74 THE ESSEX NATURALIST.
lovers would lose should an artificial system of drainage ever
be developed.
During the great drought of 1921, it was evident that these
gravels had considerable capacity for retaining large quantities
of water. In the report of the excursion secretary to the Quekett
Microscopical Club, 1921, the statement occurs that, on that
Club's visit to Chingford and Strawberry Hill on 25th June, the
pond on Chingford Plain was dry, but that by the Earl's
path contained water, as did the pond at Goldings Hill. These
examples were characteristic of the ponds throughout the
Forest: those on clay were dried up, those on the plateau gravel
below the 300 contour contained water at the end of the long
drought.
The prevalence of birch on light soils and its absence from the
clay, indicated above, refer here solely to the Forest, for birches
are known to grow on nearly every kind of soil. Warming
gives a whole series of Danish birch types, from that on peat
with Calluna and Vaccinium, to birch with meadow grass
underneath. In a communication from Dr. W. G. Smith he
writes, "I know it (the birch) from the surface of 15 feet of peat
on the Humber to the stony slopes of our Scottish hills." There
are a few birches on boggy ground at the head of streams that
run through Hangboy and other Slades, but the water in these
bogs is never quite stagnant.
Samples of soil taken at depths of 1.5", 4.5", 6.0" and 10.0"
were forwarded to Dr. E. J. Salisbury, who has published the
accompanying graphs illustrating hydrogen-ion concentration
and the percentage of humus. They show that the organic
content decreases rapidly with increasing depth, and that the
hydrogen-ion concentration attains its maximum at the surface.
The sample from Piercing Hill is not quite normal; there man
has interfered by planting Pinus sylvestris.
The fruit of the birch Betula alba is small and light in weight.
It consists of a narrow nutlet with an average length of 2.5 mm.,
while its width, including the membranous wings, approximates
4 mm. The average weight of a fruit is 0.00023 grams. From
the size of the nutlet and the breadth of the wings one realizes
that a fruit, once lifted by the wind above the height of the
surrounding trees, may be carried for a great distance during
a heavy gale of wind, yet it is a common sight to see the ground