The Root and the Soil. 



71 



vancing through the soil, does not move in a straight 

 line, but has an oscillating motion, which enables it to 

 take advantage of openings between the soil particles. 

 The force with which the root-tip is pushed forward was 

 calculated by Darwin to be at least a quarter of a pound 

 in some cases, while the increase of the root in diameter 

 may exert a much greater force. The root-tip is pro- 

 tected in its passage through the 

 soil by a thimble-like covering 

 called the root-cap* 



99. Growth of Roots in Length. 

 Since the soil offers more or less 

 resistance to the growth of roots, it 

 is evident that the roots of land 

 plants cannot elongate through their 

 whole length at once. On the con- 

 trary, the part that increases in 

 length is limited to a short portion 

 just behind the root-tip. Sachs 

 found that the part of the rootlet 

 of the broad bean that increased in 

 length by growth scarcely exceeded 

 FIG. 28. Roots of young half an inch long. In Fig. 28, the 



wheat plant. The parts , . , , 



inclosed in sand (R H) parts that are increasing in length 



are surrounded by . 



root-hairs. R T, root- are considerably shorter than the 



tips; e, older parts of 



root. One-fourth nat- root-tips (RT). 



ural size. (After Frank 



and Tschirch.) IOO . The Root-Hairs (Fig. 29 B) 



develop just behind the elongating part of the rootlet 

 and are present in nearly all plants. Their object is to 



* The root-cap is readily seen without a magnifying glass when 

 bean plant is grown in water. 



