Working with Royal Botanic Gardens, Edinburgh student Jia Dong and plant anatomist Louis Ronse De Craene has resulted in some exciting and thought-provoking images of nettle flowers. The aim of our collaboration is to understand how nettle flowers develop and in the process work out what parts they have in common and which they don’t. The samples used were from living collections at Edinburgh and RBG Kew, together with my own collections in alcohol made over several years. The results are some beautiful and very informative scanning electron micrographs which show that the part of the female flower which recieves pollen (stigma) and conducts it to the egg (style) is characterised by two classes of hairs, one comprising defensive tusk-like hairs (above) and the other receptive tubular like hairs (above & below).
Tubular hairs associated with the stigma are characteristic of all nettle flowers. They also appear very early in development. Combined this makes us think that they might have a role in pollination. Specifically in the reception of pollen. Being wind-pollinated, nettles don’t have a lot of control as to whose pollen reaches their female flowers and so there needs to be a way for them to control which pollen grains develop and fertilise the single egg. It seems likely that these hairs play a role and hopefully Jia will be able provide some more great images to test some hypotheses about this.
Funded by the Synthesys project I am studying the Cuban nettle collections of the Berlin-Dahlem Museum. The aim is to finish my account of the nettle family for the Flora of Cuba project that I started five years ago. It might seem odd that Cuba has so many nettle species that I can still be working on it, albeit in a fragmented way, for five years. Also that Berlin should be an important repository of Cuban plants, but there is a reason.
Cuba has an exceptionally rich flora and is especially important for the nettle family, Urticaceae, which is represented by about 70 species in eight genera. Together the Greater Antilles, Jamaica, Haiti/Dominican Republic, Cayman Islands and Puerto Rico, is a centre of species-richness for one group of nettles in particular, the genus Pilea. There are over 150 species of Pilea in the Greater Antilles, 60 of which are native to Cuba. The reason for such high diversity is unknown but may have something to do with the age of the islands, preponderance of limestone substrates or something else that we haven’t thought of yet. It does, however mean that I have been spending a lot of time looking at herbarium spcimens of this genus in the Berlin herbarium.
The connection between Germany and the the Greater Antilles started with botanist Ignatz Urban, who did a lot of work documenting the plant diversity of the Greater Antilles. Tragically most of his collections were destroyed in World War Two when the herbarium was bombed. During the Cold War the connection that had established between Berlin and the Caribbean switched to East Germany and the Jena herbarium impulsed by the formidable Johannes Bisse who founded the National Botanic Garden of Cuba. This lead to the foundation of the Flora of Cuba project a collaboration between the many excellent Cuban botanists and their German counterparts, initially in Jena and then from 1993 in Berlin. So that is the reason that I am in Berlin looking at Cuban nettles!
Thanks to funding from the Bentham Moxon Trust and the Guilin Botanical Garden, myself and colleagues explored five caves for plants in October of this year (2014). There are likely thousands of caves in the limestone karsts of south-east asia which contain plants. Whilst of great interest botanically and for conservation they are also beautiful in their own right and each cave is unique. I thought I would provide a portrait of each one to show how varied they are in their form, where plants grow in them and their size.
The cave above was one of the few that we spotted from the road and then were able to get to. It is also one of the few that had little evidence of human disturbance, very few footprints and areas of pure white travertine that had fallen from the roof had not been walked on or collected. The main plant-bearing cavern of the cave was about 25 m deep and the roof 15 m high and it had a well developed flora, you can see plants from the african-violet family (Gesneriaceae) in the foreground and we collected five species of nettle here.
