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 



