162 APPLIED BIOLOGY 



illustrates an important work of roots, that of holding 

 particles of soil together. Cutting a forest from a hillside 

 or sloping land allows rapid washing, and large tracts of 

 valuable land have been ruined by removal of fertile soil and 

 by formation of deep gullies. See books and pamphlets 

 on forestry. 



The same erosion often occurs when cultivated fields are 

 bare during the winter. Even if deep furrows or gullies 

 are not formed, it is evident that the muddy water which 

 always flows down bare slopes is carrying away large quanti- 

 ties of the fertile top soil. This has not been sufficiently 

 understood by many farmers in the past, but its importance 

 has been so clearly demonstrated that the scientific farmer 

 of to-day avoids leaving bare during the winter cultivated 

 soil which is sloped so that water tends to wash it away. 

 Protection against erosion is accomplished by planting in 

 the late summer some winter crop, either after harvesting 

 a cultivated crop like corn and vegetables, or between the 

 rows after the last cultivation, or in cultivated orchards 

 and vineyards. This winter crop may be wheat, rye, or 

 grasses to be harvested in the following summer, or a " cover 

 crop " of certain clovers, vetches, rye, and other plants which 

 make great root growth in the autumn, largely prevent 

 erosion by the rains of the winter and early spring, and add 

 to the fertility of the soil if plowed under in the next year's 

 cultivation. 



155. Propagation from Roots. While many stems readily 

 develop roots, most roots do not form stems. Exceptions 

 are sweet potato, which is a thickened secondary root, and 

 roots of the osage-orange tree. In order to start new plants 

 of sweet potato or osage-orange, it is simply necessary to 

 bury root-cuttings in favorable soil. A sweet potato will 

 form stems on any part of its surface ; but roots like carrot, 

 beet, horse-radish, and turnip soon die when the stem end 

 or bud is removed. Some plants (e.g., dahlia) form a cluster 



