Il6 RHARMACEUTICAL BOTANY 



Tracheids are undeveloped ducts having bordered pores and fre- 

 quently scalariform thickenings. Like tracheae their walls give the 

 characteristic lignin reaction with phloroglucin and HC1. The 

 bordered pores of coniferous tracheids (Fig. 77) exhibit a wall 

 surrounding the pore which forms a dome shaped protrusion into 

 the cell. Like tracheae, also, tracheids convey water with mineral 

 salts in solution. Tracheids and medullary rays make up most of 

 the wood of Conifers. 



PIG. 53. Transverse section of a concentric bundle from the rhizome of Iris 

 (a monocotyledon). Xylem surrounding the phloem. /, Tracheae; f 1 , proto- 

 xylem; s, sieve. tubes; g, companion cells of the internal phloem portion. (From 

 Sayre after Vines.) 



MEDULLARY RAYS 



These are bands of parenchyma cells which extend radially from 

 the cortex to the pith (primary medullary rays) or from a part of the 

 xylem to & part of the phloem (secondary medullary rays). In 

 tangential-longitudinal sections they usually appear spindle shaped 

 'while in radial-longitudinal sections they are seen crossing the other 

 elements. Their primary function is to supply the cambium and 

 wood with elaborated sap formed in the leaves and conveyed away 

 by the sieve tubes, and phloem parenchyma and to supply the cam- 



