WATER-SURFACE PLANTS AND ANIMALS. 233
of the eggs of various water creatures (e.g. Sminthurides aquaticus
is said to deposit its eggs in little holes which it makes in Lemna
fronds) the distribution of these forms is ensured by the carrying
about of the plants. In the same way many microscopic plants
and animals which attach themselves to the under sides of the
plants in question are disseminated.
There are still many miscellaneous ways in which sur-
face-tension is made use of by the water-surface organisms,
For instance, owing to the attraction between similar and the
repulsion between dissimilar capillary curves, surface plants
such as Lemna which give rise to both kinds of curves round their
margins, tend, when not too luxuriant, to space themselves out
in loosely connected irregular patterns. I may also mention in
this connection that I have seen the same spacing out into a
sort of pattern with certain small Entomostraca (Bosmina), when
caught by the surface-film and tying helpless on their sides. The
eggs of Anopheles maculipennis, which are laid singly, are also
said often to form regular patterns, which must be due, I believe,
to the attractions and repulsions of the capillary curves at their
margins when floating on the surface.
Another way in which the attraction between similar capil-
lary curves may be utilised by plants is in connection with ferti-
lisation. It has been recorded that, in its home in America, the
staminate flowers of the water-weed Elodea canadensis (which,
although so common now in this country, rarely produces male
flowers here) break off and rise to the surface where they discharge
the pollen. This floats, but remains dry and therefore must give
rise to small capillary depressions. In the pistillate flowers the
stigmas come out well over the floral envelopes and since they
are not readily wetted, they also give rise to capillary depressions
into which the pollen-grains are drawn and so come into contact
with the stigmas.
The resistance which the surface-film offers to the passage
of bodies from below, especially of such bodies as are not water.
repellent, is turned to account by certain animals which, for one
reason or another, find it advantageous to creep up water plants,
posts, &c., just above the water level. The larva of the Dipterous
insect Dixa is usually to be found in this way bent into a U-shape
just above the water's edge. Certain species of Cyclops (e.g.
C. phaleratus, &c.) and Canthocamptus (e.g. C. minutus, &c.)