ROOT STRUCTURE 



55 



they come in contact. These hairs, with thin walls lined 

 with protoplasm and containing considerable volumes of 

 cell-sap, bring the soil solution and the cell-sap into direct 

 contact, thus establishing conditions upon which the in- 

 take or absorption of water and of mineral salts from the 

 soil are absolutely dependent. Still further back from 

 the tip of the root is the part in which growth in diameter 

 may take place, a part which extends to the base of 

 the stem. 



Fig. 11. — Cross-section of Young Root, in the region of root- 

 hairs, where growth in length has ceased and absorption is most rapid. 

 t, duct in the xylem or wood; ph, sieve tube in phloem or bast; p, 

 pericycle, which bounds the vascular tissues; en, endodermis, which 

 bounds cortex, c; e, epidermis, many of the cells of which put out 

 extensions in hairs, h. 



Mechanical Results of Root Structure. — When a 

 part of a root grows in diameter, the root-hairs covering 

 its surface are crushed against the surrounding soil. In- 

 creasing diameter is ordinarily accompanied by increasing 

 strength. Thus the mechanical support which the plant 

 requires is supplied. The position of the zone of root- 



