194
THE ESSEX NATURALIST
the Lee plankton for most of the year the algae in the following
lists together make up about 20% of the total population for this
period, and may be present in small numbers during Winter and
early Spring, when Stephanodiscus is absent.
When a description and picture of an alga from the river has
been published, the reference is given in an abbreviated form with
the authors' initials only, but appears in full in the list at the end.
Although a number of rare species, including several new
British records, have been found in the Lee and are listed in this
note, it must be pointed out that the Lee is probably not unusual
among lowland rivers in the richness of its flora. It is merely that
these habitats have not been investigated properly owing to the
lack of interest in the freshwater algae in this country.
Volvocales
Spermatozopsis exsidtans Korshikov (B. & S., 1961)
This species was first noticed in a culture of plankton from
Dobb's Weir in September 1959, subsequently appearing in other
cultures. It has recently been isolated from the River Stour at
Flatford, which has a similar plankton. Among the three previous
British records is that of Scourfield (1944), who found it in bomb-
crater pools in Epping Forest.
Mesostigma viride Lauterborn (S., 1962)
A bottom-living form, found in mud samples from King's
Weir, and which has also been recorded in ponds in Richmond
Park (Lund, 1937) and in Kew Gardens (Jane, 1938).
Haematococcus droebakensis var. fastigatus Wollenweber (S.,
1959; B. & S., 1963, in the press)
This uncommon species was found a number of times in the
Rivers Lee and Stort and the New River, and it was also frequent
in the River Stour at Flatford in June 1962. The plankton of
calcareous rivers seems to have been an unrecorded habitat, the
alga having previously only been found in pools on acidic rocks.
Chlamydomonas monadina Stein
This conspicuous species is common in the plankton, where
cells with various types of pyrenoid are present, either a single
one in the form of a ring, or one to three lateral rounded ones. A
cell of the former type isolated and grown in culture gave rise to
cells with all three types of pyrenoid.
Several other unidentified species of Chlamydomonas and
Carteria were also present, but in much smaller numbers.
Chlorogonium elongatum (Dangeard) France (S., 1962)
This species occurred as scattered individuals in the plankton
of the Lee and Stort during 1958 and 1959. They each had two
apical vacuoles. The previous records of this species include one
by Scourfield (1944) from Epping Forest.