218 THE ESSEX NATURALIST.
and zoological groups which have been able to adapt them-
selves to the special conditions of life at the surface. Naturally
they have not all become equally adapted, or adapted in the
same way, to these conditions and it may be useful, in making
a tentative enumeration of the principal species, to adopt a
rough classification into groups showing more or less adaptation
to similar subdivisions of the environment.
Dealing first with the plants, it is to be noted that, of the
typical surface forms, i.e. those more completely adapted for
life at the surface, there are two distinct groups, those which
are only partly submerged, normally keeping their upper surfaces
dry, and those which are wholly submerged, although floating
quite close under the water-surface. Belonging to the first
group we have in this country five monocotyledonous plants,
namely, the Frogbit, Hydrocharis morsus-ranae, three species of
Duckweed, Lemna minor, L. gibba and L. polyrhiza, and that
smallest of our flowering plants, Wolffia arrhiza ; two species of
vascular cryptogams, namely, Azolla caroliniana and A. filicu-
loides ; and the liverwort, Ricciocarpus natans. The wholly
submerged forms are Lemna trisulca and Riccia fluitans. I am
tempted to add also Elodea (Anacharis) canadensis, for, although
it is typically rooted, it is so often found detached and floating
freely at the surface, that it has strong claim to be regarded as
one of the constituents of the surface-flora.
In addition to the foregoing typical water-surface plants,
many species of Algae become for a time surface dwellers.
Especially is this the case when certain plankton species of blue-
green algae produce the phenomenon known in some districts
as the "breaking of the meres" or more generally as "water-
bloom," the latter being simply a translation of the German
"Wasserblute." Owing to the production of gas vacuoles or
oil globules, these plants sometimes rise to the surface of a
pond or lake, forming a more or less dense layer under the
surface, The foregoing remain submerged and therefore wet,
but there are also other algae, e.g. Euglena spp., Chromulina
rosanoffii, etc., which sometimes form a filmy layer on the
surface, the upper side of which appears to be dry, and this
may possibly be true also of the bacterial scum which often
forms on the surface known by the term zoogloea.
The typical water-surface animals also fall naturally into