184 PRACTICAL BOTANY 



must be accurately longitudinal and median, i.e. the 

 section must include the organic axis of the root, around 

 which the several tissues are symmetrically arranged. 



Treat the sections for about ten minutes with dilute 

 potash, or, better, with "eau de javelle " (Appendix A), 

 and mount in glycerine. 



In a median section the following arrangement of 

 tissues will be visible : 



1. A central mass of tissue, clearly defined laterally, 

 and rounded off at its apex, which is at some distance 

 below the external apex of the root : this is the 

 plerome- cylinder. If this tissue be traced back into 

 the older part of the root, it will be found that its 

 central part is continuous with the parenchymatous pith, 

 while its peripheral part develops into the vascular ring. 

 Note rows of larger cells, which may be traced back 

 as continuous with the vessels of the xylem. In the 

 central portion of the plerome are intercellular spaces, 

 which appear black in sections from fresh material, 

 being filled with air. 



2. Surrounding the plerome is a broad band of tissue 

 with intercellular spaces, which appear as irregular 

 black lines. This tissue is the periblem, which is the 

 formative tissue of the cortex. 



3. Outside this is a single layer of cells somewhat 

 elongated radially, and with a thick outer wall : this 

 is the dermatogen, or formative tissue of the 

 piliferous layer. 



If the section be accurately median it will be possible 

 to trace (2) and (3) upwards, till, immediately above 

 the apex of the plerome, they merge into a single 

 layer of cells : thus the formative tissue from which the 



