Strange world of fleshy nettle fruits (Urticaceae)

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Top: Myrianthus arboreus, Debregeasia salicifolia, Poikilospermum suaveolens. Bottom: Dendrocnide meyeniana, Urera baccifera, Touchardia latifolia.

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.

Dendrocnide meyeniana, here brilliantly photographed in Taiwan by Flickr user ted762563. I have inserted arrows to highlight the tiny petals which later develop into the strange clasp-shaped fruit.

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.

Gyrotaenia microcarpa fruit. The fused flowering branches swell up and become white, the seeds becoming black creating a contrast which presumably attracts small birds or mammals to feed.

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.

Urera baccifera, common throughout Latin America and well known for its brightly coloured but also stinging fruit clusters.

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!

Procris wightiana from Southeast Asia. The cup-like structures at the base of the seeds are the reduced petals that have fused and become fleshy.

 

I hope that this very brief and subjective survey of nettle fruits demonstrates how innovative and surprising plants (and nettles in particular) can be.

Studying cave plants in SW China

Yangtse cave, Fengshan County, Guangxi, type locality for eight species of plant. The plant-bearing part of the cave is ca 170 m deep and 70 m wide (note the person for scale)

As part of my research on the nettle family, Urticaceae I became aware of plants growing in the entrance caverns of caves several years ago and for over a year now my collaborators at the Guangxi Institute of Botany, China lead by Professor Yigang Wei and I have been working on documenting the full diversity of this unusual flora. This lead us to think about whether these plants may have become adapted to life in caves, in particular the relatively constant climate and low light. Especially for species which grow amongst the lowest light levels at the back of caverns where they are growing in a fraction of the light they could be expected to receive in a forest. We therefore applied for a grant from the Guangxi Key Laboratory of Plant Conservation and Restoration Ecology in Karst Terrain, and the Foreign Experts Bureau to undertake some preliminary work to document the climate, light and photosynthesis of the plants in the caves.

Fu Longfei and Chen Xiaoqin taking photosynthesis measurements using an instrument called a handheld PAM which uses fluorescence to measure various aspects of photosynthesis.

We selected the Yangtse cave as we know the diversity of plants that grow there (ten species of nettle alone), we have three data-loggers recording temperature and humidity in it and it is close to a town where we can spend the night. It is also a spectacular and beautiful place to spend several days working. The aim of our work was to collect the data necessary to test the hypothesis that the plants growing within the entrance cavern of the Yangtse cave exhibit different photosynthetic performance than the same or congeneric species growing outside of the cave. To do this we randomly selected individuals of three species of nettle in the genus Elatostema, one species of Begonia and a species of fern at four different locations in the cave, the back, midway into the entrance cavern, at the entrance and outside of the cave. We also brought two species of Elatostema from the living collection at the Guangxi Institute of Botany to compare their photosynthesis performance with members of the same species that had grown up within the cave. This was to get some indication as to how plastic their response was.

Elatostema obscurinervium, one of 31 species known only from caves and recently collected from a cave in northern Vietnam.

Each plant was connected to a hand-held PAM chlorophyll fluorometer, an incredibly sensitive device that can measure several key outputs of photosynthetic reactions in the chloroplasts as they take place. By comparing our study plants to those growing outside of the cave and from the scientific literature we hope to see whether cave-dwelling plants differ from non-cave plants in some of those parameters, and whether those differences are dependent upon what kind of plant they are. These parameters include the efficiency of photosynthesis, that is how much of the light energy is harnessed by the photosynthetic reactions, how much is dissipated and how resilient the photosynthetic apparatus is to changing light intensity. If we find  a difference between cave and non-cave dwelling plants then taken together these measurements can provide some indication of which group of photosynthetic reactions are leading to these differences.

A likely new species of Pilea from the Dominican Republic

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A possible new species of Pilea characterized by large tubers which probably enable it to survive in fissures in limestone karst cliffs where it grows. Image Alex Monro.

On my recent trip collecting nettles in the Dominican Republic we came across what is very likely a new species of Pilea, a group of about succulent nettles. I thought it might be useful to outline what happens from collecting / discovering something new to it being published as a new species, from my own botanical perspective of course. In the case of this species, it has a couple of distinctive features which make it stand out from similar looking Pilea species: 1) a well developed above-ground tuber, up to the size of a small potato, and 2) relatively large male flower clusters for the group of species it is in. This gives me two diagnostic characters to check with in existing collections and in the literature. Pilea is a genus of over 700 species found in tropical and subtropical Asia, Madagascar, Africa and the Americas. All but a few species are restricted to one of these areas or a much smaller area and this knowledge enables me to delimit my search area. In this case, the Greater Antilles (Cuba, Jamaica, the Cayman Islands and Hispaniola).

Close-up of the leaves and flowers. The cluster of red flowers on the left are male and are borne on a relatively long stalk, to the right you can see a cluster of female flowers. This cluster is only a few mm long and contains several flowers.

My second job is to see if the species has already been described. To do this I need to look through the collections of the most appropriate herbaria for this area, starting with the National Herbarium of the Dominican Republic. This can take quite a long time although many collections have been digitised and are available online. I then need to follow this up by reviewing the literature for the region to see if I find any descriptions which match or are close. Once I have done this I should have a list of species that are possible or close matches. These I can then compare in more detail with my possible new species and confirm whether it is new or not. This I will do by looking at the type specimens under a microscope and comparing their key features: leaf and stem shape, the nature of their hairs, the size and shape of the flowers and of course in this case, the presence or not of a tuber. This also provides me with the information I need to write the diagnosis of the new species. Once I am sure that this is a new species I can start writing the description and think of a name. A new species description will include a detailed description of the plant, a line drawing illustrating the diagnostic features and a species conservation assessment which will provide an indication as to how threatened with extinction it is.

A cross-section through one of the tubers. The tuber likely serves as a water storage device to help the species survive dry periods growing in narrow fissures in rock.

Three new nettles from Southern China

Elatostema laevicaule, a new species of nettle from Guangxi Province in SW China. The species is a small succulent plant which grows on or amongst rocks in the forest understory

All three of the new species were described in November 2013 in the journal Phytotaxa.