THE FOREST AS A SOIL-IMPROVER 51 
are extensively used in the manufacture of baskets 
and other wickerwork. 
Even in the cold bogs of the North, the arbor-vitee 
is constantly aiding in the formation of soil. Many 
other trees and bushes are performing the same func- 
tion in almost all parts of the world. Examine eare- 
fully a still pond in the woods, around the edges of 
which there is tree-growth. The bottom of the pond 
will be covered with a thick coating of leaves which 
have fallen into the water from the overhanging 
branches. 
Besides mechanically forcing their way through 
the crevices of rocks, the roots of trees secrete acid, 
and this, combined with the acids produced by the de- 
composition of organic matter, causes the disintegra- 
tion of rocks. In limestone regions the roots of trees 
dissolve their way through rock, and thus aid in the 
formation of soil. This may be seen to good advan- 
tage in coral districts, where the soil is thin, but 
where vegetation grows luxuriantly in the solid rock. 
2. Tue Forest as a Sort-Improver 
The richness of a virgin soil is proverbial. In 
many parts of the world which have long been cul- 
tivated, the soil has lost its virginity. By virginity 
is meant freshness and richness due to an abundance 
of plant-food, and of humus or leaf-mold. Humaus, in 
