PHARMACEUTICAL BOTANY 
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Tracheids are undeveloped ducts having bordered pores and fre¬ 
quently scalariform thickenings. Like tracheae their walls give the 
characteristic lignin reaction with phloroglucin and HC 1 . 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. 
Fig. 53.—Transverse section of a concentric bundle from the rhizome of Iris 
(a monocotyledon). Xylem surrounding the phloem, t. Tracheae; t l , 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 a 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- 
