Fig.
8.3-2b. Transverse section of phloem in a cactus (Espostoopsis
dybowskii; native to Brazil with no common name). Unlike the Melocactus
peruvianus in Fig. 8.3-2a, the phloem
of this species is trapped between the expanding xylem (below the bottom of the
micrograph) and a cap of phloem fibers (such caps are common in
plants of many types). As the vascular cambium produces new sieve tube members
and companion cells, the old ones collapse and accumulate interior to the fiber
cap. Instead of producing a radial strip of collapsed cells as in Fig. 8.3-2,
they form tangential arcs of collapsed cells.