34 The pa/rt that burns away 



should do so and leave it somewhere where the rain can 

 fall on it, but cover it with a few small branches of trees 

 to prevent the wind blowing the leaves away. The heap 

 shrinks a great deal during the first few months, and in 

 the end it gives a supply of mould that will be very useful 

 if you want to grow any plants in pots. 



Some of the little hollows in the bank under a hedge, 

 especially on chalky soils, are filled with leaf mould which 

 has sometimes changed to a black powder not looking at 

 all like leaves. 



You can also find mould in holes in decayed trees ; 

 here it has formed from the wood of the tree. 



It appears, then, that dead leaves, etc., slowly change 

 into a black or brown substance, shrinking very much as 

 they do so. For this reason they do not go on piling up 

 year after year till finally they fill the wood ; instead 

 they decay or " rot down " to form leaf mould : the big 

 pile of the autumn has changed by the next summer to 

 a thin layer which mixes with the soil. 



We want now to see what happens on a common 

 or a piece of waste ground that is not cultivated. 

 Grass and wild plants grow up in summer and die 

 during winter; their stems and roots are not taken 

 away, but clearly they do not remain where they are, 

 because next year new plants grow up. We may suppose 

 that the dead roots and stems decay like the leaves did, 

 and change to a brown or black mould. It looks as if 

 we are right, because on digging a hole or examining 

 the side of a freshly cut ditch we shall find that the top 

 layer of soil, just so far as the living roots go, is darker 

 in colour than the layer below. 



We must, however, try and get some more proof, and 

 to do this we must study some of our specimens a little 



