THE PLANT CELL 79 



regions. In the dermatogen zone of the root, three 

 distinct primary tissues are usually apparent. The 

 outer layers of cells at the tip of the root form the 

 root cap and are rather thick-walled. This root cap 

 serves to protect the more delicate structures from 

 injury through contact with sharp particles of soil. 

 The epidermal cells above the root cap give rise to 

 root hairs which are of importance in the absorption 

 of materials from the soil. The root hairs are modified 

 epidermal cells and appear as thin- walled projections 

 from the latter. Portions of the root above those 

 clothed with root hairs are covered with rather thick- 

 walled epidermal cells termed the primary epidermis. 

 In the periblem zone of the root, three primary tissues 

 are present. The layer or layers of cells adjacent 

 to the primary epidermis constitute the hypodermis. 

 The hypodermal cells are usually angled and thick- 

 walled. The layer of cells adjacent to the plerom 

 zone is termed the endodermis. Endodermal cells 

 are usually thick-walled and bear a close resemblance 

 to those of the hypodermal layer. The several layers 

 of cells between the hypodermis and the endodermis 

 are termed the primary cortex or cortical parenchyma. 

 Cortical parenchyma cells represent the original peri- 

 blem tissue and show little if any differences in struc- 

 ture from the latter. The most striking changes in 

 character of cell wall due to development occur in 

 the plerom region. Certain groups of cells in this 

 region acquire thickened walls through deposition of 

 lignin, and thus form the prosenchyma or fibrous 

 elements which give strength to the plant. Con- 

 ducting elements or ducts are developed in the midst 

 of these lignified cells. Each group of lignified cells, 



