70 



Principles of Plant Culture. 



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.* 



100. Growth of Roots in Length. Since the soil offers 

 more or less resistance to the growth of roots, it is evi- 

 dent that the roots of land plants can- 

 not elongate through their whole 

 length at once. On the contrary, 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 half an inch long. In Fig. 

 28, the parts that are increasing in 

 length are considerably shorter than 

 the root-tips (ET). 



101. The Root-Hairs (Fig. 29 B.) de- 

 velop just behind the elongating part 

 of the rootlet and are present in nearly 

 all plants. Their object is to absorb 



FIG. 28. Roots of young . 



wheat plant. The parts water, with the food materials it con- 

 inciosed in sand (RH) tains. The root-hairs greatly increase 



are surrounded by root- . 



hairs. RT, root-tips; e, the absorbing surface of the roots, 

 older parts of root. One- j us ^ %$ leaves increase the absorbing 



fourth natural size. After 



Frank and Tschirch. surface of the plant above ground. 



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

 plant is grown in water. 



