The Forms and Structures of Roots 



i8s 



they are composed of pectic material which causes them to adhere 

 to the soil particles and brings them in intimate contact with the 

 water films on the soil particles. They 

 begin to develop at a short distance 

 from the tip of the root. Farther back 

 they have attained full length, and be- 

 yond this they are in a dying or dead 

 condition. Thus from day to day the 

 zone of root hairs moves forward with 

 the growth in length of the root, by 

 the continual production of new root 

 hairs just above the elongating region 

 of the root. This brings new root hairs 

 continually into contact with new sup- 

 plies of water and minerals in the soil. 

 As a plant enlarges, its root system 

 becomes more complete through re- 

 peated branching and the elongation 

 of the branches. Most of the absorp- 

 tion occurs in the root-hair zone, and 

 this is continually moved farther and 

 farther from the base of the stem. In 

 large trees this zone may be many 

 feet from the base of the trunk. The 

 fact that the outside of root hairs is 



. Fig. iio. Enlarged view of the 



composed of pectic substances is of end of a root, showing root cap, 



great interest, because these substances g^^^'i^g region, and root hairs. 

 have so strong an attraction for water. It may be of great im- 

 portance, on this account, in absorbing the last particles of 

 available water. 



Root contraction. As roots mature, they may contract in 

 length and so draw the base of the stem a slight distance into the 

 soil. In this way crevice plants on cliffs are continually held 

 firmly in place, in spite of the wearing away of the chff face by 



, Region, of 

 elongation 



Growing 

 point 



