The Essex Naturalist
115
The Stoneworts of Essex: An interim account
K.J. Adams
Dept. of Life Sciences. University of East London E15 4LZ.
Introduction
The Stoneworts or Charophytes are a very ancient group of giant freshwater,
green, plants that have heen found, as fossils, as far hack as 400 million years ago
in the Silurian period. Their heyday was in Lower Cretaceous times - hy the
Upper Cretaceous they had already begun to decline in abundance and variety,
probably due to competition with newly evolving aquatic higher plants, leaving
us only six extant genera out of the nearly 70 genera of fossil charophytes so far
described. Prior to the evolution of birds, they must have found it rather difficult
to move around rapidly from one freshwater habitat to another. Maybe
therefore, dispersal by birds combined with their rapid growth fate, enabling
them to get in quickly before higher plant aquatics arrive, has given them the
competitive edge to survive as "living fossils". In North America, tufa and marl
deposits laid down by these plants in ancient freshwater lakes, as a result of the
calcite encrustations they accumulate, are sufficiently extensive to be mined for
cement. In this country they have produced "chalk marl" or "chara marl"
deposits in the East Anglian and Irish fens up to a metre thick, that were used to
a limited extent as an agricultural lime in the nineteenth century, and in some
parts of Switzerland the living plants were dried and used as manure. Living
charophytes in this country probably provide a useful source of grazing for ducks
and geese in our larger lakes, particularly those taxa that produce rhizoidal
tubers packed with starch.
To the early naked-eye botanists their superficial resemblance to horsetails led
them to be erroneously classified with the Equisitales. The relatively recent
name "Stonewort" refers to their tendency to be encrusted with calcium
carbonate and form stonelike deposits. In France in the sixteen century they
were used as the modern equivalent of stainless-steel scourers for kitchen
utensils. Because of their large size, and because they make attractive pressed
herbarium specimens (though best pickled in 60% alcohol for subsequent
identification), they have traditionally been studied by higher plant botanists,
and included in many county floras.
Although quite common in Essex, one has to get one's eye in for them, as
they are easily overlooked. The Stoneworts have a Botanical Society of the
British Isles (BSBI) Handbook all to themselves, with illustrations of all the taxa,
and are even lucky enough to have their own JNCC Red Data Book. The
taxonomy of the stoneworts, however, is to say the least, messy, as some taxa are
quite distinct whereas others are morphologically plastic in their environmental
responses. Although the taxonomy is in a state of flux, I have followed the BSBI
handbook (Moore et al, 1986) on the assumption that most people will use it for