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.

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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.

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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.

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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!

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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.

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Lost world discovered in twilight of Chinese caves

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Elatostema obscurinervium growing in stalagmite drip zones, one of 31 species known only from caves in China and Vietnam.

I never expected to be working in caves, that is, until I started to study nettles in Southwest China with Guangxi botanist Wei Yi-Gang. When Wei suggested that we collect in a local cave I was not hopeful. However, that first visit transported me to another world, an eerie moonscape in which plants thrived in powdery ‘soil’ and perpetual twilight. I was immediately gripped and determined to explore as many caves as possible. Over the next few years we visited over 60 caves, travelling on underground rivers, hiking across rice fields or sneaking into big tourist caves. Each time I got the same thrill from entering these strange and silent places. This has culminated in a paper just published in the scientific journal PLOS ONE.

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Yangtse cave, Fengshan County, Guangxi. Eight species new to science have been described from here.

There has been very little research on plants in caves. Having come across such a widespread and diverse flora we wanted to make sure that both botanists and cave biologists were aware of it. This is because plants growing in caves could tell us a lot about how plants adapt to extreme conditions. Also, as to how connected caves are to the surrounding landscape and each other. To do so we set out to answer a series of questions. Probably the most obvious was, are plants growing in light levels distinct to those outside? Did the species evolve in caves? Or are they survivors of a previous vegetation that at some time connected caves? And, are these plants important for conservation in the region? For example, in the restoration of the karst forests lost during the 20thC.

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Cave converted to a tourist attraction.

There are thousands of caves in Southwest China and the landscape that they helped form has long been famous in Chinese culture and art. A landscape covered in dense forest until the 20thC at which time the ‘Great leap forward’ and later the ‘Cultural revolution’ resulted in massive deforestation. Once cut, the forest has not returned but instead been replaced dry scrub. The 21stC has brought new threats from cement and tourism. The rock that forms these caves is used to make cement, now in high demand, that combined with a rapid growth in tourism has meant once untouched ‘lost worlds’ are being mined or filled with walkways, bright lights and litter.

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Collaborators from the Guangxi Institute of Botany, Fu Longfei and Chen Xiaoqin taking photosynthesis measurements.

We found that some of the plants growing in caves are surviving in very low levels of light. Levels much lower than previously known. From other cave studies we also know that daily changes in temperature and humidity are very small. There is also some suggestion from other studies that the cave atmosphere may be richer in CO2. It could be that low levels of light are being offset by more stable conditions and higher CO2 – an interesting question for future research.

We also found that whilst most of the plants we document are known from forest habitats elsewhere, 31 species are only known from caves. For reasons given in our paper we do not believe that this means that they evolved in caves but rather that they are species which were restricted to the forests lost in the 20thC.

So, it looks as if the plants that we are finding in caves are relics of forests that were lost to deforestation in the 1950s and 1970s. With a current focus on restoring these lost forests in China, these surviving populations could now become a valuable source of local plant material for restoration.

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Yin Jia cave close to Gu Lin village in Yunnan. At 1600 m above sea-level this is also one of highest elevations that we have collected in caves.

Other posts by me on this topic

2016: Studying cave-plants in SW China

2014: Caves explored last month