EOOTS AND THEIK 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 1 

 (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 ? 



1 See " Forest Influences," Bulletin No. 7, Division of Forestry, U.S. Dept. 

 Agr., 1893. 



