PLANT TISSUES 
117 
bium and phloem with crude sap which passes up mainly through the 
tracheae and tracheids from the absorptive regions of the roots. 
They furthermore 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 
Fig. 54.—Diagrams illustrating the arrangement of the regions in different 
types of fibrovascular bundles. In each diagram x represents xylem; P, phloem 
and C, cambium. A, Radial bundle; B, concentric bundle of fern stem type; 
C, concentric bundle of monocotyl type; D, closed collateral bundle; E, open 
collateral bundle; F, bi-collateral bundle. 
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 
facing toward the exterior. Stems of most Monocotyledons and 
Horsetails. 
■ II. Open collateral , consisting of a mass of xylem facing toward 
the pith and a mass of phloem facing toward the exterior and sepa¬ 
rated from each other by a cambium. Stems and leaves of Dicoty¬ 
ledons and roots of Dicotyls and Gymnosperms of secondary growth. 
III. Bicollateral, characterized by a xylem mass being between 
an inner and an outer phloem mass. There are two layers of cam¬ 
bium cells, one between the xylem and inner phloem mass, the other 
