174
THE ESSEX NATURALIST
and below locks, where overflow water maintains a fairly constant
flow. Also present is the juvenile or Chantransia-stage which appears as
olive green tufts 0.5 cm. in height. This grows on weirs, usually epiphytic
on aquatic mosses.
Species of Batrachospermum have been seen twice in the Lee, above
Rye Meads lock and at King's Weir. On both occasions only one small
clump was present. The plants were fertile and appeared to belong to
the "virgatum" section of the genus.
Bangia atropurpurea (Roth) Ag. occurs in abundance on the sides
of a concrete sluice at Cheshunt and in small amounts on King's Weir.
It was found in 1956, the first record of this freshwater species in
England (Belcher, 1956). This species has been thought to favour hard
water (Smith, 1950).
A species of Asterocytis, probably A. ornata (Agardh) Hamel, has
been seen once in the Lee, but is to be found in considerable quantity
in a gravel pit close to the river at Cheshunt. Here it grows as an
epiphyte on old Cladophora filaments.
It is noteworthy that two genera of freshwater Rhodophyceae,
Lemanea and Rhodochorton (Audouinella), are completely absent from
the Lee. These occur in many other parts of the country in both acid
and calcareous waters.
REFERENCES
Belcher, J. H. (1956). On the occurrence of Bangia atropurpurea
(Roth) Ag. in a freshwater site in Britain. Hydrobiologia 8,
298-299.
Belcher, J. H., & Swale, E. M. F. (1957). Some uncommon freshwater
algae. Phycological Bulletin 5, 40-42.
Smith, G. M. (1950). Freshwater Algae of the United States. 2nd Edi-
tion. McGraw Hill.
2. THE DIATOMS OF THE RIVER LEE
BY JOHN H. BELCHER
Diatoms (or Bacillariophyceae) are microscopic unicellular plants,
yellowish brown in colour, and possessing two silica frustules or
shells which fit together as the two halves of a date box. Those
diatoms which possess a groove-like structure known as a raphe on
one or both frustules can move over a solid surface. No diatoms are
known to swim.
Diatoms are present in large numbers in the sea, in fresh water and
in the soil. They are among the dominant algae in the River Lee for
the major part of the year, both in the plankton and benthos (free-
floating and bottom-living communities). They are so abundant there
at times that they form a dark brown film over the bottom mud, or
give the water an opaque olive green colour.