PLANT TISSUES 
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). Their 
primary function is to supply the cambium and wood with elabo- 
rated sap formed in the leaves and conveyed away by the sieve tubes, 
Fig. 31. — Transverse section of a concentric bundle from the rhizome of Iris 
(a monocotyledon). Xylem surrounding the phloem, t, Tracheae; t 1 , proto- 
xylem; s, sieve tubes; g, companion cells of the internal phloem portion. ( From 
Sayre after Vines.) 
and phloem parenchyma and to supply the cambium and phloem 
with crude sap which passes up mainly through the tracheae and 
tracheids from the absorptive regions of the roots. They further- 
more serve as storage places for starch, alkaloids, resins, and other 
substances. 
Fibro-vascular Bundles are groups of fibers, vessels and cells cours- 
ing through the various organs of a plant and serving for conduction 
and support. According to the relative structural arrangement of 
their xylem and phloem masses they may be classed as follows: 
I. Closed collateral, consisting of a mass of xylem lying alongside 
of a mass of phloem, the xylem facing toward the center, the phloem 
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