are so-called because in 16th century England their ashes were
used as a source of
sodium salts in making soda-based glass. Their high salt content derives from the fact that they
are halophytes - plants that grow in very salty conditions. The glasswort above is growing on a
tidal salt-marsh, which is part of an extensive river estuary.
Glassworts can be divided into two genera: Salicornia, the annual glassworts, and Sarcocornia,
the perennial glassworts. The one illustrated apparently has a woody basal stem suggesting that it is the perennial glasswort Sarcocornia perennis. However, in annual forms when the leaves die and are shed near the older and more shaded bases of stems and branches the tough xylem core and some secondary thickening can make the stems look woody and this could be an annual Salicornia. Glassworts are tricky! We shall have a closer look below and see if we can determine this specimen.
we shall see, glasswort biology is at least as curious as the
plant's name. They are clearly
adapted to dehydrating conditions. Although water is abundant here, the water is saline and that
makes it hard for plants to extract enough water from it. The stems are succulent and the leaves
are inconspicuous, so that less water is lost in the salty coastal winds. Stomatal density is low in
this species (about 30 stomata per square mm) to reduce water loss by transpiration (see water
transport in plants).
in a Tidal Salt Marsh
Above: this is early in the season for glassworts (late July) and the flowering shoots on this specimen have only just begun to develop making it hard to determine this specimen. The full colour also develops later with species variously turning yellowish, orange or reddish-purple according to the amount of anthocyanins produced.
This is late in the season (late December) and this glasswort
finished flowering. Click images for full size.
These glassworts have a beaded appearance (due to the convex segments) and short branches mostly less than 2 cm in length and are quite strongly tinged reddish/purple. This makes me suspect that this is Salicornia ramosissima, the Purple Glasswort, a very variable annual. However, Sarcocornia perennis (Perrenial Glasswort) is another possibility since this also has short branches and a beady appearance.
These plants are tending towards being prostrate, which is again compatible with either Purple or Perennial Glasswort. The base of the stem certainly looks woody and I would have previously considered this to suggest Perennial Glasswort. However, when the fleshy leaves whither and fall from the stems of annuals then they can appear woody at the base. The stem is not visibly rooting at the nodes as one would expect for Perennial Glasswort, which can also become yellowish to reddish. (The red pigment in glassworts generally develops as the plant matures). Perennial Glasswort also typically has some 'sterile shoots' not bearing flowers and so noticeably much narrower - all the shoots here look fertile. Also, there was little visible trace of the glassworts in winter suggesting they were annuals that had died away completely. Purple Glasswort has been recorded before in this area, but perennial glasswort has also been recorded within one kilometer of here. This area is completely submerged during spring high tides. Good external link with nice photographs of perennial glasswort: http://www.nature22.com.
The red-purple colouration of Purple Glasswort increases into the season and by late summer / early Autumn they are reddest, but some individuals remain green. Although some Purple Glassworts have prostrate stems, others are erect.
A more upright (erect) specimen. Salicornia ramosissima is recorded as varying from erect to procumbent (trailing along the ground, i.e. prostrate stems) or decumbent (stems prostrate but turing up to become erect or ascending at the tips). It has been recorded that prostrate growth in glassworts may disappear in cultivation and so may be an adaptation to local conditions rather than a fixed genetic trait.
A close look at the size and number of flowers on the shoots of glassworts is informative. For these specimens:
Recall the each flower is a fleshy disc (of 3 to 4 fused tepals) with a central pore. The obviously smaller lateral flowers, almost completely covered by the bract, the convex bead-like segments and the strong reddish tinge are all compatible with Salicornia ramosissima. This plant occurs mostly in the middle and upper parts of salt marshes, as here. Note the small triangular scale leaf/bract with a papery (scarious) margin beneath the group of three flowers.
A distinctly different second type of glasswort also occurred in the vicinity, albeit in much smaller numbers. This plant was yellow-green with no obvious purple tinge in these specimens, the branches were longer (3 cm plus) and less beaded in appearance as the segments were more cylindrical and the plants were erect in habit.
The lateral flowers are not much smaller than the central one, being more than half its width. My conclusion is that this is Salicornia fragilis or Yellow Glasswort (which can sometimes be tinged red but usually turns yellow-green to yellow). Checking the distribution database for the local area, both these species: Salicornia ramosissima and Salicornia fragilis were recorded in this area recently and no others. Yellow Glasswort occurs mainly on the lower parts of saltmarshes, such as along runnels.
Above and below: Salicornia fragilis. Note the more-or-less cylindrical spikes. The terminal spike has 6-15 (occasionally up to 22) fertile segments. Actually the specimen above is a bit ambiguous and could be Salicornia dolichostachya (see below).
Globally Salicornia glassworts are distributed in the North
temperate zone, northern subtropical and subarctic boreal regions
and in the southern hemisphere they are restricted to South Africa.
Sarcocornia is distributed in warm temperate and subtropical
regions. The following discussion concerns glassworts found in the
British isles, particularly those on the south-east coast.
Glassworts are notoriously difficult to determine! I found it helpful to seek expert instruction from glasswort experts. However, there are always glasswort plants that defy classification, sometimes because they are immature and sometimes because their morphological features do not fall neatly into either species according to keys. First of all, separating Sarcocornia perennis (Perennial Glasswort) from Salicornia is relatively straightforward.
