156 THE HAUNTS OF LIFE 
ten. But it does not wholly die. Pinhead-like 
clumps of cells, called gemmules, protected 
within a sphere of beautiful capstan-like 
spicules of flint are formed throughout the 
dying body of the sponge, and these rest 
through the winter and start new sponges in 
the spring. It is interesting to compare this 
partial dying with what happens to the float- 
ing bladderwort, and to some other water- 
plants. The bulk of the plant dies, but the 
end of each shoot, heavily laden with stores of 
starch, breaks off and sinks to the floor of the 
lake, rising again, lightened, in spring, to 
start a new floating plant. 
Of great importance for living creatures is 
a peculiar property of water—almost a unique 
property. It has its maximum density—that 
is to say, is most closely packed together—at 
4 degrees centigrade. When it cools below 
this, towards freezing, it expands, instead of 
contracting as almost all other substances do 
when they pass from a liquid to a solid state. 
The expansion of the freezing water means 
that the water at the bottom of the pond rises 
to the surface as it cools below 4 degrees centi- 
grade, and there forms a protective floating 
blanket of ice. As more freezing water rises 
