STRUCTURES DEVELOPED FROM THE PERIBLEM 



139 



there is a hollow cylinder called the Periblem (b), and stil? 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 covering called the Epidermis (Fig. 422, o). 



■ The Root-cap. — The epidermis 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. 416, a-b). Toward the summit of the root-cap the 

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

 the epidermis becomes reduced to a single thickness of cells. 



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

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

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

 a more advanced stage, the outer portions also present: o, epiderm; 6, hypoderm; c, endoderm (cortex 

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

 pericycle; x, cambium of the primary phloem-bundle; y, of the primary xylera-bundle; z, of the 

 primary medullary-ray. 



The Root-hairs and Peliferous Layer. — Here it frequently develops a 

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

 perform various processes connected with absorption (Fig. 416, b-c). 

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

 as the Piliferous Layer. 



The Epidermis Proper. — Still farther up these hairs have fallen 

 away, and the single layer, after slight modificat;ions, becomes converted 

 into the epidermis proper. This has a variable duration in different 

 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- 

 oped 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 c), consisting of a 

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



