230 



General Biology 



the so-called primary tissues develop. How- 

 ever, in the early embryo, even while all the 

 cells are quite alike, it is possible to suggest 

 a division into three zones (Fig. 126), in each 

 of which certain particular structures will ulti- 

 mately grow. 



The diagram shows an outer, or dermato- 

 gen region, a more interior, or periblem region, 

 and an innermost, or plerom region. It is in 

 the dermatogen zone that the first covering- 

 tissues develop, while the periblem zone gives 

 rise to the covering-tissues of the mature 

 plant. All other structures arise in the plerom 

 zone. 



Fig. 127. 



A, longitudinal section through 

 the root tip of spiderwort, showing 

 the plerome (pi), surrounded by 

 the periblem (p), outside of peri- 

 blem the epidermis (e) which dis- 

 appears in the older parts of the 

 root, and the prominent root-cap 

 (c). (After Land.) 



B, diagram of a root hair; CM, 

 cell membrane; CS, cell sap; CW, 

 cell wall; P, protoplasm; N, 

 nucleus; S, soil particles. 



Fig. 128. Arrangement of the Pri- 

 mary Tissues in the Root. 



1. Epidermis. 2. Hypodermis. 3. 

 Primary Cortex. 4. Endodermis. 

 5. Xylem bundle. 6. Pith. 7. 

 Phloem bundle. (After C. W. Bal- 

 lard's "Vegetable Histology." Cour- 

 tesy of John Wiley and Sons.) 



The original cell-masses which constitute the three zones men- 

 tioned above, are known as fundamental tissues up to the time the pri- 

 mary tissues can be seen. 



In the dermatogen of the root, three distinct primary tissues de- 

 velop. The outermost layer at the root-tip (Fig. 127) is the root-cap. 

 This becomes thickened and protects the more delicate structures as the 

 process of growth forces the root-cap through the soil. 



The epidermal cells above the root-cap give rise to root-hairs, which 

 are important absorption organs. 



Above that portion of the root, which is covered with root-hairs, 

 there are thick-walled epidermal cells. These form the primary 

 epidermis. 



In the periblem zone there are also three primary tissues. (Fig. 



