54 MOVEMENT OF MATERIALS IN PLANTS 



land plant, serving the double purpose of absorption and 

 of mechanical support, and living and growing in a more 

 or less compact and resistant medium, possesses peculiari- 

 ties of structure closely related to the conditions in which 

 it lives. A root is a cylindrical organ, branched, it may 

 be, with a conical rather than a blunt tip, which is capped 

 or covered with cells more or less continually worn off. 

 This thimble-shaped cap (Fig. 10) protecting the tender 

 tissues, forming the growing point of the tip, is merely a 

 means of meeting the mechanical wear inescapable when 

 a point is pushed forward through abrasive material. 

 Immediately behind the thimble-shaped cap and the 

 point at which new cells are formed to replace those 





Fio. 10. — I^njdtudinal sortion of Root-tip, showing rap, enclosing 

 and protortinK tho RrowinR point, l)ohin(l which is the region of growth 

 in length. In thi.s region the tissues difTerentiMte. 



which are worn off by abra^sion, there is what is known as 

 the growing region, only two or there millimeters in 

 length, in which growth in length takes place. From this 

 arrangement it is evident that only a very short segment 

 of the root is pushed through the resistant soil. Further 

 back from the tip, where the cells have ceased to grow in 

 length, the epidermal cells branch out, forming absorbing 

 hairs (Fig. 11), which apply themselves, often very 

 closely, to the surface of the particles of soil with which 



