54 



STRUCTURE OF THE ROOT 



after their formation, so we find that the tip of the root for a 

 distance of one or two mm. is composed of small cells that are 

 in a process of division but that are enlarging to only a slight 

 degree (Fig. 34). This region is called for this reason the for- 

 mative region of the root. Back of the formative 

 region for a distance of two or four mm. the cells 

 are dividing to a less degree but are elongating 

 very rapidly and 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 

 changing in character, so that they perform dif- 

 ferent 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 with root hairs for absorption. Within 

 the epidermis is a broad zone of cells, the cortex, 

 often used for the storage of manufactured foods, 

 while in the center of the stem are the vascular 

 bundles. The woody portion of the bundle, or 

 xylem, radiates outward from the center and the 

 soft portion, or phloem, alternates with it (Fig. 

 36, X, p). The material^|absorbed from the soil 

 are largely transported up to the stem and leaves 

 through the xylem, and the foods manufactured 

 by the leaves reach the root by means of the 

 phloem cells. The branches of the root originate 

 in a very curious way from the cells just outside 

 the xylem. These cells by repeated divisions form lateral roots 

 which gradually destroy the tissues in their way and finally grow 

 out to the surface of the root (Fig. 37). By this arrangement 

 they are provided with a root cap and fully prepared to enter 

 the soil on emerging from the old root. 



25. The Transport of Water in the Root. — The inner layer of 

 the cortex, the endodermis (Fig. 36, end), consists of a layer of 

 cells which, in the older part of the roots, forms a very compact 

 and more or less cuthiized ring of cells. This tissue is supposed 



Fig- 33- 

 Diagram of a 

 section taken 

 through the 

 center of a 

 corn root : r, 

 root cap; e, 

 epidermis; c, 

 cortex; p, cen- 

 tral region. 



