The estuary of the River Stinchar in Ayrshire, south of the village of Ballantrae. The marshland is liable to tidal inundation and receives salt-laden air during storms, as well as floods after heavy rain. It provides the habitat for a number of halophytic plants. Image from Google Earth.
Last week I made my annual visit to a shingle beach, just south of Ballantrae in South Ayrshire to check the health of the Oyster Plant Mertensia maritima. It was doing fine, despite the strange weather of the last 12 months. Then I ventured onto the brackish marshland at the mouth of the River Stinchar to look for the Sea Arrow Grass Triglochin maritima. I found plenty of it among the lochans and creeks. This is a site which is frequently showered with salt-laden sea spray and sometime inundated with fresh water when the River overflows its banks.
Triglochin maritima at the edge of the lochan. The yellowish flowering and fruiting spikes are prominent. Photo: John Grace
The Sea Arrow Grass is not a grass. It belongs to a very small Family called the Juncaginaceae (not to be confused with the Juncaceae, the Rushes, although there is a resemblance to Juncus). In Britain there are only two species in the family and in the whole world there are just 30.
It is a native plant, detected as pollen in post-glacial deposits from several river estuaries, and included in Gerard’s 1597 Herbal as ‘Sea Spike-Grass’. It has wide climatic tolerances, growing around the entire coast of Britain and Ireland, thriving equally in the Northern Isles and as far south as Jersey and Guernsey. It ventures inland only where there are brine springs, for example in Cheshire (Lee 1977). It hasn’t yet been seen along salted roads.
Some diagnostic features of Sea Arrow Grass. Left hand image: small lumps at the leaf base are the squamulae intravaginales described by Agnes Arber in 1923 (but nobody seems to know their function); the inset is a hand-cut section of the mid-leaf showing the shape and also the air channels. Right hand image: the ligules are long and pointed. Photos: John Grace.
The structure of the plant was first described many years ago (Hill, 1900). Botanists of Hill’s era were steeped in the technical language of plant anatomy, and his work is hard to read today. I needed to google many of the terms, some of which have fallen into disuse. The leaves, for example, are ‘acicular’. It means needle-like (I should have paid more attention in my Latin classes). They have a half-moon cross-section and contain air channels which I presume enables oxygen to diffuse to the below-ground parts. The leaves arise from the tip of a tough rhizome which sometimes divides, thus forming a small clump. Davy and Bishop (1991) say the clumps can be 2 metres across, but at Ballantrae there are less than this.
Grazed foliage of Sea Arrow Grass. Photo: John Grace
Some of the leaves I saw were heavily grazed, presumably by the pair of mute swans that often cruise on the lochan. The plant does however have chemical defences against grazers. When cells are crushed they release the highly toxic cyanide gas by the breakdown of two compounds, one of which is called Triglochinin. Many plant species have evolved such mechanisms, but usually specific groups of herbivores have adapted to thwart the defender in the ‘evolutionary arms race’.
Stamens and developing fruit. The pollen will fertilise the flowers of other spikes, often on neighbouring plants. Photos: Chris Jeffree.
The flowers are hermaphrodite but the pollen of any one flower is released only after pollination. All flowers in a single spike may be pollinated over a period of 5-8 days. I found that spikes at all stages of development were present on any one plant. The seeds can float in water for up to 6 months, but may also be dispersed by water fowl (see Davy and Bishop 1991).
In this spike the flowering has finished and the seeds are forming. Photo: John Grace.
Triglochin maritima is another very good example of a plant well-adapted to life in salty soils and waters (halophytes). To continue to take up water from a salt solution and maintain their turgor, such species require to maintain their sap with an osmotic concentration exceeding that of the salt water. For details of how this is done, the reader is referred to Flowers and Colmer (2008). We still have much to learn about halophytes before the development of breeds of cereals that can be cultivated in sea water.
References
Arber A (1923) On the ‘squamulae intravaginales’ of the Helobieae. Annals of Botany, 37, 31-41.
Davy AJ and Bishop GF (1991) Triglochin maritima L. Journal of Ecology 79, 531-555.
Flowers TJ and Colmer TD (2008) Salinity tolerance in halophytes. New Phytologist 179, 945-963.
Lee JA (1977) The vegetation of British inland salt marshes. Journal of Ecology, 65, 673-698.
©John Grace
Botany in Scotland 