Tlie Root and the Soil. 



71 



vancing through the soil, does not move in a straight 



line, but has an oesillating 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 protected 



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 ^Jf^-JSpi^^^f^^fpartl 

 just behind the root-tip. Sachs mciosea^m^sand^m^H^) 

 found that the part of the rootlet l-''^^,-^,,\^,;,'Z'-'rS!t: 

 of the broad bean that increased in o°|t*°'"^''/r1ink^' Ind 

 length by growth scarcely exceeded Tschirch.) 



half an inch long. In Fig. 28, the parts that are in- 

 creasing in length are considerably shorter than the 

 root-tips (R T) . 



100. 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 absorb water, with 



* The root-cap is readily seen without 

 bean plant is grown in water. 



magnifying glass when a 



