217
WATER-SURFACE PLANTS AND ANIMALS;
WITH SPECIAL REFERENCE TO THE
SURFACE-TENSION FACTOR IN THEIR
ENVIRONMENT.
(Being a Presidential Address delivered to the Club on
23rd March, 1929.)
By D. J. SCOURFIELD, I.S.O., F.L.S., F.Z.S., F.R.M.S.
IN my Address last year I incidentally referred to one of
the associations of aquatic plants and animals, namely,
those living on or close to the surface of water, which appeared
to me to require and to deserve much more consideration than
it had hitherto received. With your permission I propose on
this occasion to expand that theme a little by bringing before
you in more detail some of the facts already known about these
organisms, especially in respect of the relationship of many of
them to the peculiar property of the surface-film known as
surface-tension.
But first of all I would like to make it clear which plants
and animals I am regarding in this connection as belonging
essentially to the surface association. By the phrase "water-
surface plants and animals" which I have used in the title of
this Address, I have in mind only those forms which live
continuously, or for certain periods, freely floating at the surface,
or entirely supported by the surface, either above or below it.
It does not include, therefore, any of the aquatic plants which
are rooted at the bottom, having only their leaves, or some of
them, on the surface (e.g. Water-lilies, Water-buttercups, Star-
wort, etc.), nor even those which, although not rooted, usually
remain submerged well below the surface (e.g. Hornwort,
Bladderwort, etc.). Neither does it include those aquatic
animals which only come up now and then for a breath of fresh
air (e.g. most Water-boatmen and Water-beetles, the Water-
spider, etc.), nor those much larger creatures which float and
swim at the surface, namely, the ducks and their allies.
Even in this restricted sense the water-surface plants and
animals form a fairly extensive assemblage ; an assemblage,
however, of a very miscellaneous character, consisting as a rule
of a few forms only from a number of widely separated botanical