of leaves in autumn and let them "rot down" and change
into mould. If you can in autumn collect enough leaves to make a heap
you {34} 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 {35} more closely. We will take
some leaf mould, some black mould from a hollow in the bank, some from
a tree, soils from a wood, a well-manured garden, a field and some
subsoil. All except the subsoil have a dark colour, but the wood and
garden soils are probably darker than the field soil. Now weigh out 2
grains of each of these and heat in a dish as you did the soil on p. 4;
notice that all except the subsoil go black and then begin to smoulder,
but the moulds smoulder more than the soils. Then weigh again and
calculate how much has burnt away in each case. Here are some results
that have been