Above: Sarcocornia perennis is perennial and produces a long woody stem which is horizontal for the most part (prostrate). However, the annual Salicornia may also produce a stem with a woody base due to secondary growth, and the stem of Sarcocornia perennis, which may be up to 1 m in length, is largely beneath the soil surface as a thin rhizome and not readily visible. Thus, looking for a woody stem is likely to confuse, but Sarcocornia perennis is extremely hard to uproot whereas the annual forms are easily lifted out of the soil whole. However, the presence of sterile shoots (bearing no flowers) in Sarcocornia perennis is a clear distinguishing feature: they can be seen in the above photo as very slender shoots, whereas the fertile shoots are thicker and more knobbly. The flowers occur in opposite groups of three and in Sarcocornia perennis the three are arranged more-or-less in a straight line and are similar in size (the central flower may be slightly uppermost but the two laterals never touch each other beneath the central flower and the central flower is generally slightly larger) whereas in Salicornia the flowers form a distinct triangle with the central flower uppermost and with the two laterals sometimes touching. Click the photo above for full size and zoom in to see the flowers.
Above: Sarcocornia perennis; note the anthers visible protruding from the central pores of some of the flowers.
Sarcocornia perennis also tends to occur higher up towards the top of salt marshes, though its distribution overlaps with Salicornia. Salicornia species are annuals and can be split into two principal groups within a single geographical region: the diploid species (2n = 18 chromosomes) and the tetraploid species. The diploids consist of: Salicornia pusilla, also called Salicornia disarticulata (although it is not entirely certain that these two are in fact the same species) and the Salicornia europaea aggregate (agg.) of species. S. pulsilla is readily told apart from other Salicornia since its flowers occur singly and not in groups of three and forms narrow shoots.
Above: Salicornia disarticulata (= S. pusilla) the One-flowered Glasswort - a population of small plants which characteristically turn orange-red or purplish-pink when mature and may be non-branched or much branched. Click image for full size and zoom to see the single flowers. Below: close-up of a short branch (typical of the population in this region) showing the single flowers in opposite pairs with one pair per segment. Where Salicornia ramosissima grows nearby some specimens resembling One-flowered Glasswort will have a variable 1, 2 or 3 flowers on each segment with segments bearing different numbers of flowers being found on the same plant. This is the hybrid Salicornia disarticulata x S. ramosissima (S. x marshallii). Occasionally single flowers may occur in other species, but in S. disarticulata this trait is genetic.
The Salicornia europaea agg. in this region consists of S. ramosissima (Purple Glasswort), S. europaea (Common Glasswort although not by any means as common as the Purple Glasswort, though some consider these two to be the same species) and S. obscura (Glaucous Glasswort) . Purple Glasswort occurs mainly in the middle and upper saltmarsh and are often (though not always) flushed reddish-purple by late summer / early autumn. In these forms the flowers occur in triangular arrangements of three, with two opposite pairs present on each segment (and alternating with pairs on adjacent segments). The scale leaves (bracts) form obvious triangular collars with relatively wide scarious (papery) margins. The segments are usually strongly convex, giving the shoots a beaded appearance.
The tetraploid species (4n = 36) in Britain consist (at least) of the Salicornia procumbens aggregate of species: S. emerici (Shiny Glasswort), S. fragilis and S. dolichostachya. Salicornia fragilis and S. dolichostachya occur typically in runnels and on the lower parts of saltmarshes. Also found in similar locations are forms that appear intermediate between S. fragilis and S. ramosissima, having the colour and branching habit of fragilis but the convex fertile segments and unequal flower size of ramosissima. These could be hybrids, though the difference in ploidy in the parental species (fragilis being tetraploid and ramosissima diploid) would create a barrier to interbreeding, this barrier may not be total. Otherwise I suspect these plants are a variety or form of S. ramosissima or one of the species with which I am less familiar, such as Salicornia emerici, S. europaea or S. obscura (Glaucous Glasswort). Glassworts are tricky!
Although superficially similar to Salicornia fragilis, the Long-spiked Glasswort has 12-30 fertile segments per terminal spike instead of the 6-15 (occasionally as many as 22) found in S. fragilis. Thus, if a specimen has more than 22 segments in its terminal spike, as in this case, then it can be determined as S. dolichostachya. The terminal spike is also usually tapering slightly in S. dolichostachya.
Why are Salicornia such a 'taxonomic nightmare'?
In their 2007 paper, A taxonomic nightmare comes true: phylogeny
and biogeography of glassworts (Salicornia L. Chenopodiaceae),
Kadereit et al. (Taxon 56(4): 1143-1170) report on the difficulties
of Glasswort taxonomy and utilise genetic analyses to group related
species. First of all, the reduction in stems, leaves and
flowers to a similar cylindrical form reduces the range of
characters that can be used to distinguish types. These characters
include: the convexity of segments, color, number of flowers, size
and shape of central flower and the size and form of the bracts.
Characters like anther length have in some cases been questioned.
Additionally, Glassworts do not preserve well as herbarium
specimens, hampering the analysis of large collections.
Thanks to Liam Rooney for expert advice on determining glassworts and to Sue Buckingham and The Wild Flower Society, UK, along with the Kent Botanical Recording Group (KBRG) for a guided tour of glassworts in Oare Marshes, Kent.
Salt marshes are challenging habitats for plants and the plants that thrive their have unusual adaptations, making them fascinating subjects of study. Here we review a few such species growing together as a single community on a single salt marsh.