28 THE FOOD OF PLANTS CHAP, in 



mixture of ice and salt, and leave it there for a 

 few minutes. The water in the flask will soon 

 freeze, and on lifting it carefully out, you will find 

 the flask has been split in tiny pieces by the 

 expanding water. 



Not only are the rocks thus broken up, but the 

 stones and coarser pieces of soil are crumbled finer 

 and finer every winter by the action of the frost. 

 The particles of dense clay are also separated, and 

 the surface is left in a powdery or tilthy condition 

 suitable for a seed-bed. 



When the rain falls upon this finely-powdered 

 rock the water dissolves the soluble portions, and 

 the roots can penetrate more easily in all directions 

 in search of food. 



We have learnt that the soil is a mixture 

 of broken-down rock and of decayed vegetable 

 stuff, and we have divided the mineral portion 

 roughly into sand and clay. 



We have also learnt that the soil can suck 

 up moisture, and why it is drained. 



We can now understand how the soil has 

 been slowly formed by the action of rain, air, 

 and frost, and the decaying of plants, passing 

 gradually from hard rock to rich garden 

 mould. 