Xiangshuidong Tian Keng cave in Guizhou was one of the largest caves that we have collected in, but also one of the most impacted by tourism and use by local communities. The main cavern is about 250 m deep with a roof between 45 and 30 m high and it is set within a huge cliff forming the side of a mountain. It also has a waterfall and river running across the back of it. It is in this cave that we found a very rare and unusual form of Elatostema oblongifolium that has its male flowers borne on specialised shoots but overall the plant diversity of the cave was quite low, presumably because of the large numbers of local tourists and associated trampling of much of the ground available for plants
This was the first cave that we encountered on this field trip. We found it after first being taken to a hole in the ground as what must have been a mistranslation from Mandarin into the local dialect. The cave was relatively big and had a trail running inside. The main cavern shelves very steeply meaning that very little light penetrates into the cave. Despite this and the relatively high altitude, 1500 m, we collected seven species of nettle from here.
This cave should have been perfect as it shelved gently meaning that light penetrated quite deep, it also had plenty of places for plants to grow, such as boulders and rocks. We only collected four species of nettle here, probably because the cave was heavily impacted by farmers using it as a barn to keep their water buffalo in at night. Evidenced by lots of hoof prints and dung. This is a very common use of caves and the trampling of buffalo and their herders can have a significant impact on the plants in the caves (see below).
Nettles have unisexual flowers, that is each flower functions only as a male or a female. Counter-intuitively though the flowers still retain the non-functional and often much-reduced organs of the non-functional sex. These are called pistillodes in the case of the rudimentary female organs in male flowers and staminodes in the case of the rudimentary male organs in female flowers. Inpart of the nettle family the staminodes are put to good use: ejecting the seed from the fruit.
This can be best understood by considering that nettles have male flowers which open explosively, pollen being released in tiny clouds (they are also very small). In fact one species, Pilea microphylla, is commonly known as the artillery plant for this reason. The mechanism for the explosive opening of the male flower is that the stalks (filaments) of the pollen sacks (anthers) are folded in on themselves in bud. As they develop these stalks fill with water until they are all pressing against each other within the flower bud and ready to burst. At a certain point the pressure becomes too great for the thin petals of the flower bud and they rip leading to the stalks being able to straighten explosively. This has happens incredibly quickly and although it has not been recorded for nettles, in the closely related mulberry where this also happens, the flower can open in 25 millionths of a second, moving petals to velocities in excess of half the speed of sound.
In one group of nettles which includes seven genera this explosive ability of the pollen sacks has been harnessed to release the seed from the fruit. Below you can see a close-up of the fruit of Pilea japonica showing the staminodes flexed and ready to eject the seeds (dark coloured).
I have just returned from collecting nettles in South West China with my great friend and colleague Professor Wei Yi-Gang and researcher Fu Long-Fei from the Guilin Botanic Gardens. For several years we have been working on documenting the nettle diversity of the limestone karst of this region, focusing on the poorly known cave-associated floras. Karst is a form of limestone which has been weathered by rain for millions of years resulting in finely divided and sharp surfaces and very steeply sided hills, small mountains and gorges. The karst where we have been working form part of a formation which runs from Myanmar, northern Thailand and Vietnam and across into southern China and includes the famous ‘stone forests’ of Yunnan and Guangxi. I am interested in karst because it is where nettles are most diverse, both in terms of genera (species groupings) and species. At a single location it may be possible to encounter eight genera and over a dozen species!
Because the limestone is porous it has resulted in the formation of thousands of caves whose entrances have been colonized by plants from a small group of families: nettles (Urticaceae), african violet family (Gesneriaceae), Begonias (Begoniaceae), ivy family (Araliaceae), the coffee family (Rubiaceae) together with ferns and mosses. The most diverse of these are the nettles, one group of which, Elatostema has about 1/5 of the species from this region known only from caves. As well as having very low light levels, sometimes 1/10 of 1% daylight, the caves have constant humidity and low temperatures which contrast strongly with the cave exterior. The cool air of a cave can be felt up to 20 m from the entrance, often before the cave itself can be seen. We believe that the species associated with caves have likely come from the deep shade of the forests that once dominated this area but which have since been lost to agriculture. It also possible that some of the species are relicts of a previous, cooler climate during the last ice-age. This is something that we have begun to study.
