REGIONAL SURVEY OF HATFIELD BROAD OAK. 187
very similar to those on S. antennatus. That these are glandular
and secrete some sticky material is also extremely probable.
So far as I know the agglutination of sand-grains into little
flocculent masses is a phenomenon that has not previously been
recorded. Probably it is not a very common occurrence, at
least not to the extent observed in the Hollow Pond. I suspect,
however, that it may occur, at certain times at any rate, wherever
there is a sandy margin to a small lake or pond, and it would be
very interesting to know how many different species of algae
can give rise to such occurrences.
A question which naturally arises in connection with this
phenomenon is whether the algae derive any benefit from their
intimate association with the sand-grains. A definite answer
is scarcely possible, but it may at least be suggested that the
algae inside the masses of sand-grains are protected from the
attentions of browsing Entomostraca, Rotifers, etc., in a way
which would not be possible if they were simply lying about
among the sand-grains without binding them together.
EXPLANATION OF THE FIGURES.
Figs. 1, 2 and 3. Small clusters of sand-grains linked together by
groups of cells (Coenobia) of Scenedesmus antennatus, x 650.
Fig. 4. A typical (four-celled) group of S. antennatus showing minute
particles adhering to the terminal knobs, x 1700.
Fig. 5. Do. do., showing that the cells are not arranged in a plane,
but in a curve, x 1700.
Fig. 6. A single cell attached by one end to a splinter of quartz, x 1700.
Fig. 7. A single cell highly magnified to show more clearly the extent
of the chloroplast (dotted) and the hyaline cusps with their
expanded ends. p = pyrenoid. x 3000.
THE REGIONAL SURVEY OF HATFIELD
BROAD OAK.
By Mrs. ANNIE R. HATLEY, B.Sc., F.R.G.S., Acad. Dip. Geog.
(Read January 26th, 1929.)
(With 2 Maps.)
BEFORE I commence the real business of this paper I should
like to make one or two remarks of a personal character
by way of introduction. When asked to write "The Lea and
Roding Valleys" in 1922 I realized how little I was acquainted
with the country which lies between the Roding and the Stort.