THE NANNOPLANKTON OF BOMB-CRATER POOLS. 239
varied very greatly, from a practically "pure culture" (Crypto-
monas erosa) in one case to another in which no less than 48
different species were noted, apart from bacteria and fungi,
including representatives of every one of the groups mentioned
above. Between those limits the numbers ranged from three
or four to about thirty, a moderately good collection yielding
about fifteen. There was no clear evidence that the situation of
the pools had anything to do with the number of species found in
them. For instance, both the pools just mentioned with the
smallest and largest number were completely in the open and at
some distance from any trees.
As regards the general results of the examination of the
nannoplankton of the bomb-crater pools there is not very much
to be said. From the great differences in the number and
especially in the assemblage of the species in the various pools it
seems evident that the organisms must have been introduced into
them in a purely accidental manner. They therefore throw no
new light on the mode and powers of distribution of the organisms
found in them. There is little doubt, however, that such small
organisms can be carried about in a living condition from one
piece of water to another by flying and wading animals such as
insects, birds, dogs, etc. Moreover, many, perhaps most of
them, are able to produce "cysts" or other life-preserving
equivalents, which, should they become dry, may be blown about
with the dust by the wind.
Another result appears to be that the bomb-crater pools do
not show signs of any particular character which would dis-
tinguish them as a group from other pools and small ponds in
the district. In common with these smaller pieces of water,
and thus in contrast to the larger ponds and lakes, the nanno-
plankton of the bomb-crater pools includes a considerable
number of species which are more usually regarded as bottom
forms, e.g., many of the Diatoms and the Rhizopods, or are
characteristic of waters containing appreciable amounts of
dissolved organic matter, e.g., the Euglenids. With only a few
exceptions all of the 130 odd species recorded from the bomb-
crater pools have been found in other pools and ponds in the
Epping Forest district. The only species which might perhaps
be claimed as giving a special character to the bomb-crater pools
is the peculiar spiral and typically brackish water Diatom
Cylindrotheca gracilis, some account of which has already been
given in this Journal (Scourfield, 1943). But as this was only
noted in five out of the twenty-seven pools examined it can
scarcely be regarded as characteristic of these pools.