ROOTS AND THEIR RELATION TO PLANTS 27 
growth, is one of the most serious calamities that can befall a 
country. It is especially formidable in hilly regions, which 
may become wholly uninhabitable if the forests are cut off 
and the turf on the hillsides is destroyed by too constant 
grazing and trampling of sheep or goats. Throughout south- 
ern Europe immense areas of land once valuable for timber 
and for grazing have thus been ruined, and the same process 
is under way in our own country all the way from New Eng- 
land to the Pacific coast region. One of the clearest ways in 
which the idea of loss by the washing away of the soil can 
be presented is by considering how the land is carried into 
the sea by great rivers. The delta of the Mississippi covers 
an area of more than 12,000 square miles. It consists of mate- 
rial brought down by the river in the form of mud, which 
now forms a deposit of unknown thickness, probably averag- 
ing more than 500 feet. It is calculated that the river carries 
every year enough solid matter to form a layer one foot thick 
over an area of about 268 square miles. Remembering that 
this mud consists mainly of the choicest part of the rich soil 
of the Mississippi basin, it is easy to see that the land is robbed 
every year of the material to support enormous harvests! 
(see Chapter XIX). 
25. Air supply of earth roots. Earth roots require a consid- 
erable supply of air. This is shown by the fact that most 
trees are injured or killed when the soil in which they grow is 
long flooded with water, as is often the case when a stream is 
greatly widened by the construction of a dam across it. The 
same result is seen when low fields of corn, wheat, oats, or 
cotton are flooded after heavy rains. Unglazed earthen flower- 
pots are better for house plants than glazed ones or glass jars, 
because they allow air to pass freely through the porous 
material of the pot. 
How do the earth roots of such plants as water lilies get 
their air supply ? 
1See '* Forest Influences,’’ Bulletin No. 7, Division of Forestry, U.S. Dept. 
Agr., 1893, 
