba, Hispaniola, Jamaica and Central America that has a flower which consists of the petals fused to form a tube which is fused to ovary.
I was therefore very curious to see whether Vladimir Blagoderov, Manager of the Museum’s Sackler Imaging Suite could help me generate an image that would help us resolve the mystery. He could! The two images above are each composed of about 20 images which ‘slice’ through the sample which was of young flowers collected in alcohol in Belize over 10 years ago. The resolution was amazing, each cell being visible. In fact you could even make out the rough crystalline structure on the surface of the hairs! Both of these images also helped us to answer the question, revealing that this flower does indeed consist of a tube composed of fused petals that is subsequently fused to the ovary. This we could see in both images where the clearly visible ovary is enveloped by another distinct tissue, as in the case of Gyrotaenia. It was also confirmed by the petal-like bracts at the base of the flower having stalked glands, structures not known to occur on the ovaries of Urticaceae. So a morning’s work and an idea of evolutionary relationships enabled me to answer a question that had been frustrating an albeit very small group of botanists for over 100 years!
I have just spent an amazing week with a team from the Jardín Botánico Nacional Dr. Rafael M. Moscoso and Kew’s Millennium Seed Bank collecting nettles in the Dominican Republic, thanks to funds from the Bentham Moxon Trust. The Dominican Republic represents half of the island of Hispaniola, the other half being Haiti. Thanks to a rich geological history, varied climate and very high mountains (to 2,760 m) the country is host to a very rich and diverse flora of over 5,500 species, many of which are only found on Hispaniola. We made some very interesting collections, the first two on the first day! One was of a genus, Rousselia that I have never before seen alive and of which few collections exist in herbaria, and which by coincidence I posted about a couple of months ago. The other was of a very unusual tuberous species of Pilea, a genus of ca 715 species of mainly succulent herbs, over 70 of which are found only on the island of Hispaniola.