106 PRACTICAL BOTANY 



arrangement of the meristematic tissues, is, however, the same 

 in young as in old roots ; it is therefore more convenient, and 

 quite as successful, to cut longitudinal sections of the apex 

 of the young lateral roots, which are to be found growing 

 horizontally out of the nodes of the Maize plants. Or, if 

 fitting material for this be not at hand, longitudinal sections 

 may be made of the radicle of the embryo. 



Adopting one of the above methods, cut longitudinal median 

 sections of the apex of the root. The section 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 con- 

 tinuous 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 piliferous layer and cortex are 



