PLANT WORLD CONTINUED 



229 



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. 



The original cell-masses which constitute 

 the three zones mentioned above, are known 

 as fundamental tissues up to the time the pri- 

 mary tissues can be seen. 



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 ) . 



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 epi- 

 dermis. 



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

 128.) The layer bordering on the primary epidermis is known as the 

 hypodermis ( ). This layer is made up of thick- 



