STRUCTURES DEVELOPED FROM THE PERIBLEM 



139 



there is a hollow cylinder called the Periblem (6), and still outside of 

 this and upon the surface of the root a second hollow cylinder, the 

 Dermatogen (c). 



Structures Developed from the Dermatogen. — The last mentioned 

 develops a primary covorhig called the Ki)i(k'rmis (Fig. 422, a). 



The Root-cap. — The ci)idermis consists in its earliest stage, and there- 

 fore at the very tip, of a number of layers of cells which protect the 

 apical growing point of the root, and is therefore called at that point 

 the Root-cap (Fig. 41 G, a-b). Toward the summit of the root-cap the 

 outer layers of cells successively wear off or are cast off, so that 

 the epidermis becomes reduced to a single thickness of cells. 



Fig. 420. Diagram illustrating arrangement of ground-tissues of root: a, plerom; b, periblem: c, 

 dermatogen. 421. Plerom enclosed by endodermis (c), with first appearance of bundles: e, xylem- 

 bundle; /, phloem-bundle; g, medullary-raj- : h, pericycle; i, temporary pith. 422. The same in 

 a more advanced stage, the outer portions also present: a, epiderm; b, liypoderm; c, endodcrm (cortex 

 between b and c); e, xylcm-bundles now meeting at center; /, phloem-bundle; (?, medullary-ray; h, 

 pericycle; i, cambium of the primary phloem-bundle; y, of the primary xylem-bundle; 2, of the 

 primary medullary-ray. 



The Ruot-hdirs and Pclij'croiis Layer.- — Here it frequently (lc\clops a 

 dense covering of Root-hairs which adhere tenaciously to the soil and 

 perform various processes connected with absorptit)n (Fig. 410, h-c). 

 For this reason, this portion of the epidermis of the root is known 

 as the Piliferous Layer. 



The Epidermis Proper. — Still farther uj) these hairs have fallen 

 away, and the single layer, after slight modifications, becomes converted 

 into the epidermis i)roper. This has a variable duration in difl'erent 

 plants and is consequently found covering the root for a greater or less 

 distance upward. Almost always its duration is very short. It either 

 disappears altogether, being replaced by a structure (Periderm) devel- 

 ()])c<l from the periblem, or in rare cases itself develops into the periderm. 



Structures Developed from the Periblem. — The periblem of the root 

 develops into the Cortex (Fig. 422 between b and r), consisting of a 

 number, often a large number, of layers of cells. 



