WALTER WOODHAM COMMON
173
drier, lighter soils of the area, and Quercus petraea the heavier soils.
There are also indications that Quercus hybrids are more successful
than Q. petraea in the damper situations, but the evidence for this
latter conclusion is not very strong.
REFERENCE
W. R. Masefield (1957). Woodham Walter Common. A study in plant
ecology. Essex Naturalist, 30, 22.
The Algal Flora of the River Lee
1. INTRODUCTION AND RHODOPHYCEAE
BY ERICA M. F. SWALE
The Lee, which for a large part of its length runs through the
borderland of Essex and Hertfordshire, is a calcareous river in
which the mineral nutrients necessary for plant growth never
become seriously depleted. The pH varies generally between 7.5 and 8.0
and the calcium content is about 150 p.p.m. An analysis has been
published (Belcher, 1956). Except for a few days in the year the section
of the river investigated (Rye Meads to Waltham Abbey) is not appreci-
ably polluted.
The main channel of the river is the Lee Navigation Canal, which
has numerous locks and is thus slow-flowing. There are also many
smaller and more swiftly flowing channels which leave the Navigation
Canal by means of weirs and form a network of waterways before
eventually rejoining the main river.
The river bottom is mostly muddy and after heavy rains the water
carries much suspended silt. Some of the smaller streams are floored
with small stones which the current keeps fairly clean.
Four members of the Rhodophyceae (often known as red algae,
although they are frequently not red in colour) have been found in the
Lee. The most conspicuous of these is Thorea ramosissima Bory
(Belcher and Swale, 1957). This, the largest freshwater alga in Britain,
was first discovered at King's Weir, Wormley, in 1956. It has been
seen subsequently at various places from Ware to Enfield Lock, a
distance of twelve miles. Previously Thorea had been recorded in this
country only from Walton-on-Thames in 1846-1848, where it is no
longer. The plants reach a length of 150 cm. (five feet) and are often
extremely bushy. They are found chiefly growing on the aprons of weirs