ELONGATION OF THE ROOT 



thickening goes on assisted by the addition of new fibrillae at 

 either side of the spindle until a delicate line (the-cell wall) reaches 

 across the old cell (Fig. 32, //), and the division of the mother 

 cell into two daughter cells is completed. In this way new cells 

 are being constantly added to the root. It will be noticed, if a 

 longitudinal section through the middle of the root is examined, 

 that various regions of the elongating root may be recognized 

 owing to the difference in the character of their growth. In such 

 a section (Fig. 33) we see that the tip of the root is covered with 

 a mantle of cells, the root cap. This cap protects 

 the delicate cells within like * a thimble so that they 

 are not exposed or injured as the root extends 

 through the soil. At the tip of the root, just within 

 the root cap, the cells are actively dividing and 

 adding new cells to the end of the root and some 

 cells are also added to the inner side of the root 

 cap (Fig. 34). Owing to this unique arrangement 

 it does not matter if the outer cells of the cap are 

 injured or destroyed as the root pushes through the 

 soil, because the cap is constantly being renewed 

 from within, and so always furnishes adequate pro- 

 tection to the delicate cells within. The cells that 

 are added to the tip of the root divide several times 

 after their formation, so we find that the tip of the 



root for a distance of one or two mm. is composed 



. ... 



* sma ^ c^" 5 tnat are m a process of division but 





Diagram of a 



section taken 



through the that are enlarging to only a slight degree (Fig. 34). 



center of a This region is called for this reason the formative 



corn root : r, region o f ^ root< g ac j c o f ^ formative region for 



root cap; e, 



epidermis; c a distance of two to four mm. the cells are dividing 



cortex ; p, cen- to a less degree but are elongating very rapidly and 

 tral region. changing in form (Fig. 35). This is the region of 

 rapid elongation. Still further back elongation has ceased, but 

 the walls of the cells are becoming thicker and the cells are chang- 

 ing in character, so that they perform different duties. This is 

 illustrated in the cross section of the root (Fig. 36), where we see 

 that the outer cells have become modified, forming an epidermis 



