64 



PBINCIPLES OF PLANT CULTURE 



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 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 in- 

 creasing in length are considerably 

 shorter than the root-tips {RT). 



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 food 

 materials it contains. The root-hairs 

 greatly increase the absorbing surface 

 of the roots, just as leaves increase 

 the absorbing surface of the plant 

 above ground. Each root-hair con- 

 sists of a single elongated cell (Fig. 

 .30), and is filled with protoplasm, as 

 are the cells in other living parts of 

 the plant (13). As the extremity of 

 the root advances through the soil 

 by growth, new root-hairs are formed in front of the 

 older ones, while those farthest back as rapidly die 

 off, so that only a short portion of a rootlet bears root- 

 hairs at any one time. In Fig. 27 root-hairs are visi- 

 ble in the right glass, and in Fig. 6 they may be seen 



Fig. 28. — Roots of 

 young wheat plant. 

 The parts inclosed in 

 sand (RH) are sur- 

 rounded by root- 

 hairs. RT, root- 

 tips ; e, older parts 

 of root. One-fourth 

 natural size. 



