SOIL EXHAUSTION AND ROTATION OF CROPS. 93 



brace pretty nearly all we know about this matter of the 

 growth of roots. As to depth, Schubart has made the most 

 satisfactory observations we possess on the roots of several 

 important crops growing in the field. He separated them 

 from the soil by the following expedient : an excavation was 

 made in the field to the depth of six feet, and a stream of 

 water was directed against the vertical wall until it was 

 washed away, so that the roots of the plants growing in it 

 were laid bare. The roots thus exposed in a field of rye, in 

 one of beans, and in a bed of garden peas presented the ap- 

 pearance of a mat or felt of white fibers to a depth of about 

 four feet from the surface of the ground. Roots of winter 

 wheat were observed as deep as seven feet in a light sub soil, 

 forty-seven days after sowing. The depth of tlie roots of 

 winter wheat, winter rye, and winter coleseed, as well as of clo- 

 ver, was three to four feet." Schubart further collected and 

 weighed the roots of wheat, rye, and peas, and ascertained 

 their proportion of the entire plant. Hellriegel has also pub- 

 lished some observations on the extent of the roots of barley 

 and oats. 



We have a few other observations of this sort, but not' 

 enough to enable us to determine the comparative quantity 

 and depth of the roots of our cultivated plants with any accu- 

 racy. It will not do to draw conclusions as to the length of 

 roots from such observations as these, made, it would appear, 

 in different soils, differently treated and fertilized, because 

 other observations show that the development of the root de- 

 pends not exclusively upon any impulse which it receives 

 from the plant (that is, the root must not necessarily weigh so 

 much or measure so much), but depends also upon the nature 

 of the soil. Where this is rich the roots tend to remain ; 

 they branch and ramify through all the pores of a small bulk 

 of earth. Where this is poor they stretch off and are sparsely 

 distributed through a larger space. Where they find plenty 

 of food they grow and multiply upon it ; where nourishment 

 is lacking they seem to go in search of it. All observations 

 must therefore be comparative. We know, however, in a 

 general way, that the development of roots is different in dif- 



