214 RHIZOMES AND ROOTS. 



Wood. 



The stele of monocotyledon ous rhizomes is usually composed 

 of a parenchymatous tissue resembling that of the cortex, 

 traversed by a number of collateral or concentric bundles ; the 

 ] atter are usually composed of a few vessels, together with 

 a small column of bast, the whole being often supported by a 

 sheath of sclerenchymatous fibres. The parenchymatous cells, 

 the vessels, and the fibres may all afford valuable diagnostic 

 evidence. 



In monocotyledonous roots the vessels are generally very 

 large and accompanied by numerous thick-walled, pitted fibres. 



The wood of dicotyledonous rhizomes and roots is usually 

 divided by medullary rays into wedge-shaped masses. These 

 masses may consist of thin-walled parenchymatous tissue, in 

 which vessels are scattered either singly or in small groups 

 (gentian), or the parenchymatous tissue may lignify partially 

 (liquorice), or completely (rhatany). In the centre the remains 

 of the primary wood may often be found and recognised by its 

 spiral vessels. Sometimes true vessels are entirely absent, as, 

 for instance, they are from the wood of ipecacuanha root, which 

 consists entirely of tracheids, wood fibres and wood parenchyma. 



Medullary Rays. 



These are found in dicotyledonous but not in monocotyle- 

 donous roots or rhizomes, and form, therefore, an important 

 diagnostic feature. They are generally narrow in roots that 

 have a completely lignified wood, but much wider in those that 

 have a wood consisting largely of thin-walled parenchymatous 

 tissue. They have been described, and their importance 

 indicated, in the section dealing with woods. 



Secretory Tissue. 



Tissue of this kind is by no means of uncommon occurrence. 

 As already pointed out, its presence or absence, and in the 

 former case its nature, are of great importance. Sometimes 

 two kinds of secretory tissue are present, as, for instance, in 

 ginger and allied rhizomes. Here the cortex and stele contain 

 oleo-resin cells, while the bundles contain narrow elongated 

 cells, in which a dark brown substance is secreted. Bryony 

 root contains elongated cells, analogous to laticiferous cells. 



The remarkable glandular hairs that occur in the inter- 

 cellular spaces of male fern rhizome and secrete a resinous sub- 

 stance may also be found whole or broken in the powdered 

 drug. 



