156 THE HAUNTS OF LIFE 



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 floating 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 con- 

 tracting 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 centigrade, 

 and there forms a protective floating blanket of 

 ice. As more freezing water rises the blanket 



