16 
THE BRITISH ASSOCIATION AT CARDIFF. 
Jan., 1892. 
rium, they will frequently be seen gliding along in an 
inverted position under the surface of the water with as much 
ease as flies walk on a ceiling. When the animal is heavier 
than water, a cup is formed by its foot in the surface film, 
and, by the extreme mobility of that organ, the animal alter¬ 
nately advances the cup, and then swings its body along 
underneath it; and at the same time feeding on the surface 
conferva. If the snail be forcibly submerged it at once drops 
to the bottom, and can only rise again by the aid of some 
friendly plant. Professor Miall speaks of the rigidity of the 
surface as the means of progression, but, for my part, I do 
not think that a snail could practise this mode of locomotion, 
unless it were of different specific gravity to water ; since the 
surface of liquids does not, so far as I can see, possess different 
properties to the body of the fluid, unless it be distorted 
from a true plane. 
At one of the sectional meetings Professor Miall subse¬ 
quently gave a paper on a subject intimately related to the 
foregoing, namely, “Aquatic Leaves.” Plants growing in 
water have leaves of two kinds—submerged or floating. The 
first derive their C0 2 from the water, and are commonly much 
divided ; whilst the latter breathe direct from the atmosphere 
by means of stomata on their upper surface, in a manner 
similar to those of terrestrial plants. Both kinds of leaves 
may well be studied on the water ranunculus, Now, as even 
the floating leaves are produced from below, they have to 
penetrate the surface film ; and even afterwards have to 
provide for being able to free their surfaces from water 
deposited on them by rain or agitations of the surface of the 
pond or stream in which they live. The first end is attained 
by the young leaf coming up closely curled up, and as 
sharply pointed as if it had to penetrate solid earth. The 
second condition presents more difficulties, and Professor 
Miall’s paper was more suggestive than explanatory. It 
appears to me that the end is most generally attained 
by the leaf being coated by some water-repelling varnish, 
and having a convex surface, so that the water drains 
off all round. Professor Miall, however, objects that the 
supposed varnish does not act when the leaf is dead. In 
the case of our water-lilies, the margin of the leaf is turned 
up so as to form a sharp edge, which cuts the surface of the 
water if the leaf should happen to be submerged, and chan¬ 
nels are provided to drain the leaf to the point where the 
petiole joins the blade. In a species of water-lily which I 
photographed last week at the Botanical Gardens, the edge 
of the leaf is corrugated, and a cleft divides the otherwise 
circular leaf down to the petiole. Professor Miall also stated 
