MICROSCOPIC LIFE OF THE " LEAF-CARPET." 233
organisms are indeed common to both. But on the whole the
leaf-carpet appears to be a quite distinct biological environment
and as such deserves special attention.
In its physical constitution the leaf-carpet is peculiar in that
it consists mainly of a series of thin plates, the leaves, lying over
one another in more or less close contact. Between these plates
there is, except in the uppermost levels and in very dry weather,
a persistent film of water which, by capillary attraction, helps
to hold the plates together and which, no doubt, accounts for
the possibility of the existence of the varied assortment of
microscopic aquatic organisms which are actually to be found in
the leaf-carpet.
Another peculiar feature of the leaf-carpet is that, as regards
its chemical condition, it always has a strongly acid reaction.
This can be tested quite easily with a piece of blue litmus paper
pressed on the damp leaves.
Yet another peculiar feature is that from the biological
standpoint the leaf-carpet as a whole is in a state of more or less
rapid disintegration and decay. This implies, among other
things, at least a goodly supply of bacteria, which in their turn
will furnish sustenance for many small Rhizopods and Flagel-
lates.
Before coming to the account of the organisms which I have
myself found in the leaf-carpet, a few words may be said about
the method employed for obtaining them. In the case of the
extensive work which has been carried out on the organisms of
the soil, at Rothamsted and elsewhere, rather elaborate methods
of cultivation on agar and in sterilised hay infusions, etc., in
addition to simple washing, have been largely relied upon to
obtain results. For some organisms, especially bacteria, cultiva-
tion methods are probably essential, but there is a danger in all
such methods because spores and cysts only accidentally present
in a sample may develop and so give the false impression that
these organisms were a normal part of the population.
In my case the organisms have been obtained by simply
washing samples of the leaf-carpet with or without a previous
preliminary examination of the leaves with a pocket lens.
Naturally, by the preliminary examination only a very limited
number of the microscopic organisms can be seen. Forms that