Saturday saw me attending another excellent field trip run by the South-East group of the British Bryological Society. This time we were headed to a private site just inside Kent called Hungershall Rocks. The sandstone at Hungershall Rocks is part of a large number of these outcrops stretching across the Weald from Tunbridge Wells to Ardingly and all along the ridge there are sites with a wealth of rare bryological flora.
Hungershall Rocks is a private site just outside Tunbridge Wells near High Rocks which we were lucky to get access to. Plenty of excellent bryologists have visited the site in the past and the records stretch back over more than 150 years. It has been interesting to see how the bryological flora of the site has developed over such a long time, with new species being discovered but equally species that were here in the past being lost from the site.
The rocks themselves are mainly under tree cover, however some are more exposed. Some patches are dripping wet due to the clay within the rocks and others bone dry. There are a plethora of nooks, crannies, ledges and some caves too. This wide range of rock features leads to a very diverse set of mosses and liverworts that can be found on these rocks.
It wasn’t only mosses and liverworts that were in abundance either. There were many different vascular plant species inhabiting the rocks, especially ferns. One of the best finds of the day was this beautiful, eye-catching and impressive-looking plant:
It wasn’t too easy to get to. It was growing on the wall of a small cave, where very little sunlight penetrated. There was only really room for one person inside the cave and that person had to be in a crouched, uncomfortable position. So why was this rather underwhelming (to be honest) plant worth such an effort to get to?
It is Trichomanes speciosum, the Killarney Fern. Killarney is a small town in Ireland’s County Kerry, the county where half of Ireland’s known populations of this fern are found. As you can see from the map below, this species has a predominantly Western distribution in the UK and is quite unusual in the East, being recorded in only a handful of sites in the South-East:
You might be thinking that it doesn’t look like a normal fern usually does and you’d be forgiven for thinking so because this is not the most commonly seen fern life-stage. The life-cycles of ferns are very complex and unusual and this is a seldom noticed part of it.
The green felty stuff that you can see in the image are the rhizoids of the gametophyte (also known as the prothallus), the gametophyte being the life-stage before the recognisable adult sporophytes that we most commonly come across; the rhizoids being filaments attached to the gametophyte which conduct water.
On the undersides of the fronds of adult ferns there are small, usually brown, sporangia which contain spores. These spores are released and when a suitable site is found, they grow into a gametophyte like the one shown in the image. The word gametophyte comes from the word gamete – the male and female reproductive cells (the ova and the sperm cells) that they produce.
The gametophyte features an archegonium and an antheridium. The archegonium is the female reproductive organ, which contains a single ovum. The antheridium is the male reproductive organ, which releases lots of sperm. The sperm swims through a thin film of moisture and into a nearby archegonium where the ovum is waiting to be fertilised.
Once the ovum has been fertilised, it becomes a zygote and later an embryo. The embryonic fern relies on the prothallus (gametophyte) from which it grows for its water and nutrients. Soon the embryonic plant grows into a sporophyte (the large leafy plant we most regularly recognise as a fern) and the prothallus dies.
That is fern reproduction explained as simply as I could (I can only just understand some of it myself) and there is a lot more to it. This link explains fern reproduction in much more detail, however as a warning there is certainly a lot of scientific jargon used: http://www.sas.upenn.edu/~joyellen/fernreproduction.html
The following diagram is a representation of the fern life cycle, from the same website:
So, that felty green stuff on the wall of a cave might not look like much. However clearly there is a lot of interesting information and a very complex life-cycle behind it.