THE WORK OF ROOTS AND STEMS 63 



Earth's soils are largely made from rocks. The hard 

 rock has first to be split and broken and crumbled, 

 and then gi^adually mixed with other materials; and 

 at length soil is formed, in which plants can grow. 



More powers than one go to this work of soil-making. 

 It is tremendously important work, and it takes a good 

 deal of time. 



First King Frost steps in. He splits the rocks, and 

 breaks up the stiff earth-clods, and does no end of good. 

 Where a little water happens to lie in a crack, it 

 presses the sides of the crack as it freezes farther apart, 

 and so begins to demolish the mass. Bitterly cold 

 weather, though unpleasant, is much needed each year 

 for the soil. 



Frost acts quickly and strongly; but another power, 

 which is gentle and slow in its methods, does as much 

 if not more in its own way. This is — Rain. One would 

 hardly suppose that small falling raindrops could help 

 to destroy solid rocks ; to " decompose " them is the 

 right word, and to " decompose " simply means to 

 " unmake." 



That which gives raindrops this power is the presence 

 of certain gases, such as nitric and carbonic acid gases, 

 in the water. If enough of these is present it can 

 eat its way gradually through the hardest rocks. At 

 the same time, such gases unite with other substances 

 found there, to form salts which are required for the 

 growth of plants. 



When the soil is so far ready that certain of the 

 simpler kinds, such as lichens and mosses, can begin to 

 grow in it, and when spores have been brought by wind 

 or water or animals, then such plants themselves will 



