THE ROOT AND THE SOIL 69 



them once or twice before the final planting out, for the 

 reasons named above. 



107. Root pruning (416) is sometimes employed as 

 a substitute for transplanting, and is especially useful 

 to trees that form few branch roots, as the hickory and 

 walnut. In this case, the tap root is cut off a few inches 

 below the surface of the soil the year before transplanting. 



108. The horizontal extent of roots is usually greater 

 than is generally supposed. In upright-growing plants, 

 the area occupied by the roots, as a rule, exceeds that 

 covered by the foliage, while in spreading and trailing 

 plants, the roots are probably not often less in extent 

 than the branches. It appears from the observations 

 recorded that even in such plants as the melon and squash, 

 the horizontal extent of the roots usually equals or exceeds 

 that of the runners. As the diffusion of soluble matters 

 in the soil water is probably much hindered by the soil 

 particles, the roots of plants need to travel farther after 

 food than do the branches, which develop in a freely cir- 

 culating medium. Especially is this true of plants grow- 

 ing in poor soil. 



109. Depth of roots in the soil. It appears from the 

 observations recorded that the extreme depth reached by 

 roots is generally less than their greatest horizontal extent. 

 The distance reached by the deeper roots is probably 

 governed largely by the nature of the sub-soil and the 

 depth of free ground water. But in most annual crops 

 a comparatively small part of the root system develops 

 below the plow line. At the Geneva Experiment Station 1 

 the chief root-feeding ground of the field and garden 

 crops grown in that locality appeared to be from three to 



1 See Report of New York Agricultural Experiment Station, 

 Geneva, 1886, p. 165. 