Caves represent a last refuge for several hundred species in SW China and now is a time of great change as caves come under threat from tourism, agriculture, urbanization and cement production. We want to help conserve these caves and the species that they include by first documenting their diversity and distribution across SW China. This information can be disseminated within China in the hope of raising awareness and protecting individual caves. It could also be used to identify a network of protected caves which include all of these species. Professor Wei would also like to develop protocols for cultivating the species outside of caves, something that is very difficult to achieve at the moment and which could represent a vital step for their conservation.
Since 2007 I have been working with colleagues at the Chinese Academy of Sciences Guilin Institute of Botany on documenting the unusual cave flora of SW China and Guangxi. My interest stems from the fact that one of the most common groups of plants in these caves are two particular groups of nettles, members of the succulent herbaceous genera Elatostema and Pilea. It is also heavily influenced by the presence of a very knowledgeable and dedicated botanist at the Guilin Institute of Botany, Professor Wei Yi-Gang.
More recently I have wanted to see whether it is possible to explain how, and when these plants occupied these ancient caves. Possible explanations are that they evolved in the caves, some of which are 15-25 million years old; alternatively that they represent plants which grew outside of the caves when the climate was different, during the last ice-age for example; lastly that they are relics of plants which grew in the forest understory outside of the caves prior to the arrival of agriculture in the area maybe 1,500 years ago. To try and answer these questions I have, together with a Masters student Alfred Gay, used DNA extracted from the leaves of the plants to look for patterns which may point to one of the three possible explanations above. Click here to see a slide show of the preliminary results.
Another interesting line of research would be how these nettles survive in such low light levels. In some cases 1/50th of 1% daylight! For the moment though I am focussing on documenting their diversity and describing the new species we find but in the long-term I am hoping to find collaborators to explore these other areas of research.
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).
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.
Pilea is a genus or group of over 700 species in the nettle family (Urticaceae). It is mainly comprised of succulent herbaceous plants that grow in the shade of forest and is especially diverse in the Greater Antilles and on limestone. Currently over 110 species are recorded for the island of Hispaniola which encompasses the Dominican Republic. They are very variable in their shape and form and probably most people (including many botanists) might not think that they were related to nettles. It is a group that I have been working on for over 15 years and I began work on revising their classification for neighbouring Cuba in 2010. Surveying the species in the herbarium of the National Botanic Garden in Santo Domingo helped me to see how they compared to the species in Cuba.
After a week looking at the collections and sorting them into piles I started to get an idea as to the species present on this amazing island. Of course establishing their correct names involves a whole other body of work but at least I now have an idea as to what there is in the Dominican Republic, what might be conspecific with Cuban species and where there are complexes of closely-related and potentially interbreeding species. Not bad for a week’s work.
For a week now I have been accompanying a team from the Jardín Botánico Nacional and Kew’s Millennium Seed Bank on a field trip to collect seed of plants endemic to Hispaniola for banking at the Jardín Botánico and Kew. I have been taking advantage of this trip to collect nettles but also to learn how to harvest and bank their seeds. The Greater Antilles which includes the Dominica Republic / Hispaniola is a centre for species diversity for nettles and most of the 100 or so species found here are found nowhere else. Given obvious pressure on the island’s forests both in the Dominican Republic as well as in Haiti, banking their seeds could support their reintroduction as a last ditch attempt to prevent their extinction.
The seed bank at Kew which banks the seeds of >50,000 species currently has only 14 sp of nettle which for me seemed a little on the low side given that there are about 2,000 species Worldwide. Support from the Bentham Moxon Trust is helping me to increase this figure significantly, both by enabling me to help develop seed-collecting protocols for nettles and so encourage seed banks to collect them, but also because I am learning from them how to collect seeds, something that I can do myself on future fieldwork in collaboration with other seed banks. Continue reading Collecting nettle seeds, no easy task!→
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